Merge branch 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4

[pandora-kernel.git] / fs / xfs / xfs_log_cil.c

/*
 * Copyright (c) 2010 Red Hat, 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 "xfs_fs.h"
#include "xfs_types.h"
#include "xfs_bit.h"
#include "xfs_log.h"
#include "xfs_inum.h"
#include "xfs_trans.h"
#include "xfs_trans_priv.h"
#include "xfs_log_priv.h"
#include "xfs_sb.h"
#include "xfs_ag.h"
#include "xfs_mount.h"
#include "xfs_error.h"
#include "xfs_alloc.h"

/*
 * Perform initial CIL structure initialisation. If the CIL is not
 * enabled in this filesystem, ensure the log->l_cilp is null so
 * we can check this conditional to determine if we are doing delayed
 * logging or not.
 */
int
xlog_cil_init(
        struct log      *log)
{
        struct xfs_cil  *cil;
        struct xfs_cil_ctx *ctx;

        log->l_cilp = NULL;
        if (!(log->l_mp->m_flags & XFS_MOUNT_DELAYLOG))
                return 0;

        cil = kmem_zalloc(sizeof(*cil), KM_SLEEP|KM_MAYFAIL);
        if (!cil)
                return ENOMEM;

        ctx = kmem_zalloc(sizeof(*ctx), KM_SLEEP|KM_MAYFAIL);
        if (!ctx) {
                kmem_free(cil);
                return ENOMEM;
        }

        INIT_LIST_HEAD(&cil->xc_cil);
        INIT_LIST_HEAD(&cil->xc_committing);
        spin_lock_init(&cil->xc_cil_lock);
        init_rwsem(&cil->xc_ctx_lock);
        init_waitqueue_head(&cil->xc_commit_wait);

        INIT_LIST_HEAD(&ctx->committing);
        INIT_LIST_HEAD(&ctx->busy_extents);
        ctx->sequence = 1;
        ctx->cil = cil;
        cil->xc_ctx = ctx;
        cil->xc_current_sequence = ctx->sequence;

        cil->xc_log = log;
        log->l_cilp = cil;
        return 0;
}

void
xlog_cil_destroy(
        struct log      *log)
{
        if (!log->l_cilp)
                return;

        if (log->l_cilp->xc_ctx) {
                if (log->l_cilp->xc_ctx->ticket)
                        xfs_log_ticket_put(log->l_cilp->xc_ctx->ticket);
                kmem_free(log->l_cilp->xc_ctx);
        }

        ASSERT(list_empty(&log->l_cilp->xc_cil));
        kmem_free(log->l_cilp);
}

/*
 * Allocate a new ticket. Failing to get a new ticket makes it really hard to
 * recover, so we don't allow failure here. Also, we allocate in a context that
 * we don't want to be issuing transactions from, so we need to tell the
 * allocation code this as well.
 *
 * We don't reserve any space for the ticket - we are going to steal whatever
 * space we require from transactions as they commit. To ensure we reserve all
 * the space required, we need to set the current reservation of the ticket to
 * zero so that we know to steal the initial transaction overhead from the
 * first transaction commit.
 */
static struct xlog_ticket *
xlog_cil_ticket_alloc(
        struct log      *log)
{
        struct xlog_ticket *tic;

        tic = xlog_ticket_alloc(log, 0, 1, XFS_TRANSACTION, 0,
                                KM_SLEEP|KM_NOFS);
        tic->t_trans_type = XFS_TRANS_CHECKPOINT;

        /*
         * set the current reservation to zero so we know to steal the basic
         * transaction overhead reservation from the first transaction commit.
         */
        tic->t_curr_res = 0;
        return tic;
}

