fix XEN_SAVE_RESTORE Kconfig dependencies

[pandora-kernel.git] / fs / xfs / linux-2.6 / xfs_buf.c

/*
 * Copyright (c) 2000-2006 Silicon Graphics, Inc.
 * All Rights Reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it would be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write the Free Software Foundation,
 * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
#include "xfs.h"
#include <linux/stddef.h>
#include <linux/errno.h>
#include <linux/gfp.h>
#include <linux/pagemap.h>
#include <linux/init.h>
#include <linux/vmalloc.h>
#include <linux/bio.h>
#include <linux/sysctl.h>
#include <linux/proc_fs.h>
#include <linux/workqueue.h>
#include <linux/percpu.h>
#include <linux/blkdev.h>
#include <linux/hash.h>
#include <linux/kthread.h>
#include <linux/migrate.h>
#include <linux/backing-dev.h>
#include <linux/freezer.h>
#include <linux/list_sort.h>

#include "xfs_sb.h"
#include "xfs_inum.h"
#include "xfs_log.h"
#include "xfs_ag.h"
#include "xfs_mount.h"
#include "xfs_trace.h"

static kmem_zone_t *xfs_buf_zone;
STATIC int xfsbufd(void *);
STATIC void xfs_buf_delwri_queue(xfs_buf_t *, int);

static struct workqueue_struct *xfslogd_workqueue;
struct workqueue_struct *xfsdatad_workqueue;
struct workqueue_struct *xfsconvertd_workqueue;

#ifdef XFS_BUF_LOCK_TRACKING
# define XB_SET_OWNER(bp)       ((bp)->b_last_holder = current->pid)
# define XB_CLEAR_OWNER(bp)     ((bp)->b_last_holder = -1)
# define XB_GET_OWNER(bp)       ((bp)->b_last_holder)
#else
# define XB_SET_OWNER(bp)       do { } while (0)
# define XB_CLEAR_OWNER(bp)     do { } while (0)
# define XB_GET_OWNER(bp)       do { } while (0)
#endif

#define xb_to_gfp(flags) \
        ((((flags) & XBF_READ_AHEAD) ? __GFP_NORETRY : \
          ((flags) & XBF_DONT_BLOCK) ? GFP_NOFS : GFP_KERNEL) | __GFP_NOWARN)

#define xb_to_km(flags) \
         (((flags) & XBF_DONT_BLOCK) ? KM_NOFS : KM_SLEEP)

#define xfs_buf_allocate(flags) \
        kmem_zone_alloc(xfs_buf_zone, xb_to_km(flags))
#define xfs_buf_deallocate(bp) \
        kmem_zone_free(xfs_buf_zone, (bp));

static inline int
xfs_buf_is_vmapped(
        struct xfs_buf  *bp)
{
        /*
         * Return true if the buffer is vmapped.
         *
         * The XBF_MAPPED flag is set if the buffer should be mapped, but the
         * code is clever enough to know it doesn't have to map a single page,
         * so the check has to be both for XBF_MAPPED and bp->b_page_count > 1.
         */
        return (bp->b_flags & XBF_MAPPED) && bp->b_page_count > 1;
}

static inline int
xfs_buf_vmap_len(
        struct xfs_buf  *bp)
{
        return (bp->b_page_count * PAGE_SIZE) - bp->b_offset;
}

/*
 * xfs_buf_lru_add - add a buffer to the LRU.
 *
 * The LRU takes a new reference to the buffer so that it will only be freed
 * once the shrinker takes the buffer off the LRU.
 */
STATIC void
xfs_buf_lru_add(
        struct xfs_buf  *bp)
{
        struct xfs_buftarg *btp = bp->b_target;

        spin_lock(&btp->bt_lru_lock);
        if (list_empty(&bp->b_lru)) {
                atomic_inc(&bp->b_hold);
                list_add_tail(&bp->b_lru, &btp->bt_lru);
                btp->bt_lru_nr++;
        }
        spin_unlock(&btp->bt_lru_lock);
}

/*
 * xfs_buf_lru_del - remove a buffer from the LRU
 *
 * The unlocked check is safe here because it only occurs when there are not
 * b_lru_ref counts left on the inode under the pag->pag_buf_lock. it is there
 * to optimise the shrinker removing the buffer from the LRU and calling
 * xfs_buf_free(). i.e. it removes an unnecessary round trip on the
 * bt_lru_lock.
 */
STATIC void
xfs_buf_lru_del(
        struct xfs_buf  *bp)
{
        struct xfs_buftarg *btp = bp->b_target;

        if (list_empty(&bp->b_lru))
                return;

        spin_lock(&btp->bt_lru_lock);
        if (!list_empty(&bp->b_lru)) {
                list_del_init(&bp->b_lru);
                btp->bt_lru_nr--;
        }
        spin_unlock(&btp->bt_lru_lock);
}

/*
 * When we mark a buffer stale, we remove the buffer from the LRU and clear the
 * b_lru_ref count so that the buffer is freed immediately when the buffer
 * reference count falls to zero. If the buffer is already on the LRU, we need
 * to remove the reference that LRU holds on the buffer.
 *
 * This prevents build-up of stale buffers on the LRU.
 */
void
xfs_buf_stale(
        struct xfs_buf  *bp)
{
        bp->b_flags |= XBF_STALE;
        atomic_set(&(bp)->b_lru_ref, 0);
        if (!list_empty(&bp->b_lru)) {
                struct xfs_buftarg *btp = bp->b_target;

                spin_lock(&btp->bt_lru_lock);
                if (!list_empty(&bp->b_lru)) {
                        list_del_init(&bp->b_lru);
                        btp->bt_lru_nr--;
                        atomic_dec(&bp->b_hold);
                }
                spin_unlock(&btp->bt_lru_lock);
        }
        ASSERT(atomic_read(&bp->b_hold) >= 1);
}

STATIC void
_xfs_buf_initialize(
        xfs_buf_t               *bp,
        xfs_buftarg_t           *target,
        xfs_off_t               range_base,
        size_t                  range_length,
        xfs_buf_flags_t         flags)
{
        /*
         * We don't want certain flags to appear in b_flags.
         */
        flags &= ~(XBF_LOCK|XBF_MAPPED|XBF_DONT_BLOCK|XBF_READ_AHEAD);

        memset(bp, 0, sizeof(xfs_buf_t));
        atomic_set(&bp->b_hold, 1);
        atomic_set(&bp->b_lru_ref, 1);
        init_completion(&bp->b_iowait);
        INIT_LIST_HEAD(&bp->b_lru);
        INIT_LIST_HEAD(&bp->b_list);
        RB_CLEAR_NODE(&bp->b_rbnode);
        sema_init(&bp->b_sema, 0); /* held, no waiters */
        XB_SET_OWNER(bp);
        bp->b_target = target;
        bp->b_file_offset = range_base;
        /*
         * Set buffer_length and count_desired to the same value initially.
         * I/O routines should use count_desired, which will be the same in
         * most cases but may be reset (e.g. XFS recovery).
         */
        bp->b_buffer_length = bp->b_count_desired = range_length;
        bp->b_flags = flags;
        bp->b_bn = XFS_BUF_DADDR_NULL;
        atomic_set(&bp->b_pin_count, 0);
        init_waitqueue_head(&bp->b_waiters);

