Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net

[pandora-kernel.git] / block / blk-core.c

/*
 * Copyright (C) 1991, 1992 Linus Torvalds
 * Copyright (C) 1994,      Karl Keyte: Added support for disk statistics
 * Elevator latency, (C) 2000  Andrea Arcangeli <andrea@suse.de> SuSE
 * Queue request tables / lock, selectable elevator, Jens Axboe <axboe@suse.de>
 * kernel-doc documentation started by NeilBrown <neilb@cse.unsw.edu.au>
 *      -  July2000
 * bio rewrite, highmem i/o, etc, Jens Axboe <axboe@suse.de> - may 2001
 */

/*
 * This handles all read/write requests to block devices
 */
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/backing-dev.h>
#include <linux/bio.h>
#include <linux/blkdev.h>
#include <linux/highmem.h>
#include <linux/mm.h>
#include <linux/kernel_stat.h>
#include <linux/string.h>
#include <linux/init.h>
#include <linux/completion.h>
#include <linux/slab.h>
#include <linux/swap.h>
#include <linux/writeback.h>
#include <linux/task_io_accounting_ops.h>
#include <linux/fault-inject.h>
#include <linux/list_sort.h>

#define CREATE_TRACE_POINTS
#include <trace/events/block.h>

#include "blk.h"

EXPORT_TRACEPOINT_SYMBOL_GPL(block_bio_remap);
EXPORT_TRACEPOINT_SYMBOL_GPL(block_rq_remap);
EXPORT_TRACEPOINT_SYMBOL_GPL(block_bio_complete);

static int __make_request(struct request_queue *q, struct bio *bio);

/*
 * For the allocated request tables
 */
static struct kmem_cache *request_cachep;

/*
 * For queue allocation
 */
struct kmem_cache *blk_requestq_cachep;

/*
 * Controlling structure to kblockd
 */
static struct workqueue_struct *kblockd_workqueue;

static void drive_stat_acct(struct request *rq, int new_io)
{
        struct hd_struct *part;
        int rw = rq_data_dir(rq);
        int cpu;

        if (!blk_do_io_stat(rq))
                return;

        cpu = part_stat_lock();

        if (!new_io) {
                part = rq->part;
                part_stat_inc(cpu, part, merges[rw]);
        } else {
                part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
                if (!hd_struct_try_get(part)) {
                        /*
                         * The partition is already being removed,
                         * the request will be accounted on the disk only
                         *
                         * We take a reference on disk->part0 although that
                         * partition will never be deleted, so we can treat
                         * it as any other partition.
                         */
                        part = &rq->rq_disk->part0;
                        hd_struct_get(part);
                }
                part_round_stats(cpu, part);
                part_inc_in_flight(part, rw);
                rq->part = part;
        }

        part_stat_unlock();
}

void blk_queue_congestion_threshold(struct request_queue *q)
{
        int nr;

        nr = q->nr_requests - (q->nr_requests / 8) + 1;
        if (nr > q->nr_requests)
                nr = q->nr_requests;
        q->nr_congestion_on = nr;

        nr = q->nr_requests - (q->nr_requests / 8) - (q->nr_requests / 16) - 1;
        if (nr < 1)
                nr = 1;
        q->nr_congestion_off = nr;
}

/**
 * blk_get_backing_dev_info - get the address of a queue's backing_dev_info
 * @bdev:       device
 *
 * Locates the passed device's request queue and returns the address of its
 * backing_dev_info
 *
 * Will return NULL if the request queue cannot be located.
 */
struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev)
{
        struct backing_dev_info *ret = NULL;
        struct request_queue *q = bdev_get_queue(bdev);

        if (q)
                ret = &q->backing_dev_info;
        return ret;
}
EXPORT_SYMBOL(blk_get_backing_dev_info);

void blk_rq_init(struct request_queue *q, struct request *rq)
{
        memset(rq, 0, sizeof(*rq));

        INIT_LIST_HEAD(&rq->queuelist);
        INIT_LIST_HEAD(&rq->timeout_list);
        rq->cpu = -1;
        rq->q = q;
        rq->__sector = (sector_t) -1;
        INIT_HLIST_NODE(&rq->hash);
        RB_CLEAR_NODE(&rq->rb_node);
        rq->cmd = rq->__cmd;
        rq->cmd_len = BLK_MAX_CDB;
        rq->tag = -1;
        rq->ref_count = 1;
        rq->start_time = jiffies;
        set_start_time_ns(rq);
        rq->part = NULL;
}
EXPORT_SYMBOL(blk_rq_init);

static void req_bio_endio(struct request *rq, struct bio *bio,
                          unsigned int nbytes, int error)
{
        if (error)
                clear_bit(BIO_UPTODATE, &bio->bi_flags);
        else if (!test_bit(BIO_UPTODATE, &bio->bi_flags))
                error = -EIO;

        if (unlikely(nbytes > bio->bi_size)) {
                printk(KERN_ERR "%s: want %u bytes done, %u left\n",
                       __func__, nbytes, bio->bi_size);
                nbytes = bio->bi_size;
        }

        if (unlikely(rq->cmd_flags & REQ_QUIET))
                set_bit(BIO_QUIET, &bio->bi_flags);

        bio->bi_size -= nbytes;
        bio->bi_sector += (nbytes >> 9);

        if (bio_integrity(bio))
                bio_integrity_advance(bio, nbytes);

        /* don't actually finish bio if it's part of flush sequence */
        if (bio->bi_size == 0 && !(rq->cmd_flags & REQ_FLUSH_SEQ))
                bio_endio(bio, error);
}

void blk_dump_rq_flags(struct request *rq, char *msg)
{
        int bit;

        printk(KERN_INFO "%s: dev %s: type=%x, flags=%x\n", msg,
                rq->rq_disk ? rq->rq_disk->disk_name : "?", rq->cmd_type,
                rq->cmd_flags);

        printk(KERN_INFO "  sector %llu, nr/cnr %u/%u\n",
               (unsigned long long)blk_rq_pos(rq),
               blk_rq_sectors(rq), blk_rq_cur_sectors(rq));
        printk(KERN_INFO "  bio %p, biotail %p, buffer %p, len %u\n",
               rq->bio, rq->biotail, rq->buffer, blk_rq_bytes(rq));

        if (rq->cmd_type == REQ_TYPE_BLOCK_PC) {
                printk(KERN_INFO "  cdb: ");
                for (bit = 0; bit < BLK_MAX_CDB; bit++)
                        printk("%02x ", rq->cmd[bit]);
                printk("\n");
        }
}
EXPORT_SYMBOL(blk_dump_rq_flags);

static void blk_delay_work(struct work_struct *work)
{
        struct request_queue *q;

        q = container_of(work, struct request_queue, delay_work.work);
        spin_lock_irq(q->queue_lock);
        __blk_run_queue(q);
        spin_unlock_irq(q->queue_lock);
}

/**
 * blk_delay_queue - restart queueing after defined interval
 * @q:          The &struct request_queue in question
 * @msecs:      Delay in msecs
 *
 * Description:
 *   Sometimes queueing needs to be postponed for a little while, to allow
 *   resources to come back. This function will make sure that queueing is
 *   restarted around the specified time.
 */
void blk_delay_queue(struct request_queue *q, unsigned long msecs)
{
        queue_delayed_work(kblockd_workqueue, &q->delay_work,
                                msecs_to_jiffies(msecs));
}
EXPORT_SYMBOL(blk_delay_queue);

/**
 * blk_start_queue - restart a previously stopped queue
 * @q:    The &struct request_queue in question
 *
 * Description:
 *   blk_start_queue() will clear the stop flag on the queue, and call
 *   the request_fn for the queue if it was in a stopped state when
 *   entered. Also see blk_stop_queue(). Queue lock must be held.
 **/
void blk_start_queue(struct request_queue *q)
{
        WARN_ON(!irqs_disabled());

        queue_flag_clear(QUEUE_FLAG_STOPPED, q);
        __blk_run_queue(q);
}
EXPORT_SYMBOL(blk_start_queue);

/**
 * blk_stop_queue - stop a queue
 * @q:    The &struct request_queue in question
 *
 * Description:
 *   The Linux block layer assumes that a block driver will consume all
 *   entries on the request queue when the request_fn strategy is called.
 *   Often this will not happen, because of hardware limitations (queue
 *   depth settings). If a device driver gets a 'queue full' response,
 *   or if it simply chooses not to queue more I/O at one point, it can
 *   call this function to prevent the request_fn from being called until
 *   the driver has signalled it's ready to go again. This happens by calling
 *   blk_start_queue() to restart queue operations. Queue lock must be held.
 **/
void blk_stop_queue(struct request_queue *q)
{
        __cancel_delayed_work(&q->delay_work);
        queue_flag_set(QUEUE_FLAG_STOPPED, q);
}
EXPORT_SYMBOL(blk_stop_queue);

/**
 * blk_sync_queue - cancel any pending callbacks on a queue
 * @q: the queue
 *
 * Description:
 *     The block layer may perform asynchronous callback activity
 *     on a queue, such as calling the unplug function after a timeout.
 *     A block device may call blk_sync_queue to ensure that any
 *     such activity is cancelled, thus allowing it to release resources
 *     that the callbacks might use. The caller must already have made sure
 *     that its ->make_request_fn will not re-add plugging prior to calling
 *     this function.
 *
 *     This function does not cancel any asynchronous activity arising
 *     out of elevator or throttling code. That would require elevaotor_exit()
 *     and blk_throtl_exit() to be called with queue lock initialized.
 *
 */
void blk_sync_queue(struct request_queue *q)
{
        del_timer_sync(&q->timeout);
        cancel_delayed_work_sync(&q->delay_work);
}
EXPORT_SYMBOL(blk_sync_queue);

/**
 * __blk_run_queue - run a single device queue
 * @q:  The queue to run
 *
 * Description:
 *    See @blk_run_queue. This variant must be called with the queue lock
 *    held and interrupts disabled.
 */
void __blk_run_queue(struct request_queue *q)
{
        if (unlikely(blk_queue_stopped(q)))
                return;

        q->request_fn(q);
}
EXPORT_SYMBOL(__blk_run_queue);

/**
 * blk_run_queue_async - run a single device queue in workqueue context
 * @q:  The queue to run
 *
 * Description:
 *    Tells kblockd to perform the equivalent of @blk_run_queue on behalf
 *    of us.
 */
void blk_run_queue_async(struct request_queue *q)
{
        if (likely(!blk_queue_stopped(q))) {
                __cancel_delayed_work(&q->delay_work);
                queue_delayed_work(kblockd_workqueue, &q->delay_work, 0);
        }
}
EXPORT_SYMBOL(blk_run_queue_async);

