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[linux-2.6.git] / fs / ubifs / gc.c

/*
 * This file is part of UBIFS.
 *
 * Copyright (C) 2006-2008 Nokia Corporation.
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 as published by
 * the Free Software Foundation.
 *
 * This program is distributed in the hope that it will 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 to the Free Software Foundation, Inc., 51
 * Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 *
 * Authors: Adrian Hunter
 *          Artem Bityutskiy (Битюцкий Артём)
 */

/*
 * This file implements garbage collection. The procedure for garbage collection
 * is different depending on whether a LEB as an index LEB (contains index
 * nodes) or not. For non-index LEBs, garbage collection finds a LEB which
 * contains a lot of dirty space (obsolete nodes), and copies the non-obsolete
 * nodes to the journal, at which point the garbage-collected LEB is free to be
 * reused. For index LEBs, garbage collection marks the non-obsolete index nodes
 * dirty in the TNC, and after the next commit, the garbage-collected LEB is
 * to be reused. Garbage collection will cause the number of dirty index nodes
 * to grow, however sufficient space is reserved for the index to ensure the
 * commit will never run out of space.
 *
 * Notes about dead watermark. At current UBIFS implementation we assume that
 * LEBs which have less than @c->dead_wm bytes of free + dirty space are full
 * and not worth garbage-collecting. The dead watermark is one min. I/O unit
 * size, or min. UBIFS node size, depending on what is greater. Indeed, UBIFS
 * Garbage Collector has to synchronize the GC head's write buffer before
 * returning, so this is about wasting one min. I/O unit. However, UBIFS GC can
 * actually reclaim even very small pieces of dirty space by garbage collecting
 * enough dirty LEBs, but we do not bother doing this at this implementation.
 *
 * Notes about dark watermark. The results of GC work depends on how big are
 * the UBIFS nodes GC deals with. Large nodes make GC waste more space. Indeed,
 * if GC move data from LEB A to LEB B and nodes in LEB A are large, GC would
 * have to waste large pieces of free space at the end of LEB B, because nodes
 * from LEB A would not fit. And the worst situation is when all nodes are of
 * maximum size. So dark watermark is the amount of free + dirty space in LEB
 * which are guaranteed to be reclaimable. If LEB has less space, the GC migh
 * be unable to reclaim it. So, LEBs with free + dirty greater than dark
 * watermark are "good" LEBs from GC's point of few. The other LEBs are not so
 * good, and GC takes extra care when moving them.
 */

#include <linux/pagemap.h>
#include "ubifs.h"

/*
 * GC tries to optimize the way it fit nodes to available space, and it sorts
 * nodes a little. The below constants are watermarks which define "large",
 * "medium", and "small" nodes.
 */
#define MEDIUM_NODE_WM (UBIFS_BLOCK_SIZE / 4)
#define SMALL_NODE_WM  UBIFS_MAX_DENT_NODE_SZ

/*
 * GC may need to move more than one LEB to make progress. The below constants
 * define "soft" and "hard" limits on the number of LEBs the garbage collector
 * may move.
 */
#define SOFT_LEBS_LIMIT 4
#define HARD_LEBS_LIMIT 32

/**
 * switch_gc_head - switch the garbage collection journal head.
 * @c: UBIFS file-system description object
 * @buf: buffer to write
 * @len: length of the buffer to write
 * @lnum: LEB number written is returned here
 * @offs: offset written is returned here
 *
 * This function switch the GC head to the next LEB which is reserved in
 * @c->gc_lnum. Returns %0 in case of success, %-EAGAIN if commit is required,
 * and other negative error code in case of failures.
 */
static int switch_gc_head(struct ubifs_info *c)
{
        int err, gc_lnum = c->gc_lnum;
        struct ubifs_wbuf *wbuf = &c->jheads[GCHD].wbuf;

        ubifs_assert(gc_lnum != -1);
        dbg_gc("switch GC head from LEB %d:%d to LEB %d (waste %d bytes)",
               wbuf->lnum, wbuf->offs + wbuf->used, gc_lnum,
               c->leb_size - wbuf->offs - wbuf->used);

        err = ubifs_wbuf_sync_nolock(wbuf);
        if (err)
                return err;

