Linux 4.19-rc7

[linux-2.6/btrfs-unstable.git] / fs / xfs / xfs_log_cil.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (c) 2010 Red Hat, Inc. All Rights Reserved.
 */

#include "xfs.h"
#include "xfs_fs.h"
#include "xfs_format.h"
#include "xfs_log_format.h"
#include "xfs_shared.h"
#include "xfs_trans_resv.h"
#include "xfs_mount.h"
#include "xfs_error.h"
#include "xfs_alloc.h"
#include "xfs_extent_busy.h"
#include "xfs_discard.h"
#include "xfs_trans.h"
#include "xfs_trans_priv.h"
#include "xfs_log.h"
#include "xfs_log_priv.h"
#include "xfs_trace.h"

struct workqueue_struct *xfs_discard_wq;

/*
 * 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 xlog     *log)
{
        struct xlog_ticket *tic;

        tic = xlog_ticket_alloc(log, 0, 1, XFS_TRANSACTION, 0,
                                KM_SLEEP|KM_NOFS);

        /*
         * 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 xlog     *log)
{
        log->l_cilp->xc_ctx->ticket = xlog_cil_ticket_alloc(log);
        log->l_cilp->xc_ctx->sequence = 1;
}

static inline int
xlog_cil_iovec_space(
        uint    niovecs)
{
        return round_up((sizeof(struct xfs_log_vec) +
                                        niovecs * sizeof(struct xfs_log_iovec)),
                        sizeof(uint64_t));
}

/*
 * Allocate or pin log vector buffers for CIL insertion.
 *
 * The CIL currently uses disposable buffers for copying a snapshot of the
 * modified items into the log during a push. The biggest problem with this is
 * the requirement to allocate the disposable buffer during the commit if:
 *      a) does not exist; or
 *      b) it is too small
 *
 * If we do this allocation within xlog_cil_insert_format_items(), it is done
 * under the xc_ctx_lock, which means that a CIL push cannot occur during
 * the memory allocation. This means that we have a potential deadlock situation
 * under low memory conditions when we have lots of dirty metadata pinned in
 * the CIL and we need a CIL commit to occur to free memory.
 *
 * To avoid this, we need to move the memory allocation outside the
 * xc_ctx_lock, but because the log vector buffers are disposable, that opens
 * up a TOCTOU race condition w.r.t. the CIL committing and removing the log
 * vector buffers between the check and the formatting of the item into the
 * log vector buffer within the xc_ctx_lock.
 *
 * Because the log vector buffer needs to be unchanged during the CIL push
 * process, we cannot share the buffer between the transaction commit (which
 * modifies the buffer) and the CIL push context that is writing the changes
 * into the log. This means skipping preallocation of buffer space is
 * unreliable, but we most definitely do not want to be allocating and freeing
 * buffers unnecessarily during commits when overwrites can be done safely.
 *
 * The simplest solution to this problem is to allocate a shadow buffer when a
 * log item is committed for the second time, and then to only use this buffer
 * if necessary. The buffer can remain attached to the log item until such time
 * it is needed, and this is the buffer that is reallocated to match the size of
 * the incoming modification. Then during the formatting of the item we can swap
 * the active buffer with the new one if we can't reuse the existing buffer. We
 * don't free the old buffer as it may be reused on the next modification if
 * it's size is right, otherwise we'll free and reallocate it at that point.
 *
 * 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 attaches the vector to the log item in preparation
 * for the formatting step which occurs under the xc_ctx_lock.
 *
 * While this means the memory footprint goes up, it avoids the repeated
 * alloc/free pattern that repeated modifications of an item would otherwise
 * cause, and hence minimises the CPU overhead of such behaviour.
 */
static void
xlog_cil_alloc_shadow_bufs(
        struct xlog             *log,
        struct xfs_trans        *tp)
{
        struct xfs_log_item     *lip;

        list_for_each_entry(lip, &tp->t_items, li_trans) {
                struct xfs_log_vec *lv;
                int     niovecs = 0;
                int     nbytes = 0;
                int     buf_size;
                bool    ordered = false;