/**
 * blk_run_queue - run a single device queue
 * @q: The queue to run
 *
 * Description:
 *    Invoke request handling on this queue, if it has pending work to do.
 *    May be used to restart queueing when a request has completed.
 */
void blk_run_queue(struct request_queue *q)
{
        unsigned long flags;

        spin_lock_irqsave(q->queue_lock, flags);
        __blk_run_queue(q);
        spin_unlock_irqrestore(q->queue_lock, flags);
}
EXPORT_SYMBOL(blk_run_queue);

void blk_put_queue(struct request_queue *q)
{
        kobject_put(&q->kobj);
}
EXPORT_SYMBOL(blk_put_queue);

/*
 * Note: If a driver supplied the queue lock, it should not zap that lock
 * unexpectedly as some queue cleanup components like elevator_exit() and
 * blk_throtl_exit() need queue lock.
 */
void blk_cleanup_queue(struct request_queue *q)
{
        /*
         * We know we have process context here, so we can be a little
         * cautious and ensure that pending block actions on this device
         * are done before moving on. Going into this function, we should
         * not have processes doing IO to this device.
         */
        blk_sync_queue(q);

        del_timer_sync(&q->backing_dev_info.laptop_mode_wb_timer);
        mutex_lock(&q->sysfs_lock);
        queue_flag_set_unlocked(QUEUE_FLAG_DEAD, q);
        mutex_unlock(&q->sysfs_lock);

        if (q->elevator)
                elevator_exit(q->elevator);

        blk_throtl_exit(q);

        blk_put_queue(q);
}
EXPORT_SYMBOL(blk_cleanup_queue);

static int blk_init_free_list(struct request_queue *q)
{
        struct request_list *rl = &q->rq;

        if (unlikely(rl->rq_pool))
                return 0;

        rl->count[BLK_RW_SYNC] = rl->count[BLK_RW_ASYNC] = 0;
        rl->starved[BLK_RW_SYNC] = rl->starved[BLK_RW_ASYNC] = 0;
        rl->elvpriv = 0;
        init_waitqueue_head(&rl->wait[BLK_RW_SYNC]);
        init_waitqueue_head(&rl->wait[BLK_RW_ASYNC]);

        rl->rq_pool = mempool_create_node(BLKDEV_MIN_RQ, mempool_alloc_slab,
                                mempool_free_slab, request_cachep, q->node);

        if (!rl->rq_pool)
                return -ENOMEM;

        return 0;
}

struct request_queue *blk_alloc_queue(gfp_t gfp_mask)
{
        return blk_alloc_queue_node(gfp_mask, -1);
}
EXPORT_SYMBOL(blk_alloc_queue);

struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id)
{
        struct request_queue *q;
        int err;

        q = kmem_cache_alloc_node(blk_requestq_cachep,
                                gfp_mask | __GFP_ZERO, node_id);
        if (!q)
                return NULL;

        q->backing_dev_info.ra_pages =
                        (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
        q->backing_dev_info.state = 0;
        q->backing_dev_info.capabilities = BDI_CAP_MAP_COPY;
        q->backing_dev_info.name = "block";

        err = bdi_init(&q->backing_dev_info);
        if (err) {
                kmem_cache_free(blk_requestq_cachep, q);
                return NULL;
        }

        if (blk_throtl_init(q)) {
                kmem_cache_free(blk_requestq_cachep, q);
                return NULL;
        }

        setup_timer(&q->backing_dev_info.laptop_mode_wb_timer,
                    laptop_mode_timer_fn, (unsigned long) q);
        setup_timer(&q->timeout, blk_rq_timed_out_timer, (unsigned long) q);
        INIT_LIST_HEAD(&q->timeout_list);
        INIT_LIST_HEAD(&q->flush_queue[0]);
        INIT_LIST_HEAD(&q->flush_queue[1]);
        INIT_LIST_HEAD(&q->flush_data_in_flight);
        INIT_DELAYED_WORK(&q->delay_work, blk_delay_work);

        kobject_init(&q->kobj, &blk_queue_ktype);

        mutex_init(&q->sysfs_lock);
        spin_lock_init(&q->__queue_lock);

        /*
         * By default initialize queue_lock to internal lock and driver can
         * override it later if need be.
         */
        q->queue_lock = &q->__queue_lock;

        return q;
}
EXPORT_SYMBOL(blk_alloc_queue_node);

/**
 * blk_init_queue  - prepare a request queue for use with a block device
 * @rfn:  The function to be called to process requests that have been
 *        placed on the queue.
 * @lock: Request queue spin lock
 *
 * Description:
 *    If a block device wishes to use the standard request handling procedures,
 *    which sorts requests and coalesces adjacent requests, then it must
 *    call blk_init_queue().  The function @rfn will be called when there
 *    are requests on the queue that need to be processed.  If the device
 *    supports plugging, then @rfn may not be called immediately when requests
 *    are available on the queue, but may be called at some time later instead.
 *    Plugged queues are generally unplugged when a buffer belonging to one
 *    of the requests on the queue is needed, or due to memory pressure.
 *
 *    @rfn is not required, or even expected, to remove all requests off the
 *    queue, but only as many as it can handle at a time.  If it does leave
 *    requests on the queue, it is responsible for arranging that the requests
 *    get dealt with eventually.
 *
 *    The queue spin lock must be held while manipulating the requests on the
 *    request queue; this lock will be taken also from interrupt context, so irq
 *    disabling is needed for it.
 *
 *    Function returns a pointer to the initialized request queue, or %NULL if
 *    it didn't succeed.
 *
 * Note:
 *    blk_init_queue() must be paired with a blk_cleanup_queue() call
 *    when the block device is deactivated (such as at module unload).
 **/

struct request_queue *blk_init_queue(request_fn_proc *rfn, spinlock_t *lock)
{
        return blk_init_queue_node(rfn, lock, -1);
}
EXPORT_SYMBOL(blk_init_queue);

struct request_queue *
blk_init_queue_node(request_fn_proc *rfn, spinlock_t *lock, int node_id)
{
        struct request_queue *uninit_q, *q;

        uninit_q = blk_alloc_queue_node(GFP_KERNEL, node_id);
        if (!uninit_q)
                return NULL;

        q = blk_init_allocated_queue_node(uninit_q, rfn, lock, node_id);
        if (!q)
                blk_cleanup_queue(uninit_q);

        return q;
}
EXPORT_SYMBOL(blk_init_queue_node);

struct request_queue *
blk_init_allocated_queue(struct request_queue *q, request_fn_proc *rfn,
                         spinlock_t *lock)
{
        return blk_init_allocated_queue_node(q, rfn, lock, -1);
}
EXPORT_SYMBOL(blk_init_allocated_queue);

struct request_queue *
blk_init_allocated_queue_node(struct request_queue *q, request_fn_proc *rfn,
                              spinlock_t *lock, int node_id)
{
        if (!q)
                return NULL;

        q->node = node_id;
        if (blk_init_free_list(q))
                return NULL;

        q->request_fn           = rfn;
        q->prep_rq_fn           = NULL;
        q->unprep_rq_fn         = NULL;
        q->queue_flags          = QUEUE_FLAG_DEFAULT;

        /* Override internal queue lock with supplied lock pointer */
        if (lock)
                q->queue_lock           = lock;

        /*
         * This also sets hw/phys segments, boundary and size
         */
        blk_queue_make_request(q, __make_request);

        q->sg_reserved_size = INT_MAX;

        /*
         * all done
         */
        if (!elevator_init(q, NULL)) {
                blk_queue_congestion_threshold(q);
                return q;
        }

        return NULL;
}
EXPORT_SYMBOL(blk_init_allocated_queue_node);

int blk_get_queue(struct request_queue *q)
{
        if (likely(!test_bit(QUEUE_FLAG_DEAD, &q->queue_flags))) {
                kobject_get(&q->kobj);
                return 0;
        }

        return 1;
}
EXPORT_SYMBOL(blk_get_queue);

static inline void blk_free_request(struct request_queue *q, struct request *rq)
{
        if (rq->cmd_flags & REQ_ELVPRIV)
                elv_put_request(q, rq);
        mempool_free(rq, q->rq.rq_pool);
}

static struct request *
blk_alloc_request(struct request_queue *q, int flags, int priv, gfp_t gfp_mask)
{
        struct request *rq = mempool_alloc(q->rq.rq_pool, gfp_mask);

        if (!rq)
                return NULL;

        blk_rq_init(q, rq);

        rq->cmd_flags = flags | REQ_ALLOCED;

        if (priv) {
                if (unlikely(elv_set_request(q, rq, gfp_mask))) {
                        mempool_free(rq, q->rq.rq_pool);
                        return NULL;
                }
                rq->cmd_flags |= REQ_ELVPRIV;
        }

        return rq;
}

/*
 * ioc_batching returns true if the ioc is a valid batching request and
 * should be given priority access to a request.
 */
static inline int ioc_batching(struct request_queue *q, struct io_context *ioc)
{
        if (!ioc)
                return 0;

        /*
         * Make sure the process is able to allocate at least 1 request
         * even if the batch times out, otherwise we could theoretically
         * lose wakeups.
         */
        return ioc->nr_batch_requests == q->nr_batching ||
                (ioc->nr_batch_requests > 0
                && time_before(jiffies, ioc->last_waited + BLK_BATCH_TIME));
}

/*
 * ioc_set_batching sets ioc to be a new "batcher" if it is not one. This
 * will cause the process to be a "batcher" on all queues in the system. This
 * is the behaviour we want though - once it gets a wakeup it should be given
 * a nice run.
 */
static void ioc_set_batching(struct request_queue *q, struct io_context *ioc)
{
        if (!ioc || ioc_batching(q, ioc))
                return;

        ioc->nr_batch_requests = q->nr_batching;
        ioc->last_waited = jiffies;
}

static void __freed_request(struct request_queue *q, int sync)
{
        struct request_list *rl = &q->rq;

        if (rl->count[sync] < queue_congestion_off_threshold(q))
                blk_clear_queue_congested(q, sync);

        if (rl->count[sync] + 1 <= q->nr_requests) {
                if (waitqueue_active(&rl->wait[sync]))
                        wake_up(&rl->wait[sync]);

                blk_clear_queue_full(q, sync);
        }
}

/*
 * A request has just been released.  Account for it, update the full and
 * congestion status, wake up any waiters.   Called under q->queue_lock.
 */
static void freed_request(struct request_queue *q, int sync, int priv)
{
        struct request_list *rl = &q->rq;

        rl->count[sync]--;
        if (priv)
                rl->elvpriv--;

        __freed_request(q, sync);

        if (unlikely(rl->starved[sync ^ 1]))
                __freed_request(q, sync ^ 1);
}

/*
 * Determine if elevator data should be initialized when allocating the
 * request associated with @bio.
 */
static bool blk_rq_should_init_elevator(struct bio *bio)
{
        if (!bio)
                return true;