/*
 * After the first stage of log recovery is done, we know where the head and
 * tail of the log are. We need this log initialisation done before we can
 * initialise the first CIL checkpoint context.
 *
 * Here we allocate a log ticket to track space usage during a CIL push.  This
 * ticket is passed to xlog_write() directly so that we don't slowly leak log
 * space by failing to account for space used by log headers and additional
 * region headers for split regions.
 */
void
xlog_cil_init_post_recovery(
        struct log      *log)
{
        if (!log->l_cilp)
                return;

        log->l_cilp->xc_ctx->ticket = xlog_cil_ticket_alloc(log);
        log->l_cilp->xc_ctx->sequence = 1;
        log->l_cilp->xc_ctx->commit_lsn = xlog_assign_lsn(log->l_curr_cycle,
                                                                log->l_curr_block);
}

/*
 * Format log item into a flat buffers
 *
 * For delayed logging, we need to hold a formatted buffer containing all the
 * changes on the log item. This enables us to relog the item in memory and
 * write it out asynchronously without needing to relock the object that was
 * modified at the time it gets written into the iclog.
 *
 * This function builds a vector for the changes in each log item in the
 * transaction. It then works out the length of the buffer needed for each log
 * item, allocates them and formats the vector for the item into the buffer.
 * The buffer is then attached to the log item are then inserted into the
 * Committed Item List for tracking until the next checkpoint is written out.
 *
 * We don't set up region headers during this process; we simply copy the
 * regions into the flat buffer. We can do this because we still have to do a
 * formatting step to write the regions into the iclog buffer.  Writing the
 * ophdrs during the iclog write means that we can support splitting large
 * regions across iclog boundares without needing a change in the format of the
 * item/region encapsulation.
 *
 * Hence what we need to do now is change the rewrite the vector array to point
 * to the copied region inside the buffer we just allocated. This allows us to
 * format the regions into the iclog as though they are being formatted
 * directly out of the objects themselves.
 */
static void
xlog_cil_format_items(
        struct log              *log,
        struct xfs_log_vec      *log_vector)
{
        struct xfs_log_vec *lv;

        ASSERT(log_vector);
        for (lv = log_vector; lv; lv = lv->lv_next) {
                void    *ptr;
                int     index;
                int     len = 0;

                /* build the vector array and calculate it's length */
                IOP_FORMAT(lv->lv_item, lv->lv_iovecp);
                for (index = 0; index < lv->lv_niovecs; index++)
                        len += lv->lv_iovecp[index].i_len;

                lv->lv_buf_len = len;
                lv->lv_buf = kmem_alloc(lv->lv_buf_len, KM_SLEEP|KM_NOFS);
                ptr = lv->lv_buf;

                for (index = 0; index < lv->lv_niovecs; index++) {
                        struct xfs_log_iovec *vec = &lv->lv_iovecp[index];

                        memcpy(ptr, vec->i_addr, vec->i_len);
                        vec->i_addr = ptr;
                        ptr += vec->i_len;
                }
                ASSERT(ptr == lv->lv_buf + lv->lv_buf_len);
        }
}

/*
 * Prepare the log item for insertion into the CIL. Calculate the difference in
 * log space and vectors it will consume, and if it is a new item pin it as
 * well.
 */
STATIC void
xfs_cil_prepare_item(
        struct log              *log,
        struct xfs_log_vec      *lv,
        int                     *len,
        int                     *diff_iovecs)
{
        struct xfs_log_vec      *old = lv->lv_item->li_lv;

        if (old) {
                /* existing lv on log item, space used is a delta */
                ASSERT(!list_empty(&lv->lv_item->li_cil));
                ASSERT(old->lv_buf && old->lv_buf_len && old->lv_niovecs);

                *len += lv->lv_buf_len - old->lv_buf_len;
                *diff_iovecs += lv->lv_niovecs - old->lv_niovecs;
                kmem_free(old->lv_buf);
                kmem_free(old);
        } else {
                /* new lv, must pin the log item */
                ASSERT(!lv->lv_item->li_lv);
                ASSERT(list_empty(&lv->lv_item->li_cil));

                *len += lv->lv_buf_len;
                *diff_iovecs += lv->lv_niovecs;
                IOP_PIN(lv->lv_item);

        }

        /* attach new log vector to log item */
        lv->lv_item->li_lv = lv;