                /* Skip items which aren't dirty in this transaction. */
                if (!test_bit(XFS_LI_DIRTY, &lip->li_flags))
                        continue;

                /* get number of vecs and size of data to be stored */
                lip->li_ops->iop_size(lip, &niovecs, &nbytes);

                /*
                 * Ordered items need to be tracked but we do not wish to write
                 * them. We need a logvec to track the object, but we do not
                 * need an iovec or buffer to be allocated for copying data.
                 */
                if (niovecs == XFS_LOG_VEC_ORDERED) {
                        ordered = true;
                        niovecs = 0;
                        nbytes = 0;
                }

                /*
                 * We 64-bit align the length of each iovec so that the start
                 * of the next one is naturally aligned.  We'll need to
                 * account for that slack space here. Then round nbytes up
                 * to 64-bit alignment so that the initial buffer alignment is
                 * easy to calculate and verify.
                 */
                nbytes += niovecs * sizeof(uint64_t);
                nbytes = round_up(nbytes, sizeof(uint64_t));

                /*
                 * The data buffer needs to start 64-bit aligned, so round up
                 * that space to ensure we can align it appropriately and not
                 * overrun the buffer.
                 */
                buf_size = nbytes + xlog_cil_iovec_space(niovecs);

                /*
                 * if we have no shadow buffer, or it is too small, we need to
                 * reallocate it.
                 */
                if (!lip->li_lv_shadow ||
                    buf_size > lip->li_lv_shadow->lv_size) {

                        /*
                         * We free and allocate here as a realloc would copy
                         * unecessary data. We don't use kmem_zalloc() for the
                         * same reason - we don't need to zero the data area in
                         * the buffer, only the log vector header and the iovec
                         * storage.
                         */
                        kmem_free(lip->li_lv_shadow);

                        lv = kmem_alloc_large(buf_size, KM_SLEEP | KM_NOFS);
                        memset(lv, 0, xlog_cil_iovec_space(niovecs));

                        lv->lv_item = lip;
                        lv->lv_size = buf_size;
                        if (ordered)
                                lv->lv_buf_len = XFS_LOG_VEC_ORDERED;
                        else
                                lv->lv_iovecp = (struct xfs_log_iovec *)&lv[1];
                        lip->li_lv_shadow = lv;
                } else {
                        /* same or smaller, optimise common overwrite case */
                        lv = lip->li_lv_shadow;
                        if (ordered)
                                lv->lv_buf_len = XFS_LOG_VEC_ORDERED;
                        else
                                lv->lv_buf_len = 0;
                        lv->lv_bytes = 0;
                        lv->lv_next = NULL;
                }

                /* Ensure the lv is set up according to ->iop_size */
                lv->lv_niovecs = niovecs;

                /* The allocated data region lies beyond the iovec region */
                lv->lv_buf = (char *)lv + xlog_cil_iovec_space(niovecs);
        }

}

/*
 * 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 xlog             *log,
        struct xfs_log_vec      *lv,
        struct xfs_log_vec      *old_lv,
        int                     *diff_len,
        int                     *diff_iovecs)
{
        /* Account for the new LV being passed in */
        if (lv->lv_buf_len != XFS_LOG_VEC_ORDERED) {
                *diff_len += lv->lv_bytes;
                *diff_iovecs += lv->lv_niovecs;
        }

        /*
         * If there is no old LV, this is the first time we've seen the item in
         * this CIL context and so we need to pin it. If we are replacing the
         * old_lv, then remove the space it accounts for and make it the shadow
         * buffer for later freeing. In both cases we are now switching to the
         * shadow buffer, so update the the pointer to it appropriately.
         */
        if (!old_lv) {
                lv->lv_item->li_ops->iop_pin(lv->lv_item);
                lv->lv_item->li_lv_shadow = NULL;
        } else if (old_lv != lv) {
                ASSERT(lv->lv_buf_len != XFS_LOG_VEC_ORDERED);