        /*
         * Flush requests do not use the elevator so skip initialization.
         * This allows a request to share the flush and elevator data.
         */
        if (bio->bi_rw & (REQ_FLUSH | REQ_FUA))
                return false;

        return true;
}

/*
 * Get a free request, queue_lock must be held.
 * Returns NULL on failure, with queue_lock held.
 * Returns !NULL on success, with queue_lock *not held*.
 */
static struct request *get_request(struct request_queue *q, int rw_flags,
                                   struct bio *bio, gfp_t gfp_mask)
{
        struct request *rq = NULL;
        struct request_list *rl = &q->rq;
        struct io_context *ioc = NULL;
        const bool is_sync = rw_is_sync(rw_flags) != 0;
        int may_queue, priv = 0;

        may_queue = elv_may_queue(q, rw_flags);
        if (may_queue == ELV_MQUEUE_NO)
                goto rq_starved;

        if (rl->count[is_sync]+1 >= queue_congestion_on_threshold(q)) {
                if (rl->count[is_sync]+1 >= q->nr_requests) {
                        ioc = current_io_context(GFP_ATOMIC, q->node);
                        /*
                         * The queue will fill after this allocation, so set
                         * it as full, and mark this process as "batching".
                         * This process will be allowed to complete a batch of
                         * requests, others will be blocked.
                         */
                        if (!blk_queue_full(q, is_sync)) {
                                ioc_set_batching(q, ioc);
                                blk_set_queue_full(q, is_sync);
                        } else {
                                if (may_queue != ELV_MQUEUE_MUST
                                                && !ioc_batching(q, ioc)) {
                                        /*
                                         * The queue is full and the allocating
                                         * process is not a "batcher", and not
                                         * exempted by the IO scheduler
                                         */
                                        goto out;
                                }
                        }
                }
                blk_set_queue_congested(q, is_sync);
        }

        /*
         * Only allow batching queuers to allocate up to 50% over the defined
         * limit of requests, otherwise we could have thousands of requests
         * allocated with any setting of ->nr_requests
         */
        if (rl->count[is_sync] >= (3 * q->nr_requests / 2))
                goto out;

        rl->count[is_sync]++;
        rl->starved[is_sync] = 0;

        if (blk_rq_should_init_elevator(bio)) {
                priv = !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
                if (priv)
                        rl->elvpriv++;
        }

        if (blk_queue_io_stat(q))
                rw_flags |= REQ_IO_STAT;
        spin_unlock_irq(q->queue_lock);

        rq = blk_alloc_request(q, rw_flags, priv, gfp_mask);
        if (unlikely(!rq)) {
                /*
                 * Allocation failed presumably due to memory. Undo anything
                 * we might have messed up.
                 *
                 * Allocating task should really be put onto the front of the
                 * wait queue, but this is pretty rare.
                 */
                spin_lock_irq(q->queue_lock);
                freed_request(q, is_sync, priv);

                /*
                 * in the very unlikely event that allocation failed and no
                 * requests for this direction was pending, mark us starved
                 * so that freeing of a request in the other direction will
                 * notice us. another possible fix would be to split the
                 * rq mempool into READ and WRITE
                 */
rq_starved:
                if (unlikely(rl->count[is_sync] == 0))
                        rl->starved[is_sync] = 1;

                goto out;
        }

        /*
         * ioc may be NULL here, and ioc_batching will be false. That's
         * OK, if the queue is under the request limit then requests need
         * not count toward the nr_batch_requests limit. There will always
         * be some limit enforced by BLK_BATCH_TIME.
         */
        if (ioc_batching(q, ioc))
                ioc->nr_batch_requests--;

        trace_block_getrq(q, bio, rw_flags & 1);
out:
        return rq;
}

/*
 * No available requests for this queue, wait for some requests to become
 * available.
 *
 * Called with q->queue_lock held, and returns with it unlocked.
 */
static struct request *get_request_wait(struct request_queue *q, int rw_flags,
                                        struct bio *bio)
{
        const bool is_sync = rw_is_sync(rw_flags) != 0;
        struct request *rq;

        rq = get_request(q, rw_flags, bio, GFP_NOIO);
        while (!rq) {
                DEFINE_WAIT(wait);
                struct io_context *ioc;
                struct request_list *rl = &q->rq;

                prepare_to_wait_exclusive(&rl->wait[is_sync], &wait,
                                TASK_UNINTERRUPTIBLE);

                trace_block_sleeprq(q, bio, rw_flags & 1);

                spin_unlock_irq(q->queue_lock);
                io_schedule();

                /*
                 * After sleeping, we become a "batching" process and
                 * will be able to allocate at least one request, and
                 * up to a big batch of them for a small period time.
                 * See ioc_batching, ioc_set_batching
                 */
                ioc = current_io_context(GFP_NOIO, q->node);
                ioc_set_batching(q, ioc);

                spin_lock_irq(q->queue_lock);
                finish_wait(&rl->wait[is_sync], &wait);

                rq = get_request(q, rw_flags, bio, GFP_NOIO);
        };

        return rq;
}

struct request *blk_get_request(struct request_queue *q, int rw, gfp_t gfp_mask)
{
        struct request *rq;

        if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
                return NULL;

        BUG_ON(rw != READ && rw != WRITE);

        spin_lock_irq(q->queue_lock);
        if (gfp_mask & __GFP_WAIT) {
                rq = get_request_wait(q, rw, NULL);
        } else {
                rq = get_request(q, rw, NULL, gfp_mask);
                if (!rq)
                        spin_unlock_irq(q->queue_lock);
        }
        /* q->queue_lock is unlocked at this point */

        return rq;
}
EXPORT_SYMBOL(blk_get_request);

/**
 * blk_make_request - given a bio, allocate a corresponding struct request.
 * @q: target request queue
 * @bio:  The bio describing the memory mappings that will be submitted for IO.
 *        It may be a chained-bio properly constructed by block/bio layer.
 * @gfp_mask: gfp flags to be used for memory allocation
 *
 * blk_make_request is the parallel of generic_make_request for BLOCK_PC
 * type commands. Where the struct request needs to be farther initialized by
 * the caller. It is passed a &struct bio, which describes the memory info of
 * the I/O transfer.
 *
 * The caller of blk_make_request must make sure that bi_io_vec
 * are set to describe the memory buffers. That bio_data_dir() will return
 * the needed direction of the request. (And all bio's in the passed bio-chain
 * are properly set accordingly)
 *
 * If called under none-sleepable conditions, mapped bio buffers must not
 * need bouncing, by calling the appropriate masked or flagged allocator,
 * suitable for the target device. Otherwise the call to blk_queue_bounce will
 * BUG.
 *
 * WARNING: When allocating/cloning a bio-chain, careful consideration should be
 * given to how you allocate bios. In particular, you cannot use __GFP_WAIT for
 * anything but the first bio in the chain. Otherwise you risk waiting for IO
 * completion of a bio that hasn't been submitted yet, thus resulting in a
 * deadlock. Alternatively bios should be allocated using bio_kmalloc() instead
 * of bio_alloc(), as that avoids the mempool deadlock.
 * If possible a big IO should be split into smaller parts when allocation
 * fails. Partial allocation should not be an error, or you risk a live-lock.
 */
struct request *blk_make_request(struct request_queue *q, struct bio *bio,
                                 gfp_t gfp_mask)
{
        struct request *rq = blk_get_request(q, bio_data_dir(bio), gfp_mask);

        if (unlikely(!rq))
                return ERR_PTR(-ENOMEM);

        for_each_bio(bio) {
                struct bio *bounce_bio = bio;
                int ret;

                blk_queue_bounce(q, &bounce_bio);
                ret = blk_rq_append_bio(q, rq, bounce_bio);
                if (unlikely(ret)) {
                        blk_put_request(rq);
                        return ERR_PTR(ret);
                }
        }

        return rq;
}
EXPORT_SYMBOL(blk_make_request);

/**
 * blk_requeue_request - put a request back on queue
 * @q:          request queue where request should be inserted
 * @rq:         request to be inserted
 *
 * Description:
 *    Drivers often keep queueing requests until the hardware cannot accept
 *    more, when that condition happens we need to put the request back
 *    on the queue. Must be called with queue lock held.
 */
void blk_requeue_request(struct request_queue *q, struct request *rq)
{
        blk_delete_timer(rq);
        blk_clear_rq_complete(rq);
        trace_block_rq_requeue(q, rq);

        if (blk_rq_tagged(rq))
                blk_queue_end_tag(q, rq);

        BUG_ON(blk_queued_rq(rq));

        elv_requeue_request(q, rq);
}
EXPORT_SYMBOL(blk_requeue_request);

static void add_acct_request(struct request_queue *q, struct request *rq,
                             int where)
{
        drive_stat_acct(rq, 1);
        __elv_add_request(q, rq, where);
}

/**
 * blk_insert_request - insert a special request into a request queue
 * @q:          request queue where request should be inserted
 * @rq:         request to be inserted
 * @at_head:    insert request at head or tail of queue
 * @data:       private data
 *
 * Description:
 *    Many block devices need to execute commands asynchronously, so they don't
 *    block the whole kernel from preemption during request execution.  This is
 *    accomplished normally by inserting aritficial requests tagged as
 *    REQ_TYPE_SPECIAL in to the corresponding request queue, and letting them
 *    be scheduled for actual execution by the request queue.
 *
 *    We have the option of inserting the head or the tail of the queue.
 *    Typically we use the tail for new ioctls and so forth.  We use the head
 *    of the queue for things like a QUEUE_FULL message from a device, or a
 *    host that is unable to accept a particular command.
 */
void blk_insert_request(struct request_queue *q, struct request *rq,
                        int at_head, void *data)
{
        int where = at_head ? ELEVATOR_INSERT_FRONT : ELEVATOR_INSERT_BACK;
        unsigned long flags;

        /*
         * tell I/O scheduler that this isn't a regular read/write (ie it
         * must not attempt merges on this) and that it acts as a soft
         * barrier
         */
        rq->cmd_type = REQ_TYPE_SPECIAL;

        rq->special = data;

        spin_lock_irqsave(q->queue_lock, flags);

        /*
         * If command is tagged, release the tag
         */
        if (blk_rq_tagged(rq))
                blk_queue_end_tag(q, rq);

        add_acct_request(q, rq, where);
        __blk_run_queue(q);
        spin_unlock_irqrestore(q->queue_lock, flags);
}
EXPORT_SYMBOL(blk_insert_request);

static void part_round_stats_single(int cpu, struct hd_struct *part,
                                    unsigned long now)
{
        if (now == part->stamp)
                return;

        if (part_in_flight(part)) {
                __part_stat_add(cpu, part, time_in_queue,
                                part_in_flight(part) * (now - part->stamp));
                __part_stat_add(cpu, part, io_ticks, (now - part->stamp));
        }
        part->stamp = now;
}