        /*
         * If this is the first time the item is being committed to the
         * CIL, store the sequence number on the log item so we can
         * tell in future commits whether this is the first checkpoint
         * the item is being committed into.
         */
        if (!lv->lv_item->li_seq)
                lv->lv_item->li_seq = log->l_cilp->xc_ctx->sequence;
}

/*
 * Insert the log items into the CIL and calculate the difference in space
 * consumed by the item. Add the space to the checkpoint ticket and calculate
 * if the change requires additional log metadata. If it does, take that space
 * as well. Remove the amount of space we addded to the checkpoint ticket from
 * the current transaction ticket so that the accounting works out correctly.
 */
static void
xlog_cil_insert_items(
        struct log              *log,
        struct xfs_log_vec      *log_vector,
        struct xlog_ticket      *ticket)
{
        struct xfs_cil          *cil = log->l_cilp;
        struct xfs_cil_ctx      *ctx = cil->xc_ctx;
        struct xfs_log_vec      *lv;
        int                     len = 0;
        int                     diff_iovecs = 0;
        int                     iclog_space;

        ASSERT(log_vector);

        /*
         * Do all the accounting aggregation and switching of log vectors
         * around in a separate loop to the insertion of items into the CIL.
         * Then we can do a separate loop to update the CIL within a single
         * lock/unlock pair. This reduces the number of round trips on the CIL
         * lock from O(nr_logvectors) to O(1) and greatly reduces the overall
         * hold time for the transaction commit.
         *
         * If this is the first time the item is being placed into the CIL in
         * this context, pin it so it can't be written to disk until the CIL is
         * flushed to the iclog and the iclog written to disk.
         *
         * We can do this safely because the context can't checkpoint until we
         * are done so it doesn't matter exactly how we update the CIL.
         */
        for (lv = log_vector; lv; lv = lv->lv_next)
                xfs_cil_prepare_item(log, lv, &len, &diff_iovecs);

        /* account for space used by new iovec headers  */
        len += diff_iovecs * sizeof(xlog_op_header_t);

        spin_lock(&cil->xc_cil_lock);

        /* move the items to the tail of the CIL */
        for (lv = log_vector; lv; lv = lv->lv_next)
                list_move_tail(&lv->lv_item->li_cil, &cil->xc_cil);

        ctx->nvecs += diff_iovecs;

        /*
         * Now transfer enough transaction reservation to the context ticket
         * for the checkpoint. The context ticket is special - the unit
         * reservation has to grow as well as the current reservation as we
         * steal from tickets so we can correctly determine the space used
         * during the transaction commit.
         */
        if (ctx->ticket->t_curr_res == 0) {
                /* first commit in checkpoint, steal the header reservation */
                ASSERT(ticket->t_curr_res >= ctx->ticket->t_unit_res + len);
                ctx->ticket->t_curr_res = ctx->ticket->t_unit_res;
                ticket->t_curr_res -= ctx->ticket->t_unit_res;
        }

        /* do we need space for more log record headers? */
        iclog_space = log->l_iclog_size - log->l_iclog_hsize;
        if (len > 0 && (ctx->space_used / iclog_space !=
                                (ctx->space_used + len) / iclog_space)) {
                int hdrs;

                hdrs = (len + iclog_space - 1) / iclog_space;
                /* need to take into account split region headers, too */
                hdrs *= log->l_iclog_hsize + sizeof(struct xlog_op_header);
                ctx->ticket->t_unit_res += hdrs;
                ctx->ticket->t_curr_res += hdrs;
                ticket->t_curr_res -= hdrs;
                ASSERT(ticket->t_curr_res >= len);
        }
        ticket->t_curr_res -= len;
        ctx->space_used += len;

        spin_unlock(&cil->xc_cil_lock);
}

static void
xlog_cil_free_logvec(
        struct xfs_log_vec      *log_vector)
{
        struct xfs_log_vec      *lv;

        for (lv = log_vector; lv; ) {
                struct xfs_log_vec *next = lv->lv_next;
                kmem_free(lv->lv_buf);
                kmem_free(lv);
                lv = next;
        }
}