        XFS_STATS_INC(xb_create);

        trace_xfs_buf_init(bp, _RET_IP_);
}

/*
 *      Allocate a page array capable of holding a specified number
 *      of pages, and point the page buf at it.
 */
STATIC int
_xfs_buf_get_pages(
        xfs_buf_t               *bp,
        int                     page_count,
        xfs_buf_flags_t         flags)
{
        /* Make sure that we have a page list */
        if (bp->b_pages == NULL) {
                bp->b_offset = xfs_buf_poff(bp->b_file_offset);
                bp->b_page_count = page_count;
                if (page_count <= XB_PAGES) {
                        bp->b_pages = bp->b_page_array;
                } else {
                        bp->b_pages = kmem_alloc(sizeof(struct page *) *
                                        page_count, xb_to_km(flags));
                        if (bp->b_pages == NULL)
                                return -ENOMEM;
                }
                memset(bp->b_pages, 0, sizeof(struct page *) * page_count);
        }
        return 0;
}

/*
 *      Frees b_pages if it was allocated.
 */
STATIC void
_xfs_buf_free_pages(
        xfs_buf_t       *bp)
{
        if (bp->b_pages != bp->b_page_array) {
                kmem_free(bp->b_pages);
                bp->b_pages = NULL;
        }
}

/*
 *      Releases the specified buffer.
 *
 *      The modification state of any associated pages is left unchanged.
 *      The buffer most not be on any hash - use xfs_buf_rele instead for
 *      hashed and refcounted buffers
 */
void
xfs_buf_free(
        xfs_buf_t               *bp)
{
        trace_xfs_buf_free(bp, _RET_IP_);

        ASSERT(list_empty(&bp->b_lru));

        if (bp->b_flags & _XBF_PAGES) {
                uint            i;

                if (xfs_buf_is_vmapped(bp))
                        vm_unmap_ram(bp->b_addr - bp->b_offset,
                                        bp->b_page_count);

                for (i = 0; i < bp->b_page_count; i++) {
                        struct page     *page = bp->b_pages[i];

                        __free_page(page);
                }
        } else if (bp->b_flags & _XBF_KMEM)
                kmem_free(bp->b_addr);
        _xfs_buf_free_pages(bp);
        xfs_buf_deallocate(bp);
}

/*
 * Allocates all the pages for buffer in question and builds it's page list.
 */
STATIC int
xfs_buf_allocate_memory(
        xfs_buf_t               *bp,
        uint                    flags)
{
        size_t                  size = bp->b_count_desired;
        size_t                  nbytes, offset;
        gfp_t                   gfp_mask = xb_to_gfp(flags);
        unsigned short          page_count, i;
        pgoff_t                 first;
        xfs_off_t               end;
        int                     error;

        /*
         * for buffers that are contained within a single page, just allocate
         * the memory from the heap - there's no need for the complexity of
         * page arrays to keep allocation down to order 0.
         */
        if (bp->b_buffer_length < PAGE_SIZE) {
                bp->b_addr = kmem_alloc(bp->b_buffer_length, xb_to_km(flags));
                if (!bp->b_addr) {
                        /* low memory - use alloc_page loop instead */
                        goto use_alloc_page;
                }

                if (((unsigned long)(bp->b_addr + bp->b_buffer_length - 1) &
                                                                PAGE_MASK) !=
                    ((unsigned long)bp->b_addr & PAGE_MASK)) {
                        /* b_addr spans two pages - use alloc_page instead */
                        kmem_free(bp->b_addr);
                        bp->b_addr = NULL;
                        goto use_alloc_page;
                }
                bp->b_offset = offset_in_page(bp->b_addr);
                bp->b_pages = bp->b_page_array;
                bp->b_pages[0] = virt_to_page(bp->b_addr);
                bp->b_page_count = 1;
                bp->b_flags |= XBF_MAPPED | _XBF_KMEM;
                return 0;
        }

use_alloc_page:
        end = bp->b_file_offset + bp->b_buffer_length;
        page_count = xfs_buf_btoc(end) - xfs_buf_btoct(bp->b_file_offset);
        error = _xfs_buf_get_pages(bp, page_count, flags);
        if (unlikely(error))
                return error;

        offset = bp->b_offset;
        first = bp->b_file_offset >> PAGE_SHIFT;
        bp->b_flags |= _XBF_PAGES;

        for (i = 0; i < bp->b_page_count; i++) {
                struct page     *page;
                uint            retries = 0;
retry:
                page = alloc_page(gfp_mask);
                if (unlikely(page == NULL)) {
                        if (flags & XBF_READ_AHEAD) {
                                bp->b_page_count = i;
                                error = ENOMEM;
                                goto out_free_pages;
                        }

                        /*
                         * This could deadlock.
                         *
                         * But until all the XFS lowlevel code is revamped to
                         * handle buffer allocation failures we can't do much.
                         */
                        if (!(++retries % 100))
                                xfs_err(NULL,
                "possible memory allocation deadlock in %s (mode:0x%x)",
                                        __func__, gfp_mask);

                        XFS_STATS_INC(xb_page_retries);
                        congestion_wait(BLK_RW_ASYNC, HZ/50);
                        goto retry;
                }

                XFS_STATS_INC(xb_page_found);

                nbytes = min_t(size_t, size, PAGE_SIZE - offset);
                size -= nbytes;
                bp->b_pages[i] = page;
                offset = 0;
        }
        return 0;

out_free_pages:
        for (i = 0; i < bp->b_page_count; i++)
                __free_page(bp->b_pages[i]);
        return error;
}

/*
 *      Map buffer into kernel address-space if necessary.
 */
STATIC int
_xfs_buf_map_pages(
        xfs_buf_t               *bp,
        uint                    flags)
{
        ASSERT(bp->b_flags & _XBF_PAGES);
        if (bp->b_page_count == 1) {
                /* A single page buffer is always mappable */
                bp->b_addr = page_address(bp->b_pages[0]) + bp->b_offset;
                bp->b_flags |= XBF_MAPPED;
        } else if (flags & XBF_MAPPED) {
                int retried = 0;

                do {
                        bp->b_addr = vm_map_ram(bp->b_pages, bp->b_page_count,
                                                -1, PAGE_KERNEL);
                        if (bp->b_addr)
                                break;
                        vm_unmap_aliases();
                } while (retried++ <= 1);

                if (!bp->b_addr)
                        return -ENOMEM;
                bp->b_addr += bp->b_offset;
                bp->b_flags |= XBF_MAPPED;
        }

        return 0;
}

/*
 *      Finding and Reading Buffers
 */

/*
 *      Look up, and creates if absent, a lockable buffer for
 *      a given range of an inode.  The buffer is returned
 *      locked.  If other overlapping buffers exist, they are
 *      released before the new buffer is created and locked,
 *      which may imply that this call will block until those buffers
 *      are unlocked.  No I/O is implied by this call.
 */
xfs_buf_t *
_xfs_buf_find(
        xfs_buftarg_t           *btp,   /* block device target          */
        xfs_off_t               ioff,   /* starting offset of range     */
        size_t                  isize,  /* length of range              */
        xfs_buf_flags_t         flags,
        xfs_buf_t               *new_bp)
{
        xfs_off_t               range_base;
        size_t                  range_length;
        struct xfs_perag        *pag;
        struct rb_node          **rbp;
        struct rb_node          *parent;
        xfs_buf_t               *bp;

        range_base = (ioff << BBSHIFT);
        range_length = (isize << BBSHIFT);

        /* Check for IOs smaller than the sector size / not sector aligned */
        ASSERT(!(range_length < (1 << btp->bt_sshift)));
        ASSERT(!(range_base & (xfs_off_t)btp->bt_smask));

        /* get tree root */
        pag = xfs_perag_get(btp->bt_mount,
                                xfs_daddr_to_agno(btp->bt_mount, ioff));