/**
 * part_round_stats() - Round off the performance stats on a struct disk_stats.
 * @cpu: cpu number for stats access
 * @part: target partition
 *
 * The average IO queue length and utilisation statistics are maintained
 * by observing the current state of the queue length and the amount of
 * time it has been in this state for.
 *
 * Normally, that accounting is done on IO completion, but that can result
 * in more than a second's worth of IO being accounted for within any one
 * second, leading to >100% utilisation.  To deal with that, we call this
 * function to do a round-off before returning the results when reading
 * /proc/diskstats.  This accounts immediately for all queue usage up to
 * the current jiffies and restarts the counters again.
 */
void part_round_stats(int cpu, struct hd_struct *part)
{
        unsigned long now = jiffies;

        if (part->partno)
                part_round_stats_single(cpu, &part_to_disk(part)->part0, now);
        part_round_stats_single(cpu, part, now);
}
EXPORT_SYMBOL_GPL(part_round_stats);

/*
 * queue lock must be held
 */
void __blk_put_request(struct request_queue *q, struct request *req)
{
        if (unlikely(!q))
                return;
        if (unlikely(--req->ref_count))
                return;

        elv_completed_request(q, req);

        /* this is a bio leak */
        WARN_ON(req->bio != NULL);

        /*
         * Request may not have originated from ll_rw_blk. if not,
         * it didn't come out of our reserved rq pools
         */
        if (req->cmd_flags & REQ_ALLOCED) {
                int is_sync = rq_is_sync(req) != 0;
                int priv = req->cmd_flags & REQ_ELVPRIV;

                BUG_ON(!list_empty(&req->queuelist));
                BUG_ON(!hlist_unhashed(&req->hash));

                blk_free_request(q, req);
                freed_request(q, is_sync, priv);
        }
}
EXPORT_SYMBOL_GPL(__blk_put_request);

void blk_put_request(struct request *req)
{
        unsigned long flags;
        struct request_queue *q = req->q;

        spin_lock_irqsave(q->queue_lock, flags);
        __blk_put_request(q, req);
        spin_unlock_irqrestore(q->queue_lock, flags);
}
EXPORT_SYMBOL(blk_put_request);

/**
 * blk_add_request_payload - add a payload to a request
 * @rq: request to update
 * @page: page backing the payload
 * @len: length of the payload.
 *
 * This allows to later add a payload to an already submitted request by
 * a block driver.  The driver needs to take care of freeing the payload
 * itself.
 *
 * Note that this is a quite horrible hack and nothing but handling of
 * discard requests should ever use it.
 */
void blk_add_request_payload(struct request *rq, struct page *page,
                unsigned int len)
{
        struct bio *bio = rq->bio;

        bio->bi_io_vec->bv_page = page;
        bio->bi_io_vec->bv_offset = 0;
        bio->bi_io_vec->bv_len = len;

        bio->bi_size = len;
        bio->bi_vcnt = 1;
        bio->bi_phys_segments = 1;

        rq->__data_len = rq->resid_len = len;
        rq->nr_phys_segments = 1;
        rq->buffer = bio_data(bio);
}
EXPORT_SYMBOL_GPL(blk_add_request_payload);

static bool bio_attempt_back_merge(struct request_queue *q, struct request *req,
                                   struct bio *bio)
{
        const int ff = bio->bi_rw & REQ_FAILFAST_MASK;

        if (!ll_back_merge_fn(q, req, bio))
                return false;

        trace_block_bio_backmerge(q, bio);

        if ((req->cmd_flags & REQ_FAILFAST_MASK) != ff)
                blk_rq_set_mixed_merge(req);

        req->biotail->bi_next = bio;
        req->biotail = bio;
        req->__data_len += bio->bi_size;
        req->ioprio = ioprio_best(req->ioprio, bio_prio(bio));

        drive_stat_acct(req, 0);
        elv_bio_merged(q, req, bio);
        return true;
}

static bool bio_attempt_front_merge(struct request_queue *q,
                                    struct request *req, struct bio *bio)
{
        const int ff = bio->bi_rw & REQ_FAILFAST_MASK;

        if (!ll_front_merge_fn(q, req, bio))
                return false;

        trace_block_bio_frontmerge(q, bio);

        if ((req->cmd_flags & REQ_FAILFAST_MASK) != ff)
                blk_rq_set_mixed_merge(req);

        bio->bi_next = req->bio;
        req->bio = bio;

        /*
         * may not be valid. if the low level driver said
         * it didn't need a bounce buffer then it better
         * not touch req->buffer either...
         */
        req->buffer = bio_data(bio);
        req->__sector = bio->bi_sector;
        req->__data_len += bio->bi_size;
        req->ioprio = ioprio_best(req->ioprio, bio_prio(bio));

        drive_stat_acct(req, 0);
        elv_bio_merged(q, req, bio);
        return true;
}

/*
 * Attempts to merge with the plugged list in the current process. Returns
 * true if merge was successful, otherwise false.
 */
static bool attempt_plug_merge(struct task_struct *tsk, struct request_queue *q,
                               struct bio *bio)
{
        struct blk_plug *plug;
        struct request *rq;
        bool ret = false;

        plug = tsk->plug;
        if (!plug)
                goto out;

        list_for_each_entry_reverse(rq, &plug->list, queuelist) {
                int el_ret;

                if (rq->q != q)
                        continue;

                el_ret = elv_try_merge(rq, bio);
                if (el_ret == ELEVATOR_BACK_MERGE) {
                        ret = bio_attempt_back_merge(q, rq, bio);
                        if (ret)
                                break;
                } else if (el_ret == ELEVATOR_FRONT_MERGE) {
                        ret = bio_attempt_front_merge(q, rq, bio);
                        if (ret)
                                break;
                }
        }
out:
        return ret;
}

void init_request_from_bio(struct request *req, struct bio *bio)
{
        req->cpu = bio->bi_comp_cpu;
        req->cmd_type = REQ_TYPE_FS;

        req->cmd_flags |= bio->bi_rw & REQ_COMMON_MASK;
        if (bio->bi_rw & REQ_RAHEAD)
                req->cmd_flags |= REQ_FAILFAST_MASK;

        req->errors = 0;
        req->__sector = bio->bi_sector;
        req->ioprio = bio_prio(bio);
        blk_rq_bio_prep(req->q, req, bio);
}

static int __make_request(struct request_queue *q, struct bio *bio)
{
        const bool sync = !!(bio->bi_rw & REQ_SYNC);
        struct blk_plug *plug;
        int el_ret, rw_flags, where = ELEVATOR_INSERT_SORT;
        struct request *req;

        /*
         * low level driver can indicate that it wants pages above a
         * certain limit bounced to low memory (ie for highmem, or even
         * ISA dma in theory)
         */
        blk_queue_bounce(q, &bio);

        if (bio->bi_rw & (REQ_FLUSH | REQ_FUA)) {
                spin_lock_irq(q->queue_lock);
                where = ELEVATOR_INSERT_FLUSH;
                goto get_rq;
        }

        /*
         * Check if we can merge with the plugged list before grabbing
         * any locks.
         */
        if (attempt_plug_merge(current, q, bio))
                goto out;

        spin_lock_irq(q->queue_lock);

        el_ret = elv_merge(q, &req, bio);
        if (el_ret == ELEVATOR_BACK_MERGE) {
                if (bio_attempt_back_merge(q, req, bio)) {
                        if (!attempt_back_merge(q, req))
                                elv_merged_request(q, req, el_ret);
                        goto out_unlock;
                }
        } else if (el_ret == ELEVATOR_FRONT_MERGE) {
                if (bio_attempt_front_merge(q, req, bio)) {
                        if (!attempt_front_merge(q, req))
                                elv_merged_request(q, req, el_ret);
                        goto out_unlock;
                }
        }

get_rq:
        /*
         * This sync check and mask will be re-done in init_request_from_bio(),
         * but we need to set it earlier to expose the sync flag to the
         * rq allocator and io schedulers.
         */
        rw_flags = bio_data_dir(bio);
        if (sync)
                rw_flags |= REQ_SYNC;

        /*
         * Grab a free request. This is might sleep but can not fail.
         * Returns with the queue unlocked.
         */
        req = get_request_wait(q, rw_flags, bio);

        /*
         * After dropping the lock and possibly sleeping here, our request
         * may now be mergeable after it had proven unmergeable (above).
         * We don't worry about that case for efficiency. It won't happen
         * often, and the elevators are able to handle it.
         */
        init_request_from_bio(req, bio);

        if (test_bit(QUEUE_FLAG_SAME_COMP, &q->queue_flags) ||
            bio_flagged(bio, BIO_CPU_AFFINE))
                req->cpu = raw_smp_processor_id();

        plug = current->plug;
        if (plug) {
                /*
                 * If this is the first request added after a plug, fire
                 * of a plug trace. If others have been added before, check
                 * if we have multiple devices in this plug. If so, make a
                 * note to sort the list before dispatch.
                 */
                if (list_empty(&plug->list))
                        trace_block_plug(q);
                else if (!plug->should_sort) {
                        struct request *__rq;

                        __rq = list_entry_rq(plug->list.prev);
                        if (__rq->q != q)
                                plug->should_sort = 1;
                }
                list_add_tail(&req->queuelist, &plug->list);
                plug->count++;
                drive_stat_acct(req, 1);
                if (plug->count >= BLK_MAX_REQUEST_COUNT)
                        blk_flush_plug_list(plug, false);
        } else {
                spin_lock_irq(q->queue_lock);
                add_acct_request(q, req, where);
                __blk_run_queue(q);
out_unlock:
                spin_unlock_irq(q->queue_lock);
        }
out:
        return 0;
}

/*
 * If bio->bi_dev is a partition, remap the location
 */
static inline void blk_partition_remap(struct bio *bio)
{
        struct block_device *bdev = bio->bi_bdev;

        if (bio_sectors(bio) && bdev != bdev->bd_contains) {
                struct hd_struct *p = bdev->bd_part;

                bio->bi_sector += p->start_sect;
                bio->bi_bdev = bdev->bd_contains;

                trace_block_bio_remap(bdev_get_queue(bio->bi_bdev), bio,
                                      bdev->bd_dev,
                                      bio->bi_sector - p->start_sect);
        }
}

static void handle_bad_sector(struct bio *bio)
{
        char b[BDEVNAME_SIZE];

        printk(KERN_INFO "attempt to access beyond end of device\n");
        printk(KERN_INFO "%s: rw=%ld, want=%Lu, limit=%Lu\n",
                        bdevname(bio->bi_bdev, b),
                        bio->bi_rw,
                        (unsigned long long)bio->bi_sector + bio_sectors(bio),
                        (long long)(i_size_read(bio->bi_bdev->bd_inode) >> 9));

        set_bit(BIO_EOF, &bio->bi_flags);
}

#ifdef CONFIG_FAIL_MAKE_REQUEST

static DECLARE_FAULT_ATTR(fail_make_request);

static int __init setup_fail_make_request(char *str)
{
        return setup_fault_attr(&fail_make_request, str);
}
__setup("fail_make_request=", setup_fail_make_request);

static bool should_fail_request(struct hd_struct *part, unsigned int bytes)
{
        return part->make_it_fail && should_fail(&fail_make_request, bytes);
}

static int __init fail_make_request_debugfs(void)
{
        struct dentry *dir = fault_create_debugfs_attr("fail_make_request",
                                                NULL, &fail_make_request);

        return IS_ERR(dir) ? PTR_ERR(dir) : 0;
}

late_initcall(fail_make_request_debugfs);