/*
 * Mark all items committed and clear busy extents. We free the log vector
 * chains in a separate pass so that we unpin the log items as quickly as
 * possible.
 */
static void
xlog_cil_committed(
        void    *args,
        int     abort)
{
        struct xfs_cil_ctx      *ctx = args;

        xfs_trans_committed_bulk(ctx->cil->xc_log->l_ailp, ctx->lv_chain,
                                        ctx->start_lsn, abort);

        xfs_alloc_busy_sort(&ctx->busy_extents);
        xfs_alloc_busy_clear(ctx->cil->xc_log->l_mp, &ctx->busy_extents);

        spin_lock(&ctx->cil->xc_cil_lock);
        list_del(&ctx->committing);
        spin_unlock(&ctx->cil->xc_cil_lock);

        xlog_cil_free_logvec(ctx->lv_chain);
        kmem_free(ctx);
}

/*
 * Push the Committed Item List to the log. If @push_seq flag is zero, then it
 * is a background flush and so we can chose to ignore it. Otherwise, if the
 * current sequence is the same as @push_seq we need to do a flush. If
 * @push_seq is less than the current sequence, then it has already been
 * flushed and we don't need to do anything - the caller will wait for it to
 * complete if necessary.
 *
 * @push_seq is a value rather than a flag because that allows us to do an
 * unlocked check of the sequence number for a match. Hence we can allows log
 * forces to run racily and not issue pushes for the same sequence twice. If we
 * get a race between multiple pushes for the same sequence they will block on
 * the first one and then abort, hence avoiding needless pushes.
 */
STATIC int
xlog_cil_push(
        struct log              *log,
        xfs_lsn_t               push_seq)
{
        struct xfs_cil          *cil = log->l_cilp;
        struct xfs_log_vec      *lv;
        struct xfs_cil_ctx      *ctx;
        struct xfs_cil_ctx      *new_ctx;
        struct xlog_in_core     *commit_iclog;
        struct xlog_ticket      *tic;
        int                     num_lv;
        int                     num_iovecs;
        int                     len;
        int                     error = 0;
        struct xfs_trans_header thdr;
        struct xfs_log_iovec    lhdr;
        struct xfs_log_vec      lvhdr = { NULL };
        xfs_lsn_t               commit_lsn;

        if (!cil)
                return 0;

        ASSERT(!push_seq || push_seq <= cil->xc_ctx->sequence);

        new_ctx = kmem_zalloc(sizeof(*new_ctx), KM_SLEEP|KM_NOFS);
        new_ctx->ticket = xlog_cil_ticket_alloc(log);

        /*
         * Lock out transaction commit, but don't block for background pushes
         * unless we are well over the CIL space limit. See the definition of
         * XLOG_CIL_HARD_SPACE_LIMIT() for the full explanation of the logic
         * used here.
         */
        if (!down_write_trylock(&cil->xc_ctx_lock)) {
                if (!push_seq &&
                    cil->xc_ctx->space_used < XLOG_CIL_HARD_SPACE_LIMIT(log))
                        goto out_free_ticket;
                down_write(&cil->xc_ctx_lock);
        }
        ctx = cil->xc_ctx;

        /* check if we've anything to push */
        if (list_empty(&cil->xc_cil))
                goto out_skip;

        /* check for spurious background flush */
        if (!push_seq && cil->xc_ctx->space_used < XLOG_CIL_SPACE_LIMIT(log))
                goto out_skip;

        /* check for a previously pushed seqeunce */
        if (push_seq && push_seq < cil->xc_ctx->sequence)
                goto out_skip;

        /*
         * pull all the log vectors off the items in the CIL, and
         * remove the items from the CIL. We don't need the CIL lock
         * here because it's only needed on the transaction commit
         * side which is currently locked out by the flush lock.
         */
        lv = NULL;
        num_lv = 0;
        num_iovecs = 0;
        len = 0;
        while (!list_empty(&cil->xc_cil)) {
                struct xfs_log_item     *item;
                int                     i;

                item = list_first_entry(&cil->xc_cil,
                                        struct xfs_log_item, li_cil);
                list_del_init(&item->li_cil);
                if (!ctx->lv_chain)
                        ctx->lv_chain = item->li_lv;
                else
                        lv->lv_next = item->li_lv;
                lv = item->li_lv;
                item->li_lv = NULL;

                num_lv++;
                num_iovecs += lv->lv_niovecs;
                for (i = 0; i < lv->lv_niovecs; i++)
                        len += lv->lv_iovecp[i].i_len;
        }