        /* walk tree */
        spin_lock(&pag->pag_buf_lock);
        rbp = &pag->pag_buf_tree.rb_node;
        parent = NULL;
        bp = NULL;
        while (*rbp) {
                parent = *rbp;
                bp = rb_entry(parent, struct xfs_buf, b_rbnode);

                if (range_base < bp->b_file_offset)
                        rbp = &(*rbp)->rb_left;
                else if (range_base > bp->b_file_offset)
                        rbp = &(*rbp)->rb_right;
                else {
                        /*
                         * found a block offset match. If the range doesn't
                         * match, the only way this is allowed is if the buffer
                         * in the cache is stale and the transaction that made
                         * it stale has not yet committed. i.e. we are
                         * reallocating a busy extent. Skip this buffer and
                         * continue searching to the right for an exact match.
                         */
                        if (bp->b_buffer_length != range_length) {
                                ASSERT(bp->b_flags & XBF_STALE);
                                rbp = &(*rbp)->rb_right;
                                continue;
                        }
                        atomic_inc(&bp->b_hold);
                        goto found;
                }
        }

        /* No match found */
        if (new_bp) {
                _xfs_buf_initialize(new_bp, btp, range_base,
                                range_length, flags);
                rb_link_node(&new_bp->b_rbnode, parent, rbp);
                rb_insert_color(&new_bp->b_rbnode, &pag->pag_buf_tree);
                /* the buffer keeps the perag reference until it is freed */
                new_bp->b_pag = pag;
                spin_unlock(&pag->pag_buf_lock);
        } else {
                XFS_STATS_INC(xb_miss_locked);
                spin_unlock(&pag->pag_buf_lock);
                xfs_perag_put(pag);
        }
        return new_bp;

found:
        spin_unlock(&pag->pag_buf_lock);
        xfs_perag_put(pag);

        if (xfs_buf_cond_lock(bp)) {
                /* failed, so wait for the lock if requested. */
                if (!(flags & XBF_TRYLOCK)) {
                        xfs_buf_lock(bp);
                        XFS_STATS_INC(xb_get_locked_waited);
                } else {
                        xfs_buf_rele(bp);
                        XFS_STATS_INC(xb_busy_locked);
                        return NULL;
                }
        }

        /*
         * if the buffer is stale, clear all the external state associated with
         * it. We need to keep flags such as how we allocated the buffer memory
         * intact here.
         */
        if (bp->b_flags & XBF_STALE) {
                ASSERT((bp->b_flags & _XBF_DELWRI_Q) == 0);
                bp->b_flags &= XBF_MAPPED | _XBF_KMEM | _XBF_PAGES;
        }

        trace_xfs_buf_find(bp, flags, _RET_IP_);
        XFS_STATS_INC(xb_get_locked);
        return bp;
}

/*
 *      Assembles a buffer covering the specified range.
 *      Storage in memory for all portions of the buffer will be allocated,
 *      although backing storage may not be.
 */
xfs_buf_t *
xfs_buf_get(
        xfs_buftarg_t           *target,/* target for buffer            */
        xfs_off_t               ioff,   /* starting offset of range     */
        size_t                  isize,  /* length of range              */
        xfs_buf_flags_t         flags)
{
        xfs_buf_t               *bp, *new_bp;
        int                     error = 0;

        new_bp = xfs_buf_allocate(flags);
        if (unlikely(!new_bp))
                return NULL;

        bp = _xfs_buf_find(target, ioff, isize, flags, new_bp);
        if (bp == new_bp) {
                error = xfs_buf_allocate_memory(bp, flags);
                if (error)
                        goto no_buffer;
        } else {
                xfs_buf_deallocate(new_bp);
                if (unlikely(bp == NULL))
                        return NULL;
        }

        if (!(bp->b_flags & XBF_MAPPED)) {
                error = _xfs_buf_map_pages(bp, flags);
                if (unlikely(error)) {
                        xfs_warn(target->bt_mount,
                                "%s: failed to map pages\n", __func__);
                        goto no_buffer;
                }
        }

        XFS_STATS_INC(xb_get);

        /*
         * Always fill in the block number now, the mapped cases can do
         * their own overlay of this later.
         */
        bp->b_bn = ioff;
        bp->b_count_desired = bp->b_buffer_length;

        trace_xfs_buf_get(bp, flags, _RET_IP_);
        return bp;

 no_buffer:
        if (flags & (XBF_LOCK | XBF_TRYLOCK))
                xfs_buf_unlock(bp);
        xfs_buf_rele(bp);
        return NULL;
}

STATIC int
_xfs_buf_read(
        xfs_buf_t               *bp,
        xfs_buf_flags_t         flags)
{
        int                     status;

        ASSERT(!(flags & (XBF_DELWRI|XBF_WRITE)));
        ASSERT(bp->b_bn != XFS_BUF_DADDR_NULL);

        bp->b_flags &= ~(XBF_WRITE | XBF_ASYNC | XBF_DELWRI | \
                        XBF_READ_AHEAD | _XBF_RUN_QUEUES);
        bp->b_flags |= flags & (XBF_READ | XBF_ASYNC | \
                        XBF_READ_AHEAD | _XBF_RUN_QUEUES);

        status = xfs_buf_iorequest(bp);
        if (status || XFS_BUF_ISERROR(bp) || (flags & XBF_ASYNC))
                return status;
        return xfs_buf_iowait(bp);
}

xfs_buf_t *
xfs_buf_read(
        xfs_buftarg_t           *target,
        xfs_off_t               ioff,
        size_t                  isize,
        xfs_buf_flags_t         flags)
{
        xfs_buf_t               *bp;

        flags |= XBF_READ;

        bp = xfs_buf_get(target, ioff, isize, flags);
        if (bp) {
                trace_xfs_buf_read(bp, flags, _RET_IP_);

                if (!XFS_BUF_ISDONE(bp)) {
                        XFS_STATS_INC(xb_get_read);
                        _xfs_buf_read(bp, flags);
                } else if (flags & XBF_ASYNC) {
                        /*
                         * Read ahead call which is already satisfied,
                         * drop the buffer
                         */
                        goto no_buffer;
                } else {
                        /* We do not want read in the flags */
                        bp->b_flags &= ~XBF_READ;
                }
        }

        return bp;

 no_buffer:
        if (flags & (XBF_LOCK | XBF_TRYLOCK))
                xfs_buf_unlock(bp);
        xfs_buf_rele(bp);
        return NULL;
}

/*
 *      If we are not low on memory then do the readahead in a deadlock
 *      safe manner.
 */
void
xfs_buf_readahead(
        xfs_buftarg_t           *target,
        xfs_off_t               ioff,
        size_t                  isize)
{
        struct backing_dev_info *bdi;

        if (bdi_read_congested(target->bt_bdi))
                return;

        xfs_buf_read(target, ioff, isize,
                     XBF_TRYLOCK|XBF_ASYNC|XBF_READ_AHEAD|XBF_DONT_BLOCK);
}

/*
 * Read an uncached buffer from disk. Allocates and returns a locked
 * buffer containing the disk contents or nothing.
 */
struct xfs_buf *
xfs_buf_read_uncached(
        struct xfs_mount        *mp,
        struct xfs_buftarg      *target,
        xfs_daddr_t             daddr,
        size_t                  length,
        int                     flags)
{
        xfs_buf_t               *bp;
        int                     error;

        bp = xfs_buf_get_uncached(target, length, flags);
        if (!bp)
                return NULL;