#else /* CONFIG_FAIL_MAKE_REQUEST */

static inline bool should_fail_request(struct hd_struct *part,
                                        unsigned int bytes)
{
        return false;
}

#endif /* CONFIG_FAIL_MAKE_REQUEST */

/*
 * Check whether this bio extends beyond the end of the device.
 */
static inline int bio_check_eod(struct bio *bio, unsigned int nr_sectors)
{
        sector_t maxsector;

        if (!nr_sectors)
                return 0;

        /* Test device or partition size, when known. */
        maxsector = i_size_read(bio->bi_bdev->bd_inode) >> 9;
        if (maxsector) {
                sector_t sector = bio->bi_sector;

                if (maxsector < nr_sectors || maxsector - nr_sectors < sector) {
                        /*
                         * This may well happen - the kernel calls bread()
                         * without checking the size of the device, e.g., when
                         * mounting a device.
                         */
                        handle_bad_sector(bio);
                        return 1;
                }
        }

        return 0;
}

/**
 * generic_make_request - hand a buffer to its device driver for I/O
 * @bio:  The bio describing the location in memory and on the device.
 *
 * generic_make_request() is used to make I/O requests of block
 * devices. It is passed a &struct bio, which describes the I/O that needs
 * to be done.
 *
 * generic_make_request() does not return any status.  The
 * success/failure status of the request, along with notification of
 * completion, is delivered asynchronously through the bio->bi_end_io
 * function described (one day) else where.
 *
 * The caller of generic_make_request must make sure that bi_io_vec
 * are set to describe the memory buffer, and that bi_dev and bi_sector are
 * set to describe the device address, and the
 * bi_end_io and optionally bi_private are set to describe how
 * completion notification should be signaled.
 *
 * generic_make_request and the drivers it calls may use bi_next if this
 * bio happens to be merged with someone else, and may change bi_dev and
 * bi_sector for remaps as it sees fit.  So the values of these fields
 * should NOT be depended on after the call to generic_make_request.
 */
static inline void __generic_make_request(struct bio *bio)
{
        struct request_queue *q;
        sector_t old_sector;
        int ret, nr_sectors = bio_sectors(bio);
        dev_t old_dev;
        int err = -EIO;

        might_sleep();

        if (bio_check_eod(bio, nr_sectors))
                goto end_io;

        /*
         * Resolve the mapping until finished. (drivers are
         * still free to implement/resolve their own stacking
         * by explicitly returning 0)
         *
         * NOTE: we don't repeat the blk_size check for each new device.
         * Stacking drivers are expected to know what they are doing.
         */
        old_sector = -1;
        old_dev = 0;
        do {
                char b[BDEVNAME_SIZE];
                struct hd_struct *part;

                q = bdev_get_queue(bio->bi_bdev);
                if (unlikely(!q)) {
                        printk(KERN_ERR
                               "generic_make_request: Trying to access "
                                "nonexistent block-device %s (%Lu)\n",
                                bdevname(bio->bi_bdev, b),
                                (long long) bio->bi_sector);
                        goto end_io;
                }

                if (unlikely(!(bio->bi_rw & REQ_DISCARD) &&
                             nr_sectors > queue_max_hw_sectors(q))) {
                        printk(KERN_ERR "bio too big device %s (%u > %u)\n",
                               bdevname(bio->bi_bdev, b),
                               bio_sectors(bio),
                               queue_max_hw_sectors(q));
                        goto end_io;
                }

                if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
                        goto end_io;

                part = bio->bi_bdev->bd_part;
                if (should_fail_request(part, bio->bi_size) ||
                    should_fail_request(&part_to_disk(part)->part0,
                                        bio->bi_size))
                        goto end_io;

                /*
                 * If this device has partitions, remap block n
                 * of partition p to block n+start(p) of the disk.
                 */
                blk_partition_remap(bio);

                if (bio_integrity_enabled(bio) && bio_integrity_prep(bio))
                        goto end_io;

                if (old_sector != -1)
                        trace_block_bio_remap(q, bio, old_dev, old_sector);

                old_sector = bio->bi_sector;
                old_dev = bio->bi_bdev->bd_dev;

                if (bio_check_eod(bio, nr_sectors))
                        goto end_io;

                /*
                 * Filter flush bio's early so that make_request based
                 * drivers without flush support don't have to worry
                 * about them.
                 */
                if ((bio->bi_rw & (REQ_FLUSH | REQ_FUA)) && !q->flush_flags) {
                        bio->bi_rw &= ~(REQ_FLUSH | REQ_FUA);
                        if (!nr_sectors) {
                                err = 0;
                                goto end_io;
                        }
                }

                if ((bio->bi_rw & REQ_DISCARD) &&
                    (!blk_queue_discard(q) ||
                     ((bio->bi_rw & REQ_SECURE) &&
                      !blk_queue_secdiscard(q)))) {
                        err = -EOPNOTSUPP;
                        goto end_io;
                }

                if (blk_throtl_bio(q, &bio))
                        goto end_io;

                /*
                 * If bio = NULL, bio has been throttled and will be submitted
                 * later.
                 */
                if (!bio)
                        break;

                trace_block_bio_queue(q, bio);

                ret = q->make_request_fn(q, bio);
        } while (ret);

        return;

end_io:
        bio_endio(bio, err);
}

/*
 * We only want one ->make_request_fn to be active at a time,
 * else stack usage with stacked devices could be a problem.
 * So use current->bio_list to keep a list of requests
 * submited by a make_request_fn function.
 * current->bio_list is also used as a flag to say if
 * generic_make_request is currently active in this task or not.
 * If it is NULL, then no make_request is active.  If it is non-NULL,
 * then a make_request is active, and new requests should be added
 * at the tail
 */
void generic_make_request(struct bio *bio)
{
        struct bio_list bio_list_on_stack;

        if (current->bio_list) {
                /* make_request is active */
                bio_list_add(current->bio_list, bio);
                return;
        }
        /* following loop may be a bit non-obvious, and so deserves some
         * explanation.
         * Before entering the loop, bio->bi_next is NULL (as all callers
         * ensure that) so we have a list with a single bio.
         * We pretend that we have just taken it off a longer list, so
         * we assign bio_list to a pointer to the bio_list_on_stack,
         * thus initialising the bio_list of new bios to be
         * added.  __generic_make_request may indeed add some more bios
         * through a recursive call to generic_make_request.  If it
         * did, we find a non-NULL value in bio_list and re-enter the loop
         * from the top.  In this case we really did just take the bio
         * of the top of the list (no pretending) and so remove it from
         * bio_list, and call into __generic_make_request again.
         *
         * The loop was structured like this to make only one call to
         * __generic_make_request (which is important as it is large and
         * inlined) and to keep the structure simple.
         */
        BUG_ON(bio->bi_next);
        bio_list_init(&bio_list_on_stack);
        current->bio_list = &bio_list_on_stack;
        do {
                __generic_make_request(bio);
                bio = bio_list_pop(current->bio_list);
        } while (bio);
        current->bio_list = NULL; /* deactivate */
}
EXPORT_SYMBOL(generic_make_request);

/**
 * submit_bio - submit a bio to the block device layer for I/O
 * @rw: whether to %READ or %WRITE, or maybe to %READA (read ahead)
 * @bio: The &struct bio which describes the I/O
 *
 * submit_bio() is very similar in purpose to generic_make_request(), and
 * uses that function to do most of the work. Both are fairly rough
 * interfaces; @bio must be presetup and ready for I/O.
 *
 */
void submit_bio(int rw, struct bio *bio)
{
        int count = bio_sectors(bio);

        bio->bi_rw |= rw;

        /*
         * If it's a regular read/write or a barrier with data attached,
         * go through the normal accounting stuff before submission.
         */
        if (bio_has_data(bio) && !(rw & REQ_DISCARD)) {
                if (rw & WRITE) {
                        count_vm_events(PGPGOUT, count);
                } else {
                        task_io_account_read(bio->bi_size);
                        count_vm_events(PGPGIN, count);
                }

                if (unlikely(block_dump)) {
                        char b[BDEVNAME_SIZE];
                        printk(KERN_DEBUG "%s(%d): %s block %Lu on %s (%u sectors)\n",
                        current->comm, task_pid_nr(current),
                                (rw & WRITE) ? "WRITE" : "READ",
                                (unsigned long long)bio->bi_sector,
                                bdevname(bio->bi_bdev, b),
                                count);
                }
        }

        generic_make_request(bio);
}
EXPORT_SYMBOL(submit_bio);

/**
 * blk_rq_check_limits - Helper function to check a request for the queue limit
 * @q:  the queue
 * @rq: the request being checked
 *
 * Description:
 *    @rq may have been made based on weaker limitations of upper-level queues
 *    in request stacking drivers, and it may violate the limitation of @q.
 *    Since the block layer and the underlying device driver trust @rq
 *    after it is inserted to @q, it should be checked against @q before
 *    the insertion using this generic function.
 *
 *    This function should also be useful for request stacking drivers
 *    in some cases below, so export this function.
 *    Request stacking drivers like request-based dm may change the queue
 *    limits while requests are in the queue (e.g. dm's table swapping).
 *    Such request stacking drivers should check those requests agaist
 *    the new queue limits again when they dispatch those requests,
 *    although such checkings are also done against the old queue limits
 *    when submitting requests.
 */
int blk_rq_check_limits(struct request_queue *q, struct request *rq)
{
        if (rq->cmd_flags & REQ_DISCARD)
                return 0;

        if (blk_rq_sectors(rq) > queue_max_sectors(q) ||
            blk_rq_bytes(rq) > queue_max_hw_sectors(q) << 9) {
                printk(KERN_ERR "%s: over max size limit.\n", __func__);
                return -EIO;
        }

        /*
         * queue's settings related to segment counting like q->bounce_pfn
         * may differ from that of other stacking queues.
         * Recalculate it to check the request correctly on this queue's
         * limitation.
         */
        blk_recalc_rq_segments(rq);
        if (rq->nr_phys_segments > queue_max_segments(q)) {
                printk(KERN_ERR "%s: over max segments limit.\n", __func__);
                return -EIO;
        }

        return 0;
}
EXPORT_SYMBOL_GPL(blk_rq_check_limits);

/**
 * blk_insert_cloned_request - Helper for stacking drivers to submit a request
 * @q:  the queue to submit the request
 * @rq: the request being queued
 */
int blk_insert_cloned_request(struct request_queue *q, struct request *rq)
{
        unsigned long flags;

        if (blk_rq_check_limits(q, rq))
                return -EIO;

        if (rq->rq_disk &&
            should_fail_request(&rq->rq_disk->part0, blk_rq_bytes(rq)))
                return -EIO;

        spin_lock_irqsave(q->queue_lock, flags);