        /*
         * initialise the new context and attach it to the CIL. Then attach
         * the current context to the CIL committing lsit so it can be found
         * during log forces to extract the commit lsn of the sequence that
         * needs to be forced.
         */
        INIT_LIST_HEAD(&new_ctx->committing);
        INIT_LIST_HEAD(&new_ctx->busy_extents);
        new_ctx->sequence = ctx->sequence + 1;
        new_ctx->cil = cil;
        cil->xc_ctx = new_ctx;

        /*
         * mirror the new sequence into the cil structure so that we can do
         * unlocked checks against the current sequence in log forces without
         * risking deferencing a freed context pointer.
         */
        cil->xc_current_sequence = new_ctx->sequence;

        /*
         * The switch is now done, so we can drop the context lock and move out
         * of a shared context. We can't just go straight to the commit record,
         * though - we need to synchronise with previous and future commits so
         * that the commit records are correctly ordered in the log to ensure
         * that we process items during log IO completion in the correct order.
         *
         * For example, if we get an EFI in one checkpoint and the EFD in the
         * next (e.g. due to log forces), we do not want the checkpoint with
         * the EFD to be committed before the checkpoint with the EFI.  Hence
         * we must strictly order the commit records of the checkpoints so
         * that: a) the checkpoint callbacks are attached to the iclogs in the
         * correct order; and b) the checkpoints are replayed in correct order
         * in log recovery.
         *
         * Hence we need to add this context to the committing context list so
         * that higher sequences will wait for us to write out a commit record
         * before they do.
         */
        spin_lock(&cil->xc_cil_lock);
        list_add(&ctx->committing, &cil->xc_committing);
        spin_unlock(&cil->xc_cil_lock);
        up_write(&cil->xc_ctx_lock);

        /*
         * Build a checkpoint transaction header and write it to the log to
         * begin the transaction. We need to account for the space used by the
         * transaction header here as it is not accounted for in xlog_write().
         *
         * The LSN we need to pass to the log items on transaction commit is
         * the LSN reported by the first log vector write. If we use the commit
         * record lsn then we can move the tail beyond the grant write head.
         */
        tic = ctx->ticket;
        thdr.th_magic = XFS_TRANS_HEADER_MAGIC;
        thdr.th_type = XFS_TRANS_CHECKPOINT;
        thdr.th_tid = tic->t_tid;
        thdr.th_num_items = num_iovecs;
        lhdr.i_addr = &thdr;
        lhdr.i_len = sizeof(xfs_trans_header_t);
        lhdr.i_type = XLOG_REG_TYPE_TRANSHDR;
        tic->t_curr_res -= lhdr.i_len + sizeof(xlog_op_header_t);

        lvhdr.lv_niovecs = 1;
        lvhdr.lv_iovecp = &lhdr;
        lvhdr.lv_next = ctx->lv_chain;

        error = xlog_write(log, &lvhdr, tic, &ctx->start_lsn, NULL, 0);
        if (error)
                goto out_abort_free_ticket;

        /*
         * now that we've written the checkpoint into the log, strictly
         * order the commit records so replay will get them in the right order.
         */
restart:
        spin_lock(&cil->xc_cil_lock);
        list_for_each_entry(new_ctx, &cil->xc_committing, committing) {
                /*
                 * Higher sequences will wait for this one so skip them.
                 * Don't wait for own own sequence, either.
                 */
                if (new_ctx->sequence >= ctx->sequence)
                        continue;
                if (!new_ctx->commit_lsn) {
                        /*
                         * It is still being pushed! Wait for the push to
                         * complete, then start again from the beginning.
                         */
                        xlog_wait(&cil->xc_commit_wait, &cil->xc_cil_lock);
                        goto restart;
                }
        }
        spin_unlock(&cil->xc_cil_lock);