        /* set up the buffer for a read IO */
        xfs_buf_lock(bp);
        XFS_BUF_SET_ADDR(bp, daddr);
        XFS_BUF_READ(bp);
        XFS_BUF_BUSY(bp);

        xfsbdstrat(mp, bp);
        error = xfs_buf_iowait(bp);
        if (error || bp->b_error) {
                xfs_buf_relse(bp);
                return NULL;
        }
        return bp;
}

xfs_buf_t *
xfs_buf_get_empty(
        size_t                  len,
        xfs_buftarg_t           *target)
{
        xfs_buf_t               *bp;

        bp = xfs_buf_allocate(0);
        if (bp)
                _xfs_buf_initialize(bp, target, 0, len, 0);
        return bp;
}

static inline struct page *
mem_to_page(
        void                    *addr)
{
        if ((!is_vmalloc_addr(addr))) {
                return virt_to_page(addr);
        } else {
                return vmalloc_to_page(addr);
        }
}

int
xfs_buf_associate_memory(
        xfs_buf_t               *bp,
        void                    *mem,
        size_t                  len)
{
        int                     rval;
        int                     i = 0;
        unsigned long           pageaddr;
        unsigned long           offset;
        size_t                  buflen;
        int                     page_count;

        pageaddr = (unsigned long)mem & PAGE_MASK;
        offset = (unsigned long)mem - pageaddr;
        buflen = PAGE_ALIGN(len + offset);
        page_count = buflen >> PAGE_SHIFT;

        /* Free any previous set of page pointers */
        if (bp->b_pages)
                _xfs_buf_free_pages(bp);

        bp->b_pages = NULL;
        bp->b_addr = mem;

        rval = _xfs_buf_get_pages(bp, page_count, XBF_DONT_BLOCK);
        if (rval)
                return rval;

        bp->b_offset = offset;

        for (i = 0; i < bp->b_page_count; i++) {
                bp->b_pages[i] = mem_to_page((void *)pageaddr);
                pageaddr += PAGE_SIZE;
        }

        bp->b_count_desired = len;
        bp->b_buffer_length = buflen;
        bp->b_flags |= XBF_MAPPED;

        return 0;
}

xfs_buf_t *
xfs_buf_get_uncached(
        struct xfs_buftarg      *target,
        size_t                  len,
        int                     flags)
{
        unsigned long           page_count = PAGE_ALIGN(len) >> PAGE_SHIFT;
        int                     error, i;
        xfs_buf_t               *bp;

        bp = xfs_buf_allocate(0);
        if (unlikely(bp == NULL))
                goto fail;
        _xfs_buf_initialize(bp, target, 0, len, 0);

        error = _xfs_buf_get_pages(bp, page_count, 0);
        if (error)
                goto fail_free_buf;

        for (i = 0; i < page_count; i++) {
                bp->b_pages[i] = alloc_page(xb_to_gfp(flags));
                if (!bp->b_pages[i])
                        goto fail_free_mem;
        }
        bp->b_flags |= _XBF_PAGES;

        error = _xfs_buf_map_pages(bp, XBF_MAPPED);
        if (unlikely(error)) {
                xfs_warn(target->bt_mount,
                        "%s: failed to map pages\n", __func__);
                goto fail_free_mem;
        }

        xfs_buf_unlock(bp);

        trace_xfs_buf_get_uncached(bp, _RET_IP_);
        return bp;

 fail_free_mem:
        while (--i >= 0)
                __free_page(bp->b_pages[i]);
        _xfs_buf_free_pages(bp);
 fail_free_buf:
        xfs_buf_deallocate(bp);
 fail:
        return NULL;
}

/*
 *      Increment reference count on buffer, to hold the buffer concurrently
 *      with another thread which may release (free) the buffer asynchronously.
 *      Must hold the buffer already to call this function.
 */
void
xfs_buf_hold(
        xfs_buf_t               *bp)
{
        trace_xfs_buf_hold(bp, _RET_IP_);
        atomic_inc(&bp->b_hold);
}

/*
 *      Releases a hold on the specified buffer.  If the
 *      the hold count is 1, calls xfs_buf_free.
 */
void
xfs_buf_rele(
        xfs_buf_t               *bp)
{
        struct xfs_perag        *pag = bp->b_pag;

        trace_xfs_buf_rele(bp, _RET_IP_);

        if (!pag) {
                ASSERT(list_empty(&bp->b_lru));
                ASSERT(RB_EMPTY_NODE(&bp->b_rbnode));
                if (atomic_dec_and_test(&bp->b_hold))
                        xfs_buf_free(bp);
                return;
        }

        ASSERT(!RB_EMPTY_NODE(&bp->b_rbnode));

        ASSERT(atomic_read(&bp->b_hold) > 0);
        if (atomic_dec_and_lock(&bp->b_hold, &pag->pag_buf_lock)) {
                if (!(bp->b_flags & XBF_STALE) &&
                           atomic_read(&bp->b_lru_ref)) {
                        xfs_buf_lru_add(bp);
                        spin_unlock(&pag->pag_buf_lock);
                } else {
                        xfs_buf_lru_del(bp);
                        ASSERT(!(bp->b_flags & (XBF_DELWRI|_XBF_DELWRI_Q)));
                        rb_erase(&bp->b_rbnode, &pag->pag_buf_tree);
                        spin_unlock(&pag->pag_buf_lock);
                        xfs_perag_put(pag);
                        xfs_buf_free(bp);
                }
        }
}


/*
 *      Lock a buffer object, if it is not already locked.
 *
 *      If we come across a stale, pinned, locked buffer, we know that we are
 *      being asked to lock a buffer that has been reallocated. Because it is
 *      pinned, we know that the log has not been pushed to disk and hence it
 *      will still be locked.  Rather than continuing to have trylock attempts
 *      fail until someone else pushes the log, push it ourselves before
 *      returning.  This means that the xfsaild will not get stuck trying
 *      to push on stale inode buffers.
 */
int
xfs_buf_cond_lock(
        xfs_buf_t               *bp)
{
        int                     locked;

        locked = down_trylock(&bp->b_sema) == 0;
        if (locked)
                XB_SET_OWNER(bp);
        else if (atomic_read(&bp->b_pin_count) && (bp->b_flags & XBF_STALE))
                xfs_log_force(bp->b_target->bt_mount, 0);

        trace_xfs_buf_cond_lock(bp, _RET_IP_);
        return locked ? 0 : -EBUSY;
}

int
xfs_buf_lock_value(
        xfs_buf_t               *bp)
{
        return bp->b_sema.count;
}

/*
 *      Lock a buffer object.
 *
 *      If we come across a stale, pinned, locked buffer, we know that we
 *      are being asked to lock a buffer that has been reallocated. Because
 *      it is pinned, we know that the log has not been pushed to disk and
 *      hence it will still be locked. Rather than sleeping until someone
 *      else pushes the log, push it ourselves before trying to get the lock.
 */
void
xfs_buf_lock(
        xfs_buf_t               *bp)
{
        trace_xfs_buf_lock(bp, _RET_IP_);

        if (atomic_read(&bp->b_pin_count) && (bp->b_flags & XBF_STALE))
                xfs_log_force(bp->b_target->bt_mount, 0);
        if (atomic_read(&bp->b_io_remaining))
                blk_flush_plug(current);
        down(&bp->b_sema);
        XB_SET_OWNER(bp);

        trace_xfs_buf_lock_done(bp, _RET_IP_);
}

/*
 *      Releases the lock on the buffer object.
 *      If the buffer is marked delwri but is not queued, do so before we
 *      unlock the buffer as we need to set flags correctly.  We also need to
 *      take a reference for the delwri queue because the unlocker is going to
 *      drop their's and they don't know we just queued it.
 */
void
xfs_buf_unlock(
        xfs_buf_t               *bp)
{
        if ((bp->b_flags & (XBF_DELWRI|_XBF_DELWRI_Q)) == XBF_DELWRI) {
                atomic_inc(&bp->b_hold);
                bp->b_flags |= XBF_ASYNC;
                xfs_buf_delwri_queue(bp, 0);
        }