        /*
         * Submitting request must be dequeued before calling this function
         * because it will be linked to another request_queue
         */
        BUG_ON(blk_queued_rq(rq));

        add_acct_request(q, rq, ELEVATOR_INSERT_BACK);
        spin_unlock_irqrestore(q->queue_lock, flags);

        return 0;
}
EXPORT_SYMBOL_GPL(blk_insert_cloned_request);

/**
 * blk_rq_err_bytes - determine number of bytes till the next failure boundary
 * @rq: request to examine
 *
 * Description:
 *     A request could be merge of IOs which require different failure
 *     handling.  This function determines the number of bytes which
 *     can be failed from the beginning of the request without
 *     crossing into area which need to be retried further.
 *
 * Return:
 *     The number of bytes to fail.
 *
 * Context:
 *     queue_lock must be held.
 */
unsigned int blk_rq_err_bytes(const struct request *rq)
{
        unsigned int ff = rq->cmd_flags & REQ_FAILFAST_MASK;
        unsigned int bytes = 0;
        struct bio *bio;

        if (!(rq->cmd_flags & REQ_MIXED_MERGE))
                return blk_rq_bytes(rq);

        /*
         * Currently the only 'mixing' which can happen is between
         * different fastfail types.  We can safely fail portions
         * which have all the failfast bits that the first one has -
         * the ones which are at least as eager to fail as the first
         * one.
         */
        for (bio = rq->bio; bio; bio = bio->bi_next) {
                if ((bio->bi_rw & ff) != ff)
                        break;
                bytes += bio->bi_size;
        }

        /* this could lead to infinite loop */
        BUG_ON(blk_rq_bytes(rq) && !bytes);
        return bytes;
}
EXPORT_SYMBOL_GPL(blk_rq_err_bytes);

static void blk_account_io_completion(struct request *req, unsigned int bytes)
{
        if (blk_do_io_stat(req)) {
                const int rw = rq_data_dir(req);
                struct hd_struct *part;
                int cpu;

                cpu = part_stat_lock();
                part = req->part;
                part_stat_add(cpu, part, sectors[rw], bytes >> 9);
                part_stat_unlock();
        }
}

static void blk_account_io_done(struct request *req)
{
        /*
         * Account IO completion.  flush_rq isn't accounted as a
         * normal IO on queueing nor completion.  Accounting the
         * containing request is enough.
         */
        if (blk_do_io_stat(req) && !(req->cmd_flags & REQ_FLUSH_SEQ)) {
                unsigned long duration = jiffies - req->start_time;
                const int rw = rq_data_dir(req);
                struct hd_struct *part;
                int cpu;

                cpu = part_stat_lock();
                part = req->part;

                part_stat_inc(cpu, part, ios[rw]);
                part_stat_add(cpu, part, ticks[rw], duration);
                part_round_stats(cpu, part);
                part_dec_in_flight(part, rw);

                hd_struct_put(part);
                part_stat_unlock();
        }
}

/**
 * blk_peek_request - peek at the top of a request queue
 * @q: request queue to peek at
 *
 * Description:
 *     Return the request at the top of @q.  The returned request
 *     should be started using blk_start_request() before LLD starts
 *     processing it.
 *
 * Return:
 *     Pointer to the request at the top of @q if available.  Null
 *     otherwise.
 *
 * Context:
 *     queue_lock must be held.
 */
struct request *blk_peek_request(struct request_queue *q)
{
        struct request *rq;
        int ret;

        while ((rq = __elv_next_request(q)) != NULL) {
                if (!(rq->cmd_flags & REQ_STARTED)) {
                        /*
                         * This is the first time the device driver
                         * sees this request (possibly after
                         * requeueing).  Notify IO scheduler.
                         */
                        if (rq->cmd_flags & REQ_SORTED)
                                elv_activate_rq(q, rq);

                        /*
                         * just mark as started even if we don't start
                         * it, a request that has been delayed should
                         * not be passed by new incoming requests
                         */
                        rq->cmd_flags |= REQ_STARTED;
                        trace_block_rq_issue(q, rq);
                }

                if (!q->boundary_rq || q->boundary_rq == rq) {
                        q->end_sector = rq_end_sector(rq);
                        q->boundary_rq = NULL;
                }

                if (rq->cmd_flags & REQ_DONTPREP)
                        break;

                if (q->dma_drain_size && blk_rq_bytes(rq)) {
                        /*
                         * make sure space for the drain appears we
                         * know we can do this because max_hw_segments
                         * has been adjusted to be one fewer than the
                         * device can handle
                         */
                        rq->nr_phys_segments++;
                }

                if (!q->prep_rq_fn)
                        break;

                ret = q->prep_rq_fn(q, rq);
                if (ret == BLKPREP_OK) {
                        break;
                } else if (ret == BLKPREP_DEFER) {
                        /*
                         * the request may have been (partially) prepped.
                         * we need to keep this request in the front to
                         * avoid resource deadlock.  REQ_STARTED will
                         * prevent other fs requests from passing this one.
                         */
                        if (q->dma_drain_size && blk_rq_bytes(rq) &&
                            !(rq->cmd_flags & REQ_DONTPREP)) {
                                /*
                                 * remove the space for the drain we added
                                 * so that we don't add it again
                                 */
                                --rq->nr_phys_segments;
                        }

                        rq = NULL;
                        break;
                } else if (ret == BLKPREP_KILL) {
                        rq->cmd_flags |= REQ_QUIET;
                        /*
                         * Mark this request as started so we don't trigger
                         * any debug logic in the end I/O path.
                         */
                        blk_start_request(rq);
                        __blk_end_request_all(rq, -EIO);
                } else {
                        printk(KERN_ERR "%s: bad return=%d\n", __func__, ret);
                        break;
                }
        }

        return rq;
}
EXPORT_SYMBOL(blk_peek_request);

void blk_dequeue_request(struct request *rq)
{
        struct request_queue *q = rq->q;

        BUG_ON(list_empty(&rq->queuelist));
        BUG_ON(ELV_ON_HASH(rq));

        list_del_init(&rq->queuelist);

        /*
         * the time frame between a request being removed from the lists
         * and to it is freed is accounted as io that is in progress at
         * the driver side.
         */
        if (blk_account_rq(rq)) {
                q->in_flight[rq_is_sync(rq)]++;
                set_io_start_time_ns(rq);
        }
}

/**
 * blk_start_request - start request processing on the driver
 * @req: request to dequeue
 *
 * Description:
 *     Dequeue @req and start timeout timer on it.  This hands off the
 *     request to the driver.
 *
 *     Block internal functions which don't want to start timer should
 *     call blk_dequeue_request().
 *
 * Context:
 *     queue_lock must be held.
 */
void blk_start_request(struct request *req)
{
        blk_dequeue_request(req);

        /*
         * We are now handing the request to the hardware, initialize
         * resid_len to full count and add the timeout handler.
         */
        req->resid_len = blk_rq_bytes(req);
        if (unlikely(blk_bidi_rq(req)))
                req->next_rq->resid_len = blk_rq_bytes(req->next_rq);

        blk_add_timer(req);
}
EXPORT_SYMBOL(blk_start_request);

/**
 * blk_fetch_request - fetch a request from a request queue
 * @q: request queue to fetch a request from
 *
 * Description:
 *     Return the request at the top of @q.  The request is started on
 *     return and LLD can start processing it immediately.
 *
 * Return:
 *     Pointer to the request at the top of @q if available.  Null
 *     otherwise.
 *
 * Context:
 *     queue_lock must be held.
 */
struct request *blk_fetch_request(struct request_queue *q)
{
        struct request *rq;

        rq = blk_peek_request(q);
        if (rq)
                blk_start_request(rq);
        return rq;
}
EXPORT_SYMBOL(blk_fetch_request);

/**
 * blk_update_request - Special helper function for request stacking drivers
 * @req:      the request being processed
 * @error:    %0 for success, < %0 for error
 * @nr_bytes: number of bytes to complete @req
 *
 * Description:
 *     Ends I/O on a number of bytes attached to @req, but doesn't complete
 *     the request structure even if @req doesn't have leftover.
 *     If @req has leftover, sets it up for the next range of segments.
 *
 *     This special helper function is only for request stacking drivers
 *     (e.g. request-based dm) so that they can handle partial completion.
 *     Actual device drivers should use blk_end_request instead.
 *
 *     Passing the result of blk_rq_bytes() as @nr_bytes guarantees
 *     %false return from this function.
 *
 * Return:
 *     %false - this request doesn't have any more data
 *     %true  - this request has more data
 **/
bool blk_update_request(struct request *req, int error, unsigned int nr_bytes)
{
        int total_bytes, bio_nbytes, next_idx = 0;
        struct bio *bio;

        if (!req->bio)
                return false;

        trace_block_rq_complete(req->q, req);

        /*
         * For fs requests, rq is just carrier of independent bio's
         * and each partial completion should be handled separately.
         * Reset per-request error on each partial completion.
         *
         * TODO: tj: This is too subtle.  It would be better to let
         * low level drivers do what they see fit.
         */
        if (req->cmd_type == REQ_TYPE_FS)
                req->errors = 0;

        if (error && req->cmd_type == REQ_TYPE_FS &&
            !(req->cmd_flags & REQ_QUIET)) {
                char *error_type;

                switch (error) {
                case -ENOLINK:
                        error_type = "recoverable transport";
                        break;
                case -EREMOTEIO:
                        error_type = "critical target";
                        break;
                case -EBADE:
                        error_type = "critical nexus";
                        break;
                case -EIO:
                default:
                        error_type = "I/O";
                        break;
                }
                printk(KERN_ERR "end_request: %s error, dev %s, sector %llu\n",
                       error_type, req->rq_disk ? req->rq_disk->disk_name : "?",
                       (unsigned long long)blk_rq_pos(req));
        }

        blk_account_io_completion(req, nr_bytes);

        total_bytes = bio_nbytes = 0;
        while ((bio = req->bio) != NULL) {
                int nbytes;

                if (nr_bytes >= bio->bi_size) {
                        req->bio = bio->bi_next;
                        nbytes = bio->bi_size;
                        req_bio_endio(req, bio, nbytes, error);
                        next_idx = 0;
                        bio_nbytes = 0;
                } else {
                        int idx = bio->bi_idx + next_idx;

                        if (unlikely(idx >= bio->bi_vcnt)) {
                                blk_dump_rq_flags(req, "__end_that");
                                printk(KERN_ERR "%s: bio idx %d >= vcnt %d\n",
                                       __func__, idx, bio->bi_vcnt);
                                break;
                        }

                        nbytes = bio_iovec_idx(bio, idx)->bv_len;
                        BIO_BUG_ON(nbytes > bio->bi_size);