                *diff_len -= old_lv->lv_bytes;
                *diff_iovecs -= old_lv->lv_niovecs;
                lv->lv_item->li_lv_shadow = old_lv;
        }

        /* 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;
}

/*
 * 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 takes the prepared log vectors attached to each log item, and
 * formats the changes into the log vector buffer. The buffer it uses is
 * dependent on the current state of the vector in the CIL - the shadow lv is
 * guaranteed to be large enough for the current modification, but we will only
 * use that if we can't reuse the existing lv. If we can't reuse the existing
 * lv, then simple swap it out for the shadow lv. We don't free it - that is
 * done lazily either by th enext modification or the freeing of the log item.
 *
 * 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_insert_format_items(
        struct xlog             *log,
        struct xfs_trans        *tp,
        int                     *diff_len,
        int                     *diff_iovecs)
{
        struct xfs_log_item     *lip;


        /* Bail out if we didn't find a log item.  */
        if (list_empty(&tp->t_items)) {
                ASSERT(0);
                return;
        }

        list_for_each_entry(lip, &tp->t_items, li_trans) {
                struct xfs_log_vec *lv;
                struct xfs_log_vec *old_lv = NULL;
                struct xfs_log_vec *shadow;
                bool    ordered = false;

                /* Skip items which aren't dirty in this transaction. */
                if (!test_bit(XFS_LI_DIRTY, &lip->li_flags))
                        continue;

                /*
                 * The formatting size information is already attached to
                 * the shadow lv on the log item.
                 */
                shadow = lip->li_lv_shadow;
                if (shadow->lv_buf_len == XFS_LOG_VEC_ORDERED)
                        ordered = true;

                /* Skip items that do not have any vectors for writing */
                if (!shadow->lv_niovecs && !ordered)
                        continue;

                /* compare to existing item size */
                old_lv = lip->li_lv;
                if (lip->li_lv && shadow->lv_size <= lip->li_lv->lv_size) {
                        /* same or smaller, optimise common overwrite case */
                        lv = lip->li_lv;
                        lv->lv_next = NULL;

                        if (ordered)
                                goto insert;

                        /*
                         * set the item up as though it is a new insertion so
                         * that the space reservation accounting is correct.
                         */
                        *diff_iovecs -= lv->lv_niovecs;
                        *diff_len -= lv->lv_bytes;

                        /* Ensure the lv is set up according to ->iop_size */
                        lv->lv_niovecs = shadow->lv_niovecs;

                        /* reset the lv buffer information for new formatting */
                        lv->lv_buf_len = 0;
                        lv->lv_bytes = 0;
                        lv->lv_buf = (char *)lv +
                                        xlog_cil_iovec_space(lv->lv_niovecs);
                } else {
                        /* switch to shadow buffer! */
                        lv = shadow;
                        lv->lv_item = lip;
                        if (ordered) {
                                /* track as an ordered logvec */
                                ASSERT(lip->li_lv == NULL);
                                goto insert;
                        }
                }

                ASSERT(IS_ALIGNED((unsigned long)lv->lv_buf, sizeof(uint64_t)));
                lip->li_ops->iop_format(lip, lv);
insert:
                xfs_cil_prepare_item(log, lv, old_lv, diff_len, diff_iovecs);
        }
}

/*
 * 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 added to the checkpoint ticket from
 * the current transaction ticket so that the accounting works out correctly.
 */
static void
xlog_cil_insert_items(
        struct xlog             *log,
        struct xfs_trans        *tp)
{
        struct xfs_cil          *cil = log->l_cilp;
        struct xfs_cil_ctx      *ctx = cil->xc_ctx;
        struct xfs_log_item     *lip;
        int                     len = 0;
        int                     diff_iovecs = 0;
        int                     iclog_space;
        int                     iovhdr_res = 0, split_res = 0, ctx_res = 0;