        XB_CLEAR_OWNER(bp);
        up(&bp->b_sema);

        trace_xfs_buf_unlock(bp, _RET_IP_);
}

STATIC void
xfs_buf_wait_unpin(
        xfs_buf_t               *bp)
{
        DECLARE_WAITQUEUE       (wait, current);

        if (atomic_read(&bp->b_pin_count) == 0)
                return;

        add_wait_queue(&bp->b_waiters, &wait);
        for (;;) {
                set_current_state(TASK_UNINTERRUPTIBLE);
                if (atomic_read(&bp->b_pin_count) == 0)
                        break;
                io_schedule();
        }
        remove_wait_queue(&bp->b_waiters, &wait);
        set_current_state(TASK_RUNNING);
}

/*
 *      Buffer Utility Routines
 */

STATIC void
xfs_buf_iodone_work(
        struct work_struct      *work)
{
        xfs_buf_t               *bp =
                container_of(work, xfs_buf_t, b_iodone_work);

        if (bp->b_iodone)
                (*(bp->b_iodone))(bp);
        else if (bp->b_flags & XBF_ASYNC)
                xfs_buf_relse(bp);
}

void
xfs_buf_ioend(
        xfs_buf_t               *bp,
        int                     schedule)
{
        trace_xfs_buf_iodone(bp, _RET_IP_);

        bp->b_flags &= ~(XBF_READ | XBF_WRITE | XBF_READ_AHEAD);
        if (bp->b_error == 0)
                bp->b_flags |= XBF_DONE;

        if ((bp->b_iodone) || (bp->b_flags & XBF_ASYNC)) {
                if (schedule) {
                        INIT_WORK(&bp->b_iodone_work, xfs_buf_iodone_work);
                        queue_work(xfslogd_workqueue, &bp->b_iodone_work);
                } else {
                        xfs_buf_iodone_work(&bp->b_iodone_work);
                }
        } else {
                complete(&bp->b_iowait);
        }
}

void
xfs_buf_ioerror(
        xfs_buf_t               *bp,
        int                     error)
{
        ASSERT(error >= 0 && error <= 0xffff);
        bp->b_error = (unsigned short)error;
        trace_xfs_buf_ioerror(bp, error, _RET_IP_);
}

int
xfs_bwrite(
        struct xfs_mount        *mp,
        struct xfs_buf          *bp)
{
        int                     error;

        bp->b_flags |= XBF_WRITE;
        bp->b_flags &= ~(XBF_ASYNC | XBF_READ);

        xfs_buf_delwri_dequeue(bp);
        xfs_bdstrat_cb(bp);

        error = xfs_buf_iowait(bp);
        if (error)
                xfs_force_shutdown(mp, SHUTDOWN_META_IO_ERROR);
        xfs_buf_relse(bp);
        return error;
}

void
xfs_bdwrite(
        void                    *mp,
        struct xfs_buf          *bp)
{
        trace_xfs_buf_bdwrite(bp, _RET_IP_);

        bp->b_flags &= ~XBF_READ;
        bp->b_flags |= (XBF_DELWRI | XBF_ASYNC);

        xfs_buf_delwri_queue(bp, 1);
}

/*
 * Called when we want to stop a buffer from getting written or read.
 * We attach the EIO error, muck with its flags, and call xfs_buf_ioend
 * so that the proper iodone callbacks get called.
 */
STATIC int
xfs_bioerror(
        xfs_buf_t *bp)
{
#ifdef XFSERRORDEBUG
        ASSERT(XFS_BUF_ISREAD(bp) || bp->b_iodone);
#endif

        /*
         * No need to wait until the buffer is unpinned, we aren't flushing it.
         */
        XFS_BUF_ERROR(bp, EIO);

        /*
         * We're calling xfs_buf_ioend, so delete XBF_DONE flag.
         */
        XFS_BUF_UNREAD(bp);
        XFS_BUF_UNDELAYWRITE(bp);
        XFS_BUF_UNDONE(bp);
        XFS_BUF_STALE(bp);

        xfs_buf_ioend(bp, 0);

        return EIO;
}

/*
 * Same as xfs_bioerror, except that we are releasing the buffer
 * here ourselves, and avoiding the xfs_buf_ioend call.
 * This is meant for userdata errors; metadata bufs come with
 * iodone functions attached, so that we can track down errors.
 */
STATIC int
xfs_bioerror_relse(
        struct xfs_buf  *bp)
{
        int64_t         fl = XFS_BUF_BFLAGS(bp);
        /*
         * No need to wait until the buffer is unpinned.
         * We aren't flushing it.
         *
         * chunkhold expects B_DONE to be set, whether
         * we actually finish the I/O or not. We don't want to
         * change that interface.
         */
        XFS_BUF_UNREAD(bp);
        XFS_BUF_UNDELAYWRITE(bp);
        XFS_BUF_DONE(bp);
        XFS_BUF_STALE(bp);
        XFS_BUF_CLR_IODONE_FUNC(bp);
        if (!(fl & XBF_ASYNC)) {
                /*
                 * Mark b_error and B_ERROR _both_.
                 * Lot's of chunkcache code assumes that.
                 * There's no reason to mark error for
                 * ASYNC buffers.
                 */
                XFS_BUF_ERROR(bp, EIO);
                XFS_BUF_FINISH_IOWAIT(bp);
        } else {
                xfs_buf_relse(bp);
        }

        return EIO;
}


/*
 * All xfs metadata buffers except log state machine buffers
 * get this attached as their b_bdstrat callback function.
 * This is so that we can catch a buffer
 * after prematurely unpinning it to forcibly shutdown the filesystem.
 */
int
xfs_bdstrat_cb(
        struct xfs_buf  *bp)
{
        if (XFS_FORCED_SHUTDOWN(bp->b_target->bt_mount)) {
                trace_xfs_bdstrat_shut(bp, _RET_IP_);
                /*
                 * Metadata write that didn't get logged but
                 * written delayed anyway. These aren't associated
                 * with a transaction, and can be ignored.
                 */
                if (!bp->b_iodone && !XFS_BUF_ISREAD(bp))
                        return xfs_bioerror_relse(bp);
                else
                        return xfs_bioerror(bp);
        }

        xfs_buf_iorequest(bp);
        return 0;
}

/*
 * Wrapper around bdstrat so that we can stop data from going to disk in case
 * we are shutting down the filesystem.  Typically user data goes thru this
 * path; one of the exceptions is the superblock.
 */
void
xfsbdstrat(
        struct xfs_mount        *mp,
        struct xfs_buf          *bp)
{
        if (XFS_FORCED_SHUTDOWN(mp)) {
                trace_xfs_bdstrat_shut(bp, _RET_IP_);
                xfs_bioerror_relse(bp);
                return;
        }

        xfs_buf_iorequest(bp);
}

STATIC void
_xfs_buf_ioend(
        xfs_buf_t               *bp,
        int                     schedule)
{
        if (atomic_dec_and_test(&bp->b_io_remaining) == 1)
                xfs_buf_ioend(bp, schedule);
}