                        /*
                         * not a complete bvec done
                         */
                        if (unlikely(nbytes > nr_bytes)) {
                                bio_nbytes += nr_bytes;
                                total_bytes += nr_bytes;
                                break;
                        }

                        /*
                         * advance to the next vector
                         */
                        next_idx++;
                        bio_nbytes += nbytes;
                }

                total_bytes += nbytes;
                nr_bytes -= nbytes;

                bio = req->bio;
                if (bio) {
                        /*
                         * end more in this run, or just return 'not-done'
                         */
                        if (unlikely(nr_bytes <= 0))
                                break;
                }
        }

        /*
         * completely done
         */
        if (!req->bio) {
                /*
                 * Reset counters so that the request stacking driver
                 * can find how many bytes remain in the request
                 * later.
                 */
                req->__data_len = 0;
                return false;
        }

        /*
         * if the request wasn't completed, update state
         */
        if (bio_nbytes) {
                req_bio_endio(req, bio, bio_nbytes, error);
                bio->bi_idx += next_idx;
                bio_iovec(bio)->bv_offset += nr_bytes;
                bio_iovec(bio)->bv_len -= nr_bytes;
        }

        req->__data_len -= total_bytes;
        req->buffer = bio_data(req->bio);

        /* update sector only for requests with clear definition of sector */
        if (req->cmd_type == REQ_TYPE_FS || (req->cmd_flags & REQ_DISCARD))
                req->__sector += total_bytes >> 9;

        /* mixed attributes always follow the first bio */
        if (req->cmd_flags & REQ_MIXED_MERGE) {
                req->cmd_flags &= ~REQ_FAILFAST_MASK;
                req->cmd_flags |= req->bio->bi_rw & REQ_FAILFAST_MASK;
        }

        /*
         * If total number of sectors is less than the first segment
         * size, something has gone terribly wrong.
         */
        if (blk_rq_bytes(req) < blk_rq_cur_bytes(req)) {
                blk_dump_rq_flags(req, "request botched");
                req->__data_len = blk_rq_cur_bytes(req);
        }

        /* recalculate the number of segments */
        blk_recalc_rq_segments(req);

        return true;
}
EXPORT_SYMBOL_GPL(blk_update_request);

static bool blk_update_bidi_request(struct request *rq, int error,
                                    unsigned int nr_bytes,
                                    unsigned int bidi_bytes)
{
        if (blk_update_request(rq, error, nr_bytes))
                return true;

        /* Bidi request must be completed as a whole */
        if (unlikely(blk_bidi_rq(rq)) &&
            blk_update_request(rq->next_rq, error, bidi_bytes))
                return true;

        if (blk_queue_add_random(rq->q))
                add_disk_randomness(rq->rq_disk);

        return false;
}

/**
 * blk_unprep_request - unprepare a request
 * @req:        the request
 *
 * This function makes a request ready for complete resubmission (or
 * completion).  It happens only after all error handling is complete,
 * so represents the appropriate moment to deallocate any resources
 * that were allocated to the request in the prep_rq_fn.  The queue
 * lock is held when calling this.
 */
void blk_unprep_request(struct request *req)
{
        struct request_queue *q = req->q;

        req->cmd_flags &= ~REQ_DONTPREP;
        if (q->unprep_rq_fn)
                q->unprep_rq_fn(q, req);
}
EXPORT_SYMBOL_GPL(blk_unprep_request);

/*
 * queue lock must be held
 */
static void blk_finish_request(struct request *req, int error)
{
        if (blk_rq_tagged(req))
                blk_queue_end_tag(req->q, req);

        BUG_ON(blk_queued_rq(req));

        if (unlikely(laptop_mode) && req->cmd_type == REQ_TYPE_FS)
                laptop_io_completion(&req->q->backing_dev_info);

        blk_delete_timer(req);

        if (req->cmd_flags & REQ_DONTPREP)
                blk_unprep_request(req);


        blk_account_io_done(req);

        if (req->end_io)
                req->end_io(req, error);
        else {
                if (blk_bidi_rq(req))
                        __blk_put_request(req->next_rq->q, req->next_rq);

                __blk_put_request(req->q, req);
        }
}

/**
 * blk_end_bidi_request - Complete a bidi request
 * @rq:         the request to complete
 * @error:      %0 for success, < %0 for error
 * @nr_bytes:   number of bytes to complete @rq
 * @bidi_bytes: number of bytes to complete @rq->next_rq
 *
 * Description:
 *     Ends I/O on a number of bytes attached to @rq and @rq->next_rq.
 *     Drivers that supports bidi can safely call this member for any
 *     type of request, bidi or uni.  In the later case @bidi_bytes is
 *     just ignored.
 *
 * Return:
 *     %false - we are done with this request
 *     %true  - still buffers pending for this request
 **/
static bool blk_end_bidi_request(struct request *rq, int error,
                                 unsigned int nr_bytes, unsigned int bidi_bytes)
{
        struct request_queue *q = rq->q;
        unsigned long flags;

        if (blk_update_bidi_request(rq, error, nr_bytes, bidi_bytes))
                return true;

        spin_lock_irqsave(q->queue_lock, flags);
        blk_finish_request(rq, error);
        spin_unlock_irqrestore(q->queue_lock, flags);

        return false;
}

/**
 * __blk_end_bidi_request - Complete a bidi request with queue lock held
 * @rq:         the request to complete
 * @error:      %0 for success, < %0 for error
 * @nr_bytes:   number of bytes to complete @rq
 * @bidi_bytes: number of bytes to complete @rq->next_rq
 *
 * Description:
 *     Identical to blk_end_bidi_request() except that queue lock is
 *     assumed to be locked on entry and remains so on return.
 *
 * Return:
 *     %false - we are done with this request
 *     %true  - still buffers pending for this request
 **/
static bool __blk_end_bidi_request(struct request *rq, int error,
                                   unsigned int nr_bytes, unsigned int bidi_bytes)
{
        if (blk_update_bidi_request(rq, error, nr_bytes, bidi_bytes))
                return true;

        blk_finish_request(rq, error);

        return false;
}

/**
 * blk_end_request - Helper function for drivers to complete the request.
 * @rq:       the request being processed
 * @error:    %0 for success, < %0 for error
 * @nr_bytes: number of bytes to complete
 *
 * Description:
 *     Ends I/O on a number of bytes attached to @rq.
 *     If @rq has leftover, sets it up for the next range of segments.
 *
 * Return:
 *     %false - we are done with this request
 *     %true  - still buffers pending for this request
 **/
bool blk_end_request(struct request *rq, int error, unsigned int nr_bytes)
{
        return blk_end_bidi_request(rq, error, nr_bytes, 0);
}
EXPORT_SYMBOL(blk_end_request);

/**
 * blk_end_request_all - Helper function for drives to finish the request.
 * @rq: the request to finish
 * @error: %0 for success, < %0 for error
 *
 * Description:
 *     Completely finish @rq.
 */
void blk_end_request_all(struct request *rq, int error)
{
        bool pending;
        unsigned int bidi_bytes = 0;

        if (unlikely(blk_bidi_rq(rq)))
                bidi_bytes = blk_rq_bytes(rq->next_rq);

        pending = blk_end_bidi_request(rq, error, blk_rq_bytes(rq), bidi_bytes);
        BUG_ON(pending);
}
EXPORT_SYMBOL(blk_end_request_all);

/**
 * blk_end_request_cur - Helper function to finish the current request chunk.
 * @rq: the request to finish the current chunk for
 * @error: %0 for success, < %0 for error
 *
 * Description:
 *     Complete the current consecutively mapped chunk from @rq.
 *
 * Return:
 *     %false - we are done with this request
 *     %true  - still buffers pending for this request
 */
bool blk_end_request_cur(struct request *rq, int error)
{
        return blk_end_request(rq, error, blk_rq_cur_bytes(rq));
}
EXPORT_SYMBOL(blk_end_request_cur);

/**
 * blk_end_request_err - Finish a request till the next failure boundary.
 * @rq: the request to finish till the next failure boundary for
 * @error: must be negative errno
 *
 * Description:
 *     Complete @rq till the next failure boundary.
 *
 * Return:
 *     %false - we are done with this request
 *     %true  - still buffers pending for this request
 */
bool blk_end_request_err(struct request *rq, int error)
{
        WARN_ON(error >= 0);
        return blk_end_request(rq, error, blk_rq_err_bytes(rq));
}
EXPORT_SYMBOL_GPL(blk_end_request_err);

/**
 * __blk_end_request - Helper function for drivers to complete the request.
 * @rq:       the request being processed
 * @error:    %0 for success, < %0 for error
 * @nr_bytes: number of bytes to complete
 *
 * Description:
 *     Must be called with queue lock held unlike blk_end_request().
 *
 * Return:
 *     %false - we are done with this request
 *     %true  - still buffers pending for this request
 **/
bool __blk_end_request(struct request *rq, int error, unsigned int nr_bytes)
{
        return __blk_end_bidi_request(rq, error, nr_bytes, 0);
}
EXPORT_SYMBOL(__blk_end_request);

/**
 * __blk_end_request_all - Helper function for drives to finish the request.
 * @rq: the request to finish
 * @error: %0 for success, < %0 for error
 *
 * Description:
 *     Completely finish @rq.  Must be called with queue lock held.
 */
void __blk_end_request_all(struct request *rq, int error)
{
        bool pending;
        unsigned int bidi_bytes = 0;

        if (unlikely(blk_bidi_rq(rq)))
                bidi_bytes = blk_rq_bytes(rq->next_rq);

        pending = __blk_end_bidi_request(rq, error, blk_rq_bytes(rq), bidi_bytes);
        BUG_ON(pending);
}
EXPORT_SYMBOL(__blk_end_request_all);

/**
 * __blk_end_request_cur - Helper function to finish the current request chunk.
 * @rq: the request to finish the current chunk for
 * @error: %0 for success, < %0 for error
 *
 * Description:
 *     Complete the current consecutively mapped chunk from @rq.  Must
 *     be called with queue lock held.
 *
 * Return:
 *     %false - we are done with this request
 *     %true  - still buffers pending for this request
 */
bool __blk_end_request_cur(struct request *rq, int error)
{
        return __blk_end_request(rq, error, blk_rq_cur_bytes(rq));
}
EXPORT_SYMBOL(__blk_end_request_cur);