        ASSERT(tp);

        /*
         * 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.
         */
        xlog_cil_insert_format_items(log, tp, &len, &diff_iovecs);

        spin_lock(&cil->xc_cil_lock);

        /* account for space used by new iovec headers  */
        iovhdr_res = diff_iovecs * sizeof(xlog_op_header_t);
        len += iovhdr_res;
        ctx->nvecs += diff_iovecs;

        /* attach the transaction to the CIL if it has any busy extents */
        if (!list_empty(&tp->t_busy))
                list_splice_init(&tp->t_busy, &ctx->busy_extents);

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

        /*
         * If we've overrun the reservation, dump the tx details before we move
         * the log items. Shutdown is imminent...
         */
        if (WARN_ON(tp->t_ticket->t_curr_res < 0)) {
                xfs_warn(log->l_mp, "Transaction log reservation overrun:");
                xfs_warn(log->l_mp,
                         "  log items: %d bytes (iov hdrs: %d bytes)",
                         len, iovhdr_res);
                xfs_warn(log->l_mp, "  split region headers: %d bytes",
                         split_res);
                xfs_warn(log->l_mp, "  ctx ticket: %d bytes", ctx_res);
                xlog_print_trans(tp);
        }

        /*
         * Now (re-)position everything modified at the tail of the CIL.
         * We do this here so we only need to take the CIL lock once during
         * the transaction commit.
         */
        list_for_each_entry(lip, &tp->t_items, li_trans) {

                /* Skip items which aren't dirty in this transaction. */
                if (!test_bit(XFS_LI_DIRTY, &lip->li_flags))
                        continue;

                /*
                 * Only move the item if it isn't already at the tail. This is
                 * to prevent a transient list_empty() state when reinserting
                 * an item that is already the only item in the CIL.
                 */
                if (!list_is_last(&lip->li_cil, &cil->xc_cil))
                        list_move_tail(&lip->li_cil, &cil->xc_cil);
        }

        spin_unlock(&cil->xc_cil_lock);

        if (tp->t_ticket->t_curr_res < 0)
                xfs_force_shutdown(log->l_mp, SHUTDOWN_LOG_IO_ERROR);
}

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 = next;
        }
}

static void
xlog_discard_endio_work(
        struct work_struct      *work)
{
        struct xfs_cil_ctx      *ctx =
                container_of(work, struct xfs_cil_ctx, discard_endio_work);
        struct xfs_mount        *mp = ctx->cil->xc_log->l_mp;

        xfs_extent_busy_clear(mp, &ctx->busy_extents, false);
        kmem_free(ctx);
}

/*
 * Queue up the actual completion to a thread to avoid IRQ-safe locking for
 * pagb_lock.  Note that we need a unbounded workqueue, otherwise we might
 * get the execution delayed up to 30 seconds for weird reasons.
 */
static void
xlog_discard_endio(
        struct bio              *bio)
{
        struct xfs_cil_ctx      *ctx = bio->bi_private;

        INIT_WORK(&ctx->discard_endio_work, xlog_discard_endio_work);
        queue_work(xfs_discard_wq, &ctx->discard_endio_work);
        bio_put(bio);
}

static void
xlog_discard_busy_extents(
        struct xfs_mount        *mp,
        struct xfs_cil_ctx      *ctx)
{
        struct list_head        *list = &ctx->busy_extents;
        struct xfs_extent_busy  *busyp;
        struct bio              *bio = NULL;
        struct blk_plug         plug;
        int                     error = 0;