STATIC void
xfs_buf_bio_end_io(
        struct bio              *bio,
        int                     error)
{
        xfs_buf_t               *bp = (xfs_buf_t *)bio->bi_private;

        xfs_buf_ioerror(bp, -error);

        if (!error && xfs_buf_is_vmapped(bp) && (bp->b_flags & XBF_READ))
                invalidate_kernel_vmap_range(bp->b_addr, xfs_buf_vmap_len(bp));

        _xfs_buf_ioend(bp, 1);
        bio_put(bio);
}

STATIC void
_xfs_buf_ioapply(
        xfs_buf_t               *bp)
{
        int                     rw, map_i, total_nr_pages, nr_pages;
        struct bio              *bio;
        int                     offset = bp->b_offset;
        int                     size = bp->b_count_desired;
        sector_t                sector = bp->b_bn;

        total_nr_pages = bp->b_page_count;
        map_i = 0;

        if (bp->b_flags & XBF_ORDERED) {
                ASSERT(!(bp->b_flags & XBF_READ));
                rw = WRITE_FLUSH_FUA;
        } else if (bp->b_flags & XBF_LOG_BUFFER) {
                ASSERT(!(bp->b_flags & XBF_READ_AHEAD));
                bp->b_flags &= ~_XBF_RUN_QUEUES;
                rw = (bp->b_flags & XBF_WRITE) ? WRITE_SYNC : READ_SYNC;
        } else if (bp->b_flags & _XBF_RUN_QUEUES) {
                ASSERT(!(bp->b_flags & XBF_READ_AHEAD));
                bp->b_flags &= ~_XBF_RUN_QUEUES;
                rw = (bp->b_flags & XBF_WRITE) ? WRITE_META : READ_META;
        } else {
                rw = (bp->b_flags & XBF_WRITE) ? WRITE :
                     (bp->b_flags & XBF_READ_AHEAD) ? READA : READ;
        }


next_chunk:
        atomic_inc(&bp->b_io_remaining);
        nr_pages = BIO_MAX_SECTORS >> (PAGE_SHIFT - BBSHIFT);
        if (nr_pages > total_nr_pages)
                nr_pages = total_nr_pages;

        bio = bio_alloc(GFP_NOIO, nr_pages);
        bio->bi_bdev = bp->b_target->bt_bdev;
        bio->bi_sector = sector;
        bio->bi_end_io = xfs_buf_bio_end_io;
        bio->bi_private = bp;


        for (; size && nr_pages; nr_pages--, map_i++) {
                int     rbytes, nbytes = PAGE_SIZE - offset;

                if (nbytes > size)
                        nbytes = size;

                rbytes = bio_add_page(bio, bp->b_pages[map_i], nbytes, offset);
                if (rbytes < nbytes)
                        break;

                offset = 0;
                sector += nbytes >> BBSHIFT;
                size -= nbytes;
                total_nr_pages--;
        }

        if (likely(bio->bi_size)) {
                if (xfs_buf_is_vmapped(bp)) {
                        flush_kernel_vmap_range(bp->b_addr,
                                                xfs_buf_vmap_len(bp));
                }
                submit_bio(rw, bio);
                if (size)
                        goto next_chunk;
        } else {
                xfs_buf_ioerror(bp, EIO);
                bio_put(bio);
        }
}

int
xfs_buf_iorequest(
        xfs_buf_t               *bp)
{
        trace_xfs_buf_iorequest(bp, _RET_IP_);

        if (bp->b_flags & XBF_DELWRI) {
                xfs_buf_delwri_queue(bp, 1);
                return 0;
        }

        if (bp->b_flags & XBF_WRITE) {
                xfs_buf_wait_unpin(bp);
        }

        xfs_buf_hold(bp);

        /* Set the count to 1 initially, this will stop an I/O
         * completion callout which happens before we have started
         * all the I/O from calling xfs_buf_ioend too early.
         */
        atomic_set(&bp->b_io_remaining, 1);
        _xfs_buf_ioapply(bp);
        _xfs_buf_ioend(bp, 0);

        xfs_buf_rele(bp);
        return 0;
}

/*
 *      Waits for I/O to complete on the buffer supplied.
 *      It returns immediately if no I/O is pending.
 *      It returns the I/O error code, if any, or 0 if there was no error.
 */
int
xfs_buf_iowait(
        xfs_buf_t               *bp)
{
        trace_xfs_buf_iowait(bp, _RET_IP_);

        if (atomic_read(&bp->b_io_remaining))
                blk_flush_plug(current);
        wait_for_completion(&bp->b_iowait);

        trace_xfs_buf_iowait_done(bp, _RET_IP_);
        return bp->b_error;
}

xfs_caddr_t
xfs_buf_offset(
        xfs_buf_t               *bp,
        size_t                  offset)
{
        struct page             *page;

        if (bp->b_flags & XBF_MAPPED)
                return XFS_BUF_PTR(bp) + offset;

        offset += bp->b_offset;
        page = bp->b_pages[offset >> PAGE_SHIFT];
        return (xfs_caddr_t)page_address(page) + (offset & (PAGE_SIZE-1));
}

/*
 *      Move data into or out of a buffer.
 */
void
xfs_buf_iomove(
        xfs_buf_t               *bp,    /* buffer to process            */
        size_t                  boff,   /* starting buffer offset       */
        size_t                  bsize,  /* length to copy               */
        void                    *data,  /* data address                 */
        xfs_buf_rw_t            mode)   /* read/write/zero flag         */
{
        size_t                  bend, cpoff, csize;
        struct page             *page;

        bend = boff + bsize;
        while (boff < bend) {
                page = bp->b_pages[xfs_buf_btoct(boff + bp->b_offset)];
                cpoff = xfs_buf_poff(boff + bp->b_offset);
                csize = min_t(size_t,
                              PAGE_SIZE-cpoff, bp->b_count_desired-boff);

                ASSERT(((csize + cpoff) <= PAGE_SIZE));

                switch (mode) {
                case XBRW_ZERO:
                        memset(page_address(page) + cpoff, 0, csize);
                        break;
                case XBRW_READ:
                        memcpy(data, page_address(page) + cpoff, csize);
                        break;
                case XBRW_WRITE:
                        memcpy(page_address(page) + cpoff, data, csize);
                }

                boff += csize;
                data += csize;
        }
}

/*
 *      Handling of buffer targets (buftargs).
 */

/*
 * Wait for any bufs with callbacks that have been submitted but have not yet
 * returned. These buffers will have an elevated hold count, so wait on those
 * while freeing all the buffers only held by the LRU.
 */
void
xfs_wait_buftarg(
        struct xfs_buftarg      *btp)
{
        struct xfs_buf          *bp;

restart:
        spin_lock(&btp->bt_lru_lock);
        while (!list_empty(&btp->bt_lru)) {
                bp = list_first_entry(&btp->bt_lru, struct xfs_buf, b_lru);
                if (atomic_read(&bp->b_hold) > 1) {
                        spin_unlock(&btp->bt_lru_lock);
                        delay(100);
                        goto restart;
                }
                /*
                 * clear the LRU reference count so the bufer doesn't get
                 * ignored in xfs_buf_rele().
                 */
                atomic_set(&bp->b_lru_ref, 0);
                spin_unlock(&btp->bt_lru_lock);
                xfs_buf_rele(bp);
                spin_lock(&btp->bt_lru_lock);
        }
        spin_unlock(&btp->bt_lru_lock);
}

int
xfs_buftarg_shrink(
        struct shrinker         *shrink,
        int                     nr_to_scan,
        gfp_t                   mask)
{
        struct xfs_buftarg      *btp = container_of(shrink,
                                        struct xfs_buftarg, bt_shrinker);
        struct xfs_buf          *bp;
        LIST_HEAD(dispose);

        if (!nr_to_scan)
                return btp->bt_lru_nr;

        spin_lock(&btp->bt_lru_lock);
        while (!list_empty(&btp->bt_lru)) {
                if (nr_to_scan-- <= 0)
                        break;

                bp = list_first_entry(&btp->bt_lru, struct xfs_buf, b_lru);