        /* xfs_log_done always frees the ticket on error. */
        commit_lsn = xfs_log_done(log->l_mp, tic, &commit_iclog, 0);
        if (commit_lsn == -1)
                goto out_abort;

        /* attach all the transactions w/ busy extents to iclog */
        ctx->log_cb.cb_func = xlog_cil_committed;
        ctx->log_cb.cb_arg = ctx;
        error = xfs_log_notify(log->l_mp, commit_iclog, &ctx->log_cb);
        if (error)
                goto out_abort;

        /*
         * now the checkpoint commit is complete and we've attached the
         * callbacks to the iclog we can assign the commit LSN to the context
         * and wake up anyone who is waiting for the commit to complete.
         */
        spin_lock(&cil->xc_cil_lock);
        ctx->commit_lsn = commit_lsn;
        wake_up_all(&cil->xc_commit_wait);
        spin_unlock(&cil->xc_cil_lock);

        /* release the hounds! */
        return xfs_log_release_iclog(log->l_mp, commit_iclog);

out_skip:
        up_write(&cil->xc_ctx_lock);
out_free_ticket:
        xfs_log_ticket_put(new_ctx->ticket);
        kmem_free(new_ctx);
        return 0;

out_abort_free_ticket:
        xfs_log_ticket_put(tic);
out_abort:
        xlog_cil_committed(ctx, XFS_LI_ABORTED);
        return XFS_ERROR(EIO);
}

/*
 * Commit a transaction with the given vector to the Committed Item List.
 *
 * To do this, we need to format the item, pin it in memory if required and
 * account for the space used by the transaction. Once we have done that we
 * need to release the unused reservation for the transaction, attach the
 * transaction to the checkpoint context so we carry the busy extents through
 * to checkpoint completion, and then unlock all the items in the transaction.
 *
 * For more specific information about the order of operations in
 * xfs_log_commit_cil() please refer to the comments in
 * xfs_trans_commit_iclog().
 *
 * Called with the context lock already held in read mode to lock out
 * background commit, returns without it held once background commits are
 * allowed again.
 */
void
xfs_log_commit_cil(
        struct xfs_mount        *mp,
        struct xfs_trans        *tp,
        struct xfs_log_vec      *log_vector,
        xfs_lsn_t               *commit_lsn,
        int                     flags)
{
        struct log              *log = mp->m_log;
        int                     log_flags = 0;
        int                     push = 0;

        if (flags & XFS_TRANS_RELEASE_LOG_RES)
                log_flags = XFS_LOG_REL_PERM_RESERV;

        /*
         * do all the hard work of formatting items (including memory
         * allocation) outside the CIL context lock. This prevents stalling CIL
         * pushes when we are low on memory and a transaction commit spends a
         * lot of time in memory reclaim.
         */
        xlog_cil_format_items(log, log_vector);

        /* lock out background commit */
        down_read(&log->l_cilp->xc_ctx_lock);
        if (commit_lsn)
                *commit_lsn = log->l_cilp->xc_ctx->sequence;

        xlog_cil_insert_items(log, log_vector, tp->t_ticket);

        /* check we didn't blow the reservation */
        if (tp->t_ticket->t_curr_res < 0)
                xlog_print_tic_res(log->l_mp, tp->t_ticket);

        /* attach the transaction to the CIL if it has any busy extents */
        if (!list_empty(&tp->t_busy)) {
                spin_lock(&log->l_cilp->xc_cil_lock);
                list_splice_init(&tp->t_busy,
                                        &log->l_cilp->xc_ctx->busy_extents);
                spin_unlock(&log->l_cilp->xc_cil_lock);
        }

        tp->t_commit_lsn = *commit_lsn;
        xfs_log_done(mp, tp->t_ticket, NULL, log_flags);
        xfs_trans_unreserve_and_mod_sb(tp);