        ASSERT(mp->m_flags & XFS_MOUNT_DISCARD);

        blk_start_plug(&plug);
        list_for_each_entry(busyp, list, list) {
                trace_xfs_discard_extent(mp, busyp->agno, busyp->bno,
                                         busyp->length);

                error = __blkdev_issue_discard(mp->m_ddev_targp->bt_bdev,
                                XFS_AGB_TO_DADDR(mp, busyp->agno, busyp->bno),
                                XFS_FSB_TO_BB(mp, busyp->length),
                                GFP_NOFS, 0, &bio);
                if (error && error != -EOPNOTSUPP) {
                        xfs_info(mp,
         "discard failed for extent [0x%llx,%u], error %d",
                                 (unsigned long long)busyp->bno,
                                 busyp->length,
                                 error);
                        break;
                }
        }

        if (bio) {
                bio->bi_private = ctx;
                bio->bi_end_io = xlog_discard_endio;
                submit_bio(bio);
        } else {
                xlog_discard_endio_work(&ctx->discard_endio_work);
        }
        blk_finish_plug(&plug);
}

/*
 * 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;
        struct xfs_mount        *mp = ctx->cil->xc_log->l_mp;

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

        xfs_extent_busy_sort(&ctx->busy_extents);
        xfs_extent_busy_clear(mp, &ctx->busy_extents,
                             (mp->m_flags & XFS_MOUNT_DISCARD) && !abort);

        /*
         * If we are aborting the commit, wake up anyone waiting on the
         * committing list.  If we don't, then a shutdown we can leave processes
         * waiting in xlog_cil_force_lsn() waiting on a sequence commit that
         * will never happen because we aborted it.
         */
        spin_lock(&ctx->cil->xc_push_lock);
        if (abort)
                wake_up_all(&ctx->cil->xc_commit_wait);
        list_del(&ctx->committing);
        spin_unlock(&ctx->cil->xc_push_lock);

        xlog_cil_free_logvec(ctx->lv_chain);

        if (!list_empty(&ctx->busy_extents))
                xlog_discard_busy_extents(mp, ctx);
        else
                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 xlog             *log)
{
        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_iovecs;
        int                     error = 0;
        struct xfs_trans_header thdr;
        struct xfs_log_iovec    lhdr;
        struct xfs_log_vec      lvhdr = { NULL };
        xfs_lsn_t               commit_lsn;
        xfs_lsn_t               push_seq;

        if (!cil)
                return 0;

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

        down_write(&cil->xc_ctx_lock);
        ctx = cil->xc_ctx;

        spin_lock(&cil->xc_push_lock);
        push_seq = cil->xc_push_seq;
        ASSERT(push_seq <= ctx->sequence);

        /*
         * Check if we've anything to push. If there is nothing, then we don't
         * move on to a new sequence number and so we have to be able to push
         * this sequence again later.
         */
        if (list_empty(&cil->xc_cil)) {
                cil->xc_push_seq = 0;
                spin_unlock(&cil->xc_push_lock);
                goto out_skip;
        }


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

        /*
         * We are now going to push this context, so add it to the committing
         * list before we do anything else. This ensures that anyone waiting on
         * this push can easily detect the difference between a "push in
         * progress" and "CIL is empty, nothing to do".
         *
         * IOWs, a wait loop can now check for:
         *      the current sequence not being found on the committing list;
         *      an empty CIL; and
         *      an unchanged sequence number
         * to detect a push that had nothing to do and therefore does not need
         * waiting on. If the CIL is not empty, we get put on the committing
         * list before emptying the CIL and bumping the sequence number. Hence
         * an empty CIL and an unchanged sequence number means we jumped out
         * above after doing nothing.
         *
         * Hence the waiter will either find the commit sequence on the
         * committing list or the sequence number will be unchanged and the CIL
         * still dirty. In that latter case, the push has not yet started, and
         * so the waiter will have to continue trying to check the CIL
         * committing list until it is found. In extreme cases of delay, the
         * sequence may fully commit between the attempts the wait makes to wait
         * on the commit sequence.
         */
        list_add(&ctx->committing, &cil->xc_committing);
        spin_unlock(&cil->xc_push_lock);

        /*
         * 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_iovecs = 0;
        while (!list_empty(&cil->xc_cil)) {
                struct xfs_log_item     *item;

                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_iovecs += lv->lv_niovecs;
        }