        /*
         * The GC write-buffer was synchronized, we may safely unmap
         * 'c->gc_lnum'.
         */
        err = ubifs_leb_unmap(c, gc_lnum);
        if (err)
                return err;

        err = ubifs_add_bud_to_log(c, GCHD, gc_lnum, 0);
        if (err)
                return err;

        c->gc_lnum = -1;
        err = ubifs_wbuf_seek_nolock(wbuf, gc_lnum, 0, UBI_LONGTERM);
        return err;
}

/**
 * joinup - bring data nodes for an inode together.
 * @c: UBIFS file-system description object
 * @sleb: describes scanned LEB
 * @inum: inode number
 * @blk: block number
 * @data: list to which to add data nodes
 *
 * This function looks at the first few nodes in the scanned LEB @sleb and adds
 * them to @data if they are data nodes from @inum and have a larger block
 * number than @blk. This function returns %0 on success and a negative error
 * code on failure.
 */
static int joinup(struct ubifs_info *c, struct ubifs_scan_leb *sleb, ino_t inum,
                  unsigned int blk, struct list_head *data)
{
        int err, cnt = 6, lnum = sleb->lnum, offs;
        struct ubifs_scan_node *snod, *tmp;
        union ubifs_key *key;

        list_for_each_entry_safe(snod, tmp, &sleb->nodes, list) {
                key = &snod->key;
                if (key_inum(c, key) == inum &&
                    key_type(c, key) == UBIFS_DATA_KEY &&
                    key_block(c, key) > blk) {
                        offs = snod->offs;
                        err = ubifs_tnc_has_node(c, key, 0, lnum, offs, 0);
                        if (err < 0)
                                return err;
                        list_del(&snod->list);
                        if (err) {
                                list_add_tail(&snod->list, data);
                                blk = key_block(c, key);
                        } else
                                kfree(snod);
                        cnt = 6;
                } else if (--cnt == 0)
                        break;
        }
        return 0;
}

/**
 * move_nodes - move nodes.
 * @c: UBIFS file-system description object
 * @sleb: describes nodes to move
 *
 * This function moves valid nodes from data LEB described by @sleb to the GC
 * journal head. The obsolete nodes are dropped.
 *
 * When moving nodes we have to deal with classical bin-packing problem: the
 * space in the current GC journal head LEB and in @c->gc_lnum are the "bins",
 * where the nodes in the @sleb->nodes list are the elements which should be
 * fit optimally to the bins. This function uses the "first fit decreasing"
 * strategy, although it does not really sort the nodes but just split them on
 * 3 classes - large, medium, and small, so they are roughly sorted.
 *
 * This function returns zero in case of success, %-EAGAIN if commit is
 * required, and other negative error codes in case of other failures.
 */
static int move_nodes(struct ubifs_info *c, struct ubifs_scan_leb *sleb)
{
        struct ubifs_scan_node *snod, *tmp;
        struct list_head data, large, medium, small;
        struct ubifs_wbuf *wbuf = &c->jheads[GCHD].wbuf;
        int avail, err, min = INT_MAX;
        unsigned int blk = 0;
        ino_t inum = 0;

        INIT_LIST_HEAD(&data);
        INIT_LIST_HEAD(&large);
        INIT_LIST_HEAD(&medium);
        INIT_LIST_HEAD(&small);

        while (!list_empty(&sleb->nodes)) {
                struct list_head *lst = sleb->nodes.next;

                snod = list_entry(lst, struct ubifs_scan_node, list);

                ubifs_assert(snod->type != UBIFS_IDX_NODE);
                ubifs_assert(snod->type != UBIFS_REF_NODE);
                ubifs_assert(snod->type != UBIFS_CS_NODE);

                err = ubifs_tnc_has_node(c, &snod->key, 0, sleb->lnum,
                                         snod->offs, 0);
                if (err < 0)
                        goto out;

                list_del(lst);
                if (!err) {
                        /* The node is obsolete, remove it from the list */
                        kfree(snod);
                        continue;
                }