        /*
         * Once all the items of the transaction have been copied to the CIL,
         * the items can be unlocked and freed.
         *
         * This needs to be done before we drop the CIL context lock because we
         * have to update state in the log items and unlock them before they go
         * to disk. If we don't, then the CIL checkpoint can race with us and
         * we can run checkpoint completion before we've updated and unlocked
         * the log items. This affects (at least) processing of stale buffers,
         * inodes and EFIs.
         */
        xfs_trans_free_items(tp, *commit_lsn, 0);

        /* check for background commit before unlock */
        if (log->l_cilp->xc_ctx->space_used > XLOG_CIL_SPACE_LIMIT(log))
                push = 1;

        up_read(&log->l_cilp->xc_ctx_lock);

        /*
         * We need to push CIL every so often so we don't cache more than we
         * can fit in the log. The limit really is that a checkpoint can't be
         * more than half the log (the current checkpoint is not allowed to
         * overwrite the previous checkpoint), but commit latency and memory
         * usage limit this to a smaller size in most cases.
         */
        if (push)
                xlog_cil_push(log, 0);
}

/*
 * Conditionally push the CIL based on the sequence passed in.
 *
 * We only need to push if we haven't already pushed the sequence
 * number given. Hence the only time we will trigger a push here is
 * if the push sequence is the same as the current context.
 *
 * We return the current commit lsn to allow the callers to determine if a
 * iclog flush is necessary following this call.
 *
 * XXX: Initially, just push the CIL unconditionally and return whatever
 * commit lsn is there. It'll be empty, so this is broken for now.
 */
xfs_lsn_t
xlog_cil_force_lsn(
        struct log      *log,
        xfs_lsn_t       sequence)
{
        struct xfs_cil          *cil = log->l_cilp;
        struct xfs_cil_ctx      *ctx;
        xfs_lsn_t               commit_lsn = NULLCOMMITLSN;

        ASSERT(sequence <= cil->xc_current_sequence);

        /*
         * check to see if we need to force out the current context.
         * xlog_cil_push() handles racing pushes for the same sequence,
         * so no need to deal with it here.
         */
        if (sequence == cil->xc_current_sequence)
                xlog_cil_push(log, sequence);

        /*
         * See if we can find a previous sequence still committing.
         * We need to wait for all previous sequence commits to complete
         * before allowing the force of push_seq to go ahead. Hence block
         * on commits for those as well.
         */
restart:
        spin_lock(&cil->xc_cil_lock);
        list_for_each_entry(ctx, &cil->xc_committing, committing) {
                if (ctx->sequence > sequence)
                        continue;
                if (!ctx->commit_lsn) {
                        /*
                         * It is still being pushed! Wait for the push to
                         * complete, then start again from the beginning.
                         */
                        xlog_wait(&cil->xc_commit_wait, &cil->xc_cil_lock);
                        goto restart;
                }
                if (ctx->sequence != sequence)
                        continue;
                /* found it! */
                commit_lsn = ctx->commit_lsn;
        }
        spin_unlock(&cil->xc_cil_lock);
        return commit_lsn;
}

/*
 * Check if the current log item was first committed in this sequence.
 * We can't rely on just the log item being in the CIL, we have to check
 * the recorded commit sequence number.
 *
 * Note: for this to be used in a non-racy manner, it has to be called with
 * CIL flushing locked out. As a result, it should only be used during the
 * transaction commit process when deciding what to format into the item.
 */
bool
xfs_log_item_in_current_chkpt(
        struct xfs_log_item *lip)
{
        struct xfs_cil_ctx *ctx;

        if (!(lip->li_mountp->m_flags & XFS_MOUNT_DELAYLOG))
                return false;
        if (list_empty(&lip->li_cil))
                return false;

        ctx = lip->li_mountp->m_log->l_cilp->xc_ctx;

        /*
         * li_seq is written on the first commit of a log item to record the
         * first checkpoint it is written to. Hence if it is different to the
         * current sequence, we're in a new checkpoint.
         */
        if (XFS_LSN_CMP(lip->li_seq, ctx->sequence) != 0)
                return false;
        return true;
}