        /*
         * 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;

        /*
         * 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.
         *
         * xfs_log_force_lsn requires us to mirror the new sequence into the cil
         * structure atomically with the addition of this sequence to the
         * committing list. This also ensures that we can do unlocked checks
         * against the current sequence in log forces without risking
         * deferencing a freed context pointer.
         */
        spin_lock(&cil->xc_push_lock);
        cil->xc_current_sequence = new_ctx->sequence;
        spin_unlock(&cil->xc_push_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_push_lock);
        list_for_each_entry(new_ctx, &cil->xc_committing, committing) {
                /*
                 * Avoid getting stuck in this loop because we were woken by the
                 * shutdown, but then went back to sleep once already in the
                 * shutdown state.
                 */
                if (XLOG_FORCED_SHUTDOWN(log)) {
                        spin_unlock(&cil->xc_push_lock);
                        goto out_abort_free_ticket;
                }

                /*
                 * Higher sequences will wait for this one so skip them.
                 * Don't wait for our 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_push_lock);
                        goto restart;
                }
        }
        spin_unlock(&cil->xc_push_lock);

        /* xfs_log_done always frees the ticket on error. */
        commit_lsn = xfs_log_done(log->l_mp, tic, &commit_iclog, false);
        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(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_push_lock);
        ctx->commit_lsn = commit_lsn;
        wake_up_all(&cil->xc_commit_wait);
        spin_unlock(&cil->xc_push_lock);

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

out_skip:
        up_write(&cil->xc_ctx_lock);
        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 -EIO;
}

static void
xlog_cil_push_work(
        struct work_struct      *work)
{
        struct xfs_cil          *cil = container_of(work, struct xfs_cil,
                                                        xc_push_work);
        xlog_cil_push(cil->xc_log);
}

/*
 * 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.
 */
static void
xlog_cil_push_background(
        struct xlog     *log)
{
        struct xfs_cil  *cil = log->l_cilp;

        /*
         * The cil won't be empty because we are called while holding the
         * context lock so whatever we added to the CIL will still be there
         */
        ASSERT(!list_empty(&cil->xc_cil));

        /*
         * don't do a background push if we haven't used up all the
         * space available yet.
         */
        if (cil->xc_ctx->space_used < XLOG_CIL_SPACE_LIMIT(log))
                return;

        spin_lock(&cil->xc_push_lock);
        if (cil->xc_push_seq < cil->xc_current_sequence) {
                cil->xc_push_seq = cil->xc_current_sequence;
                queue_work(log->l_mp->m_cil_workqueue, &cil->xc_push_work);
        }
        spin_unlock(&cil->xc_push_lock);

}

/*
 * xlog_cil_push_now() is used to trigger an immediate CIL push to the sequence
 * number that is passed. When it returns, the work will be queued for
 * @push_seq, but it won't be completed. The caller is expected to do any
 * waiting for push_seq to complete if it is required.
 */
static void
xlog_cil_push_now(
        struct xlog     *log,
        xfs_lsn_t       push_seq)
{
        struct xfs_cil  *cil = log->l_cilp;

        if (!cil)
                return;

        ASSERT(push_seq && push_seq <= cil->xc_current_sequence);

        /* start on any pending background push to minimise wait time on it */
        flush_work(&cil->xc_push_work);

        /*
         * If the CIL is empty or we've already pushed the sequence then
         * there's no work we need to do.
         */
        spin_lock(&cil->xc_push_lock);
        if (list_empty(&cil->xc_cil) || push_seq <= cil->xc_push_seq) {
                spin_unlock(&cil->xc_push_lock);
                return;
        }

        cil->xc_push_seq = push_seq;
        queue_work(log->l_mp->m_cil_workqueue, &cil->xc_push_work);
        spin_unlock(&cil->xc_push_lock);
}

bool
xlog_cil_empty(
        struct xlog     *log)
{
        struct xfs_cil  *cil = log->l_cilp;
        bool            empty = false;

        spin_lock(&cil->xc_push_lock);
        if (list_empty(&cil->xc_cil))
                empty = true;
        spin_unlock(&cil->xc_push_lock);
        return empty;
}