                /*
                 * Decrement the b_lru_ref count unless the value is already
                 * zero. If the value is already zero, we need to reclaim the
                 * buffer, otherwise it gets another trip through the LRU.
                 */
                if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
                        list_move_tail(&bp->b_lru, &btp->bt_lru);
                        continue;
                }

                /*
                 * remove the buffer from the LRU now to avoid needing another
                 * lock round trip inside xfs_buf_rele().
                 */
                list_move(&bp->b_lru, &dispose);
                btp->bt_lru_nr--;
        }
        spin_unlock(&btp->bt_lru_lock);

        while (!list_empty(&dispose)) {
                bp = list_first_entry(&dispose, struct xfs_buf, b_lru);
                list_del_init(&bp->b_lru);
                xfs_buf_rele(bp);
        }

        return btp->bt_lru_nr;
}

void
xfs_free_buftarg(
        struct xfs_mount        *mp,
        struct xfs_buftarg      *btp)
{
        unregister_shrinker(&btp->bt_shrinker);

        xfs_flush_buftarg(btp, 1);
        if (mp->m_flags & XFS_MOUNT_BARRIER)
                xfs_blkdev_issue_flush(btp);

        kthread_stop(btp->bt_task);
        kmem_free(btp);
}

STATIC int
xfs_setsize_buftarg_flags(
        xfs_buftarg_t           *btp,
        unsigned int            blocksize,
        unsigned int            sectorsize,
        int                     verbose)
{
        btp->bt_bsize = blocksize;
        btp->bt_sshift = ffs(sectorsize) - 1;
        btp->bt_smask = sectorsize - 1;

        if (set_blocksize(btp->bt_bdev, sectorsize)) {
                xfs_warn(btp->bt_mount,
                        "Cannot set_blocksize to %u on device %s\n",
                        sectorsize, XFS_BUFTARG_NAME(btp));
                return EINVAL;
        }

        return 0;
}

/*
 *      When allocating the initial buffer target we have not yet
 *      read in the superblock, so don't know what sized sectors
 *      are being used is at this early stage.  Play safe.
 */
STATIC int
xfs_setsize_buftarg_early(
        xfs_buftarg_t           *btp,
        struct block_device     *bdev)
{
        return xfs_setsize_buftarg_flags(btp,
                        PAGE_SIZE, bdev_logical_block_size(bdev), 0);
}

int
xfs_setsize_buftarg(
        xfs_buftarg_t           *btp,
        unsigned int            blocksize,
        unsigned int            sectorsize)
{
        return xfs_setsize_buftarg_flags(btp, blocksize, sectorsize, 1);
}

STATIC int
xfs_alloc_delwrite_queue(
        xfs_buftarg_t           *btp,
        const char              *fsname)
{
        INIT_LIST_HEAD(&btp->bt_delwrite_queue);
        spin_lock_init(&btp->bt_delwrite_lock);
        btp->bt_flags = 0;
        btp->bt_task = kthread_run(xfsbufd, btp, "xfsbufd/%s", fsname);
        if (IS_ERR(btp->bt_task))
                return PTR_ERR(btp->bt_task);
        return 0;
}

xfs_buftarg_t *
xfs_alloc_buftarg(
        struct xfs_mount        *mp,
        struct block_device     *bdev,
        int                     external,
        const char              *fsname)
{
        xfs_buftarg_t           *btp;

        btp = kmem_zalloc(sizeof(*btp), KM_SLEEP);

        btp->bt_mount = mp;
        btp->bt_dev =  bdev->bd_dev;
        btp->bt_bdev = bdev;
        btp->bt_bdi = blk_get_backing_dev_info(bdev);
        if (!btp->bt_bdi)
                goto error;

        INIT_LIST_HEAD(&btp->bt_lru);
        spin_lock_init(&btp->bt_lru_lock);
        if (xfs_setsize_buftarg_early(btp, bdev))
                goto error;
        if (xfs_alloc_delwrite_queue(btp, fsname))
                goto error;
        btp->bt_shrinker.shrink = xfs_buftarg_shrink;
        btp->bt_shrinker.seeks = DEFAULT_SEEKS;
        register_shrinker(&btp->bt_shrinker);
        return btp;

error:
        kmem_free(btp);
        return NULL;
}


/*
 *      Delayed write buffer handling
 */
STATIC void
xfs_buf_delwri_queue(
        xfs_buf_t               *bp,
        int                     unlock)
{
        struct list_head        *dwq = &bp->b_target->bt_delwrite_queue;
        spinlock_t              *dwlk = &bp->b_target->bt_delwrite_lock;

        trace_xfs_buf_delwri_queue(bp, _RET_IP_);

        ASSERT((bp->b_flags&(XBF_DELWRI|XBF_ASYNC)) == (XBF_DELWRI|XBF_ASYNC));

        spin_lock(dwlk);
        /* If already in the queue, dequeue and place at tail */
        if (!list_empty(&bp->b_list)) {
                ASSERT(bp->b_flags & _XBF_DELWRI_Q);
                if (unlock)
                        atomic_dec(&bp->b_hold);
                list_del(&bp->b_list);
        }

        if (list_empty(dwq)) {
                /* start xfsbufd as it is about to have something to do */
                wake_up_process(bp->b_target->bt_task);
        }

        bp->b_flags |= _XBF_DELWRI_Q;
        list_add_tail(&bp->b_list, dwq);
        bp->b_queuetime = jiffies;
        spin_unlock(dwlk);

        if (unlock)
                xfs_buf_unlock(bp);
}

void
xfs_buf_delwri_dequeue(
        xfs_buf_t               *bp)
{
        spinlock_t              *dwlk = &bp->b_target->bt_delwrite_lock;
        int                     dequeued = 0;

        spin_lock(dwlk);
        if ((bp->b_flags & XBF_DELWRI) && !list_empty(&bp->b_list)) {
                ASSERT(bp->b_flags & _XBF_DELWRI_Q);
                list_del_init(&bp->b_list);
                dequeued = 1;
        }
        bp->b_flags &= ~(XBF_DELWRI|_XBF_DELWRI_Q);
        spin_unlock(dwlk);

        if (dequeued)
                xfs_buf_rele(bp);

        trace_xfs_buf_delwri_dequeue(bp, _RET_IP_);
}

/*
 * If a delwri buffer needs to be pushed before it has aged out, then promote
 * it to the head of the delwri queue so that it will be flushed on the next
 * xfsbufd run. We do this by resetting the queuetime of the buffer to be older
 * than the age currently needed to flush the buffer. Hence the next time the
 * xfsbufd sees it is guaranteed to be considered old enough to flush.
 */
void
xfs_buf_delwri_promote(
        struct xfs_buf  *bp)
{
        struct xfs_buftarg *btp = bp->b_target;
        long            age = xfs_buf_age_centisecs * msecs_to_jiffies(10) + 1;

        ASSERT(bp->b_flags & XBF_DELWRI);
        ASSERT(bp->b_flags & _XBF_DELWRI_Q);