/**
 * __blk_end_request_err - Finish a request till the next failure boundary.
 * @rq: the request to finish till the next failure boundary for
 * @error: must be negative errno
 *
 * Description:
 *     Complete @rq till the next failure boundary.  Must be called
 *     with queue lock held.
 *
 * Return:
 *     %false - we are done with this request
 *     %true  - still buffers pending for this request
 */
bool __blk_end_request_err(struct request *rq, int error)
{
        WARN_ON(error >= 0);
        return __blk_end_request(rq, error, blk_rq_err_bytes(rq));
}
EXPORT_SYMBOL_GPL(__blk_end_request_err);

void blk_rq_bio_prep(struct request_queue *q, struct request *rq,
                     struct bio *bio)
{
        /* Bit 0 (R/W) is identical in rq->cmd_flags and bio->bi_rw */
        rq->cmd_flags |= bio->bi_rw & REQ_WRITE;

        if (bio_has_data(bio)) {
                rq->nr_phys_segments = bio_phys_segments(q, bio);
                rq->buffer = bio_data(bio);
        }
        rq->__data_len = bio->bi_size;
        rq->bio = rq->biotail = bio;

        if (bio->bi_bdev)
                rq->rq_disk = bio->bi_bdev->bd_disk;
}

#if ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE
/**
 * rq_flush_dcache_pages - Helper function to flush all pages in a request
 * @rq: the request to be flushed
 *
 * Description:
 *     Flush all pages in @rq.
 */
void rq_flush_dcache_pages(struct request *rq)
{
        struct req_iterator iter;
        struct bio_vec *bvec;

        rq_for_each_segment(bvec, rq, iter)
                flush_dcache_page(bvec->bv_page);
}
EXPORT_SYMBOL_GPL(rq_flush_dcache_pages);
#endif

/**
 * blk_lld_busy - Check if underlying low-level drivers of a device are busy
 * @q : the queue of the device being checked
 *
 * Description:
 *    Check if underlying low-level drivers of a device are busy.
 *    If the drivers want to export their busy state, they must set own
 *    exporting function using blk_queue_lld_busy() first.
 *
 *    Basically, this function is used only by request stacking drivers
 *    to stop dispatching requests to underlying devices when underlying
 *    devices are busy.  This behavior helps more I/O merging on the queue
 *    of the request stacking driver and prevents I/O throughput regression
 *    on burst I/O load.
 *
 * Return:
 *    0 - Not busy (The request stacking driver should dispatch request)
 *    1 - Busy (The request stacking driver should stop dispatching request)
 */
int blk_lld_busy(struct request_queue *q)
{
        if (q->lld_busy_fn)
                return q->lld_busy_fn(q);

        return 0;
}
EXPORT_SYMBOL_GPL(blk_lld_busy);

/**
 * blk_rq_unprep_clone - Helper function to free all bios in a cloned request
 * @rq: the clone request to be cleaned up
 *
 * Description:
 *     Free all bios in @rq for a cloned request.
 */
void blk_rq_unprep_clone(struct request *rq)
{
        struct bio *bio;

        while ((bio = rq->bio) != NULL) {
                rq->bio = bio->bi_next;

                bio_put(bio);
        }
}
EXPORT_SYMBOL_GPL(blk_rq_unprep_clone);

/*
 * Copy attributes of the original request to the clone request.
 * The actual data parts (e.g. ->cmd, ->buffer, ->sense) are not copied.
 */
static void __blk_rq_prep_clone(struct request *dst, struct request *src)
{
        dst->cpu = src->cpu;
        dst->cmd_flags = (src->cmd_flags & REQ_CLONE_MASK) | REQ_NOMERGE;
        dst->cmd_type = src->cmd_type;
        dst->__sector = blk_rq_pos(src);
        dst->__data_len = blk_rq_bytes(src);
        dst->nr_phys_segments = src->nr_phys_segments;
        dst->ioprio = src->ioprio;
        dst->extra_len = src->extra_len;
}

/**
 * blk_rq_prep_clone - Helper function to setup clone request
 * @rq: the request to be setup
 * @rq_src: original request to be cloned
 * @bs: bio_set that bios for clone are allocated from
 * @gfp_mask: memory allocation mask for bio
 * @bio_ctr: setup function to be called for each clone bio.
 *           Returns %0 for success, non %0 for failure.
 * @data: private data to be passed to @bio_ctr
 *
 * Description:
 *     Clones bios in @rq_src to @rq, and copies attributes of @rq_src to @rq.
 *     The actual data parts of @rq_src (e.g. ->cmd, ->buffer, ->sense)
 *     are not copied, and copying such parts is the caller's responsibility.
 *     Also, pages which the original bios are pointing to are not copied
 *     and the cloned bios just point same pages.
 *     So cloned bios must be completed before original bios, which means
 *     the caller must complete @rq before @rq_src.
 */
int blk_rq_prep_clone(struct request *rq, struct request *rq_src,
                      struct bio_set *bs, gfp_t gfp_mask,
                      int (*bio_ctr)(struct bio *, struct bio *, void *),
                      void *data)
{
        struct bio *bio, *bio_src;

        if (!bs)
                bs = fs_bio_set;

        blk_rq_init(NULL, rq);

        __rq_for_each_bio(bio_src, rq_src) {
                bio = bio_alloc_bioset(gfp_mask, bio_src->bi_max_vecs, bs);
                if (!bio)
                        goto free_and_out;

                __bio_clone(bio, bio_src);

                if (bio_integrity(bio_src) &&
                    bio_integrity_clone(bio, bio_src, gfp_mask, bs))
                        goto free_and_out;

                if (bio_ctr && bio_ctr(bio, bio_src, data))
                        goto free_and_out;

                if (rq->bio) {
                        rq->biotail->bi_next = bio;
                        rq->biotail = bio;
                } else
                        rq->bio = rq->biotail = bio;
        }

        __blk_rq_prep_clone(rq, rq_src);

        return 0;

free_and_out:
        if (bio)
                bio_free(bio, bs);
        blk_rq_unprep_clone(rq);

        return -ENOMEM;
}
EXPORT_SYMBOL_GPL(blk_rq_prep_clone);

int kblockd_schedule_work(struct request_queue *q, struct work_struct *work)
{
        return queue_work(kblockd_workqueue, work);
}
EXPORT_SYMBOL(kblockd_schedule_work);

int kblockd_schedule_delayed_work(struct request_queue *q,
                        struct delayed_work *dwork, unsigned long delay)
{
        return queue_delayed_work(kblockd_workqueue, dwork, delay);
}
EXPORT_SYMBOL(kblockd_schedule_delayed_work);

#define PLUG_MAGIC      0x91827364

void blk_start_plug(struct blk_plug *plug)
{
        struct task_struct *tsk = current;

        plug->magic = PLUG_MAGIC;
        INIT_LIST_HEAD(&plug->list);
        INIT_LIST_HEAD(&plug->cb_list);
        plug->should_sort = 0;
        plug->count = 0;

        /*
         * If this is a nested plug, don't actually assign it. It will be
         * flushed on its own.
         */
        if (!tsk->plug) {
                /*
                 * Store ordering should not be needed here, since a potential
                 * preempt will imply a full memory barrier
                 */
                tsk->plug = plug;
        }
}
EXPORT_SYMBOL(blk_start_plug);

static int plug_rq_cmp(void *priv, struct list_head *a, struct list_head *b)
{
        struct request *rqa = container_of(a, struct request, queuelist);
        struct request *rqb = container_of(b, struct request, queuelist);

        return !(rqa->q <= rqb->q);
}

/*
 * If 'from_schedule' is true, then postpone the dispatch of requests
 * until a safe kblockd context. We due this to avoid accidental big
 * additional stack usage in driver dispatch, in places where the originally
 * plugger did not intend it.
 */
static void queue_unplugged(struct request_queue *q, unsigned int depth,
                            bool from_schedule)
        __releases(q->queue_lock)
{
        trace_block_unplug(q, depth, !from_schedule);

        /*
         * If we are punting this to kblockd, then we can safely drop
         * the queue_lock before waking kblockd (which needs to take
         * this lock).
         */
        if (from_schedule) {
                spin_unlock(q->queue_lock);
                blk_run_queue_async(q);
        } else {
                __blk_run_queue(q);
                spin_unlock(q->queue_lock);
        }

}

static void flush_plug_callbacks(struct blk_plug *plug)
{
        LIST_HEAD(callbacks);

        if (list_empty(&plug->cb_list))
                return;

        list_splice_init(&plug->cb_list, &callbacks);

        while (!list_empty(&callbacks)) {
                struct blk_plug_cb *cb = list_first_entry(&callbacks,
                                                          struct blk_plug_cb,
                                                          list);
                list_del(&cb->list);
                cb->callback(cb);
        }
}

void blk_flush_plug_list(struct blk_plug *plug, bool from_schedule)
{
        struct request_queue *q;
        unsigned long flags;
        struct request *rq;
        LIST_HEAD(list);
        unsigned int depth;

        BUG_ON(plug->magic != PLUG_MAGIC);

        flush_plug_callbacks(plug);
        if (list_empty(&plug->list))
                return;

        list_splice_init(&plug->list, &list);
        plug->count = 0;

        if (plug->should_sort) {
                list_sort(NULL, &list, plug_rq_cmp);
                plug->should_sort = 0;
        }

        q = NULL;
        depth = 0;

        /*
         * Save and disable interrupts here, to avoid doing it for every
         * queue lock we have to take.
         */
        local_irq_save(flags);
        while (!list_empty(&list)) {
                rq = list_entry_rq(list.next);
                list_del_init(&rq->queuelist);
                BUG_ON(!rq->q);
                if (rq->q != q) {
                        /*
                         * This drops the queue lock
                         */
                        if (q)
                                queue_unplugged(q, depth, from_schedule);
                        q = rq->q;
                        depth = 0;
                        spin_lock(q->queue_lock);
                }
                /*
                 * rq is already accounted, so use raw insert
                 */
                if (rq->cmd_flags & (REQ_FLUSH | REQ_FUA))
                        __elv_add_request(q, rq, ELEVATOR_INSERT_FLUSH);
                else
                        __elv_add_request(q, rq, ELEVATOR_INSERT_SORT_MERGE);

                depth++;
        }

        /*
         * This drops the queue lock
         */
        if (q)
                queue_unplugged(q, depth, from_schedule);

        local_irq_restore(flags);
}

void blk_finish_plug(struct blk_plug *plug)
{
        blk_flush_plug_list(plug, false);

        if (plug == current->plug)
                current->plug = NULL;
}
EXPORT_SYMBOL(blk_finish_plug);

int __init blk_dev_init(void)
{
        BUILD_BUG_ON(__REQ_NR_BITS > 8 *
                        sizeof(((struct request *)0)->cmd_flags));

        /* used for unplugging and affects IO latency/throughput - HIGHPRI */
        kblockd_workqueue = alloc_workqueue("kblockd",
                                            WQ_MEM_RECLAIM | WQ_HIGHPRI, 0);
        if (!kblockd_workqueue)
                panic("Failed to create kblockd\n");

        request_cachep = kmem_cache_create("blkdev_requests",
                        sizeof(struct request), 0, SLAB_PANIC, NULL);

        blk_requestq_cachep = kmem_cache_create("blkdev_queue",
                        sizeof(struct request_queue), 0, SLAB_PANIC, NULL);

        return 0;
}