/*
 * 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.
 *
 * 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,
        xfs_lsn_t               *commit_lsn,
        bool                    regrant)
{
        struct xlog             *log = mp->m_log;
        struct xfs_cil          *cil = log->l_cilp;
        xfs_lsn_t               xc_commit_lsn;

        /*
         * Do all necessary memory allocation before we lock the CIL.
         * This ensures the allocation does not deadlock with a CIL
         * push in memory reclaim (e.g. from kswapd).
         */
        xlog_cil_alloc_shadow_bufs(log, tp);

        /* lock out background commit */
        down_read(&cil->xc_ctx_lock);

        xlog_cil_insert_items(log, tp);

        xc_commit_lsn = cil->xc_ctx->sequence;
        if (commit_lsn)
                *commit_lsn = xc_commit_lsn;

        xfs_log_done(mp, tp->t_ticket, NULL, regrant);
        tp->t_ticket = NULL;
        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, xc_commit_lsn, false);

        xlog_cil_push_background(log);

        up_read(&cil->xc_ctx_lock);
}

/*
 * 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.
 */
xfs_lsn_t
xlog_cil_force_lsn(
        struct xlog     *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.
         */
restart:
        xlog_cil_push_now(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.
         */
        spin_lock(&cil->xc_push_lock);
        list_for_each_entry(ctx, &cil->xc_committing, committing) {
                /*
                 * Avoid getting stuck in this loop because we were woken by the
                 * shutdown, but then went back to sleep once already in the
                 * shutdown state.
                 */
                if (XLOG_FORCED_SHUTDOWN(log))
                        goto out_shutdown;
                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_push_lock);
                        goto restart;
                }
                if (ctx->sequence != sequence)
                        continue;
                /* found it! */
                commit_lsn = ctx->commit_lsn;
        }

        /*
         * The call to xlog_cil_push_now() executes the push in the background.
         * Hence by the time we have got here it our sequence may not have been
         * pushed yet. This is true if the current sequence still matches the
         * push sequence after the above wait loop and the CIL still contains
         * dirty objects. This is guaranteed by the push code first adding the
         * context to the committing list before emptying the CIL.
         *
         * Hence if we don't find the context in the committing list and the
         * current sequence number is unchanged then the CIL contents are
         * significant.  If the CIL is empty, if means there was nothing to push
         * and that means there is nothing to wait for. If the CIL is not empty,
         * it means we haven't yet started the push, because if it had started
         * we would have found the context on the committing list.
         */
        if (sequence == cil->xc_current_sequence &&
            !list_empty(&cil->xc_cil)) {
                spin_unlock(&cil->xc_push_lock);
                goto restart;
        }

        spin_unlock(&cil->xc_push_lock);
        return commit_lsn;

        /*
         * We detected a shutdown in progress. We need to trigger the log force
         * to pass through it's iclog state machine error handling, even though
         * we are already in a shutdown state. Hence we can't return
         * NULLCOMMITLSN here as that has special meaning to log forces (i.e.
         * LSN is already stable), so we return a zero LSN instead.
         */
out_shutdown:
        spin_unlock(&cil->xc_push_lock);
        return 0;
}

/*
 * 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 (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;
}

/*
 * Perform initial CIL structure initialisation.
 */
int
xlog_cil_init(
        struct xlog     *log)
{
        struct xfs_cil  *cil;
        struct xfs_cil_ctx *ctx;

        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_WORK(&cil->xc_push_work, xlog_cil_push_work);
        INIT_LIST_HEAD(&cil->xc_cil);
        INIT_LIST_HEAD(&cil->xc_committing);
        spin_lock_init(&cil->xc_cil_lock);
        spin_lock_init(&cil->xc_push_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 xlog     *log)
{
        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);
}