        /*
         * Check the buffer age before locking the delayed write queue as we
         * don't need to promote buffers that are already past the flush age.
         */
        if (bp->b_queuetime < jiffies - age)
                return;
        bp->b_queuetime = jiffies - age;
        spin_lock(&btp->bt_delwrite_lock);
        list_move(&bp->b_list, &btp->bt_delwrite_queue);
        spin_unlock(&btp->bt_delwrite_lock);
}

STATIC void
xfs_buf_runall_queues(
        struct workqueue_struct *queue)
{
        flush_workqueue(queue);
}

/*
 * Move as many buffers as specified to the supplied list
 * idicating if we skipped any buffers to prevent deadlocks.
 */
STATIC int
xfs_buf_delwri_split(
        xfs_buftarg_t   *target,
        struct list_head *list,
        unsigned long   age)
{
        xfs_buf_t       *bp, *n;
        struct list_head *dwq = &target->bt_delwrite_queue;
        spinlock_t      *dwlk = &target->bt_delwrite_lock;
        int             skipped = 0;
        int             force;

        force = test_and_clear_bit(XBT_FORCE_FLUSH, &target->bt_flags);
        INIT_LIST_HEAD(list);
        spin_lock(dwlk);
        list_for_each_entry_safe(bp, n, dwq, b_list) {
                ASSERT(bp->b_flags & XBF_DELWRI);

                if (!XFS_BUF_ISPINNED(bp) && !xfs_buf_cond_lock(bp)) {
                        if (!force &&
                            time_before(jiffies, bp->b_queuetime + age)) {
                                xfs_buf_unlock(bp);
                                break;
                        }

                        bp->b_flags &= ~(XBF_DELWRI|_XBF_DELWRI_Q|
                                         _XBF_RUN_QUEUES);
                        bp->b_flags |= XBF_WRITE;
                        list_move_tail(&bp->b_list, list);
                        trace_xfs_buf_delwri_split(bp, _RET_IP_);
                } else
                        skipped++;
        }
        spin_unlock(dwlk);

        return skipped;

}

/*
 * Compare function is more complex than it needs to be because
 * the return value is only 32 bits and we are doing comparisons
 * on 64 bit values
 */
static int
xfs_buf_cmp(
        void            *priv,
        struct list_head *a,
        struct list_head *b)
{
        struct xfs_buf  *ap = container_of(a, struct xfs_buf, b_list);
        struct xfs_buf  *bp = container_of(b, struct xfs_buf, b_list);
        xfs_daddr_t             diff;

        diff = ap->b_bn - bp->b_bn;
        if (diff < 0)
                return -1;
        if (diff > 0)
                return 1;
        return 0;
}

void
xfs_buf_delwri_sort(
        xfs_buftarg_t   *target,
        struct list_head *list)
{
        list_sort(NULL, list, xfs_buf_cmp);
}

STATIC int
xfsbufd(
        void            *data)
{
        xfs_buftarg_t   *target = (xfs_buftarg_t *)data;

        current->flags |= PF_MEMALLOC;

        set_freezable();

        do {
                long    age = xfs_buf_age_centisecs * msecs_to_jiffies(10);
                long    tout = xfs_buf_timer_centisecs * msecs_to_jiffies(10);
                int     count = 0;
                struct list_head tmp;

                if (unlikely(freezing(current))) {
                        set_bit(XBT_FORCE_SLEEP, &target->bt_flags);
                        refrigerator();
                } else {
                        clear_bit(XBT_FORCE_SLEEP, &target->bt_flags);
                }

                /* sleep for a long time if there is nothing to do. */
                if (list_empty(&target->bt_delwrite_queue))
                        tout = MAX_SCHEDULE_TIMEOUT;
                schedule_timeout_interruptible(tout);

                xfs_buf_delwri_split(target, &tmp, age);
                list_sort(NULL, &tmp, xfs_buf_cmp);
                while (!list_empty(&tmp)) {
                        struct xfs_buf *bp;
                        bp = list_first_entry(&tmp, struct xfs_buf, b_list);
                        list_del_init(&bp->b_list);
                        xfs_bdstrat_cb(bp);
                        count++;
                }
                if (count)
                        blk_flush_plug(current);

        } while (!kthread_should_stop());

        return 0;
}

/*
 *      Go through all incore buffers, and release buffers if they belong to
 *      the given device. This is used in filesystem error handling to
 *      preserve the consistency of its metadata.
 */
int
xfs_flush_buftarg(
        xfs_buftarg_t   *target,
        int             wait)
{
        xfs_buf_t       *bp;
        int             pincount = 0;
        LIST_HEAD(tmp_list);
        LIST_HEAD(wait_list);

        xfs_buf_runall_queues(xfsconvertd_workqueue);
        xfs_buf_runall_queues(xfsdatad_workqueue);
        xfs_buf_runall_queues(xfslogd_workqueue);

        set_bit(XBT_FORCE_FLUSH, &target->bt_flags);
        pincount = xfs_buf_delwri_split(target, &tmp_list, 0);

        /*
         * Dropped the delayed write list lock, now walk the temporary list.
         * All I/O is issued async and then if we need to wait for completion
         * we do that after issuing all the IO.
         */
        list_sort(NULL, &tmp_list, xfs_buf_cmp);
        while (!list_empty(&tmp_list)) {
                bp = list_first_entry(&tmp_list, struct xfs_buf, b_list);
                ASSERT(target == bp->b_target);
                list_del_init(&bp->b_list);
                if (wait) {
                        bp->b_flags &= ~XBF_ASYNC;
                        list_add(&bp->b_list, &wait_list);
                }
                xfs_bdstrat_cb(bp);
        }

        if (wait) {
                /* Expedite and wait for IO to complete. */
                blk_flush_plug(current);
                while (!list_empty(&wait_list)) {
                        bp = list_first_entry(&wait_list, struct xfs_buf, b_list);

                        list_del_init(&bp->b_list);
                        xfs_buf_iowait(bp);
                        xfs_buf_relse(bp);
                }
        }

        return pincount;
}

int __init
xfs_buf_init(void)
{
        xfs_buf_zone = kmem_zone_init_flags(sizeof(xfs_buf_t), "xfs_buf",
                                                KM_ZONE_HWALIGN, NULL);
        if (!xfs_buf_zone)
                goto out;

        xfslogd_workqueue = alloc_workqueue("xfslogd",
                                        WQ_MEM_RECLAIM | WQ_HIGHPRI, 1);
        if (!xfslogd_workqueue)
                goto out_free_buf_zone;

        xfsdatad_workqueue = alloc_workqueue("xfsdatad", WQ_MEM_RECLAIM, 1);
        if (!xfsdatad_workqueue)
                goto out_destroy_xfslogd_workqueue;

        xfsconvertd_workqueue = alloc_workqueue("xfsconvertd",
                                                WQ_MEM_RECLAIM, 1);
        if (!xfsconvertd_workqueue)
                goto out_destroy_xfsdatad_workqueue;

        return 0;

 out_destroy_xfsdatad_workqueue:
        destroy_workqueue(xfsdatad_workqueue);
 out_destroy_xfslogd_workqueue:
        destroy_workqueue(xfslogd_workqueue);
 out_free_buf_zone:
        kmem_zone_destroy(xfs_buf_zone);
 out:
        return -ENOMEM;
}

void
xfs_buf_terminate(void)
{
        destroy_workqueue(xfsconvertd_workqueue);
        destroy_workqueue(xfsdatad_workqueue);
        destroy_workqueue(xfslogd_workqueue);
        kmem_zone_destroy(xfs_buf_zone);
}

#ifdef CONFIG_KDB_MODULES
struct list_head *
xfs_get_buftarg_list(void)
{
        return &xfs_buftarg_list;
}
#endif