                /*
                 * Sort the list of nodes so that data nodes go first, large
                 * nodes go second, and small nodes go last.
                 */
                if (key_type(c, &snod->key) == UBIFS_DATA_KEY) {
                        if (inum != key_inum(c, &snod->key)) {
                                if (inum) {
                                        /*
                                         * Try to move data nodes from the same
                                         * inode together.
                                         */
                                        err = joinup(c, sleb, inum, blk, &data);
                                        if (err)
                                                goto out;
                                }
                                inum = key_inum(c, &snod->key);
                                blk = key_block(c, &snod->key);
                        }
                        list_add_tail(lst, &data);
                } else if (snod->len > MEDIUM_NODE_WM)
                        list_add_tail(lst, &large);
                else if (snod->len > SMALL_NODE_WM)
                        list_add_tail(lst, &medium);
                else
                        list_add_tail(lst, &small);

                /* And find the smallest node */
                if (snod->len < min)
                        min = snod->len;
        }

        /*
         * Join the tree lists so that we'd have one roughly sorted list
         * ('large' will be the head of the joined list).
         */
        list_splice(&data, &large);
        list_splice(&medium, large.prev);
        list_splice(&small, large.prev);

        if (wbuf->lnum == -1) {
                /*
                 * The GC journal head is not set, because it is the first GC
                 * invocation since mount.
                 */
                err = switch_gc_head(c);
                if (err)
                        goto out;
        }

        /* Write nodes to their new location. Use the first-fit strategy */
        while (1) {
                avail = c->leb_size - wbuf->offs - wbuf->used;
                list_for_each_entry_safe(snod, tmp, &large, list) {
                        int new_lnum, new_offs;

                        if (avail < min)
                                break;

                        if (snod->len > avail)
                                /* This node does not fit */
                                continue;

                        cond_resched();

                        new_lnum = wbuf->lnum;
                        new_offs = wbuf->offs + wbuf->used;
                        err = ubifs_wbuf_write_nolock(wbuf, snod->node,
                                                      snod->len);
                        if (err)
                                goto out;
                        err = ubifs_tnc_replace(c, &snod->key, sleb->lnum,
                                                snod->offs, new_lnum, new_offs,
                                                snod->len);
                        if (err)
                                goto out;

                        avail = c->leb_size - wbuf->offs - wbuf->used;
                        list_del(&snod->list);
                        kfree(snod);
                }

                if (list_empty(&large))
                        break;

                /*
                 * Waste the rest of the space in the LEB and switch to the
                 * next LEB.
                 */
                err = switch_gc_head(c);
                if (err)
                        goto out;
        }

        return 0;

out:
        list_for_each_entry_safe(snod, tmp, &large, list) {
                list_del(&snod->list);
                kfree(snod);
        }
        return err;
}

/**
 * gc_sync_wbufs - sync write-buffers for GC.
 * @c: UBIFS file-system description object
 *
 * We must guarantee that obsoleting nodes are on flash. Unfortunately they may
 * be in a write-buffer instead. That is, a node could be written to a
 * write-buffer, obsoleting another node in a LEB that is GC'd. If that LEB is
 * erased before the write-buffer is sync'd and then there is an unclean
 * unmount, then an existing node is lost. To avoid this, we sync all
 * write-buffers.
 *
 * This function returns %0 on success or a negative error code on failure.
 */
static int gc_sync_wbufs(struct ubifs_info *c)
{
        int err, i;

        for (i = 0; i < c->jhead_cnt; i++) {
                if (i == GCHD)
                        continue;
                err = ubifs_wbuf_sync(&c->jheads[i].wbuf);
                if (err)
                        return err;
        }
        return 0;
}

/**
 * ubifs_garbage_collect_leb - garbage-collect a logical eraseblock.
 * @c: UBIFS file-system description object
 * @lp: describes the LEB to garbage collect
 *
 * This function garbage-collects an LEB and returns one of the @LEB_FREED,
 * @LEB_RETAINED, etc positive codes in case of success, %-EAGAIN if commit is
 * required, and other negative error codes in case of failures.
 */
int ubifs_garbage_collect_leb(struct ubifs_info *c, struct ubifs_lprops *lp)
{
        struct ubifs_scan_leb *sleb;
        struct ubifs_scan_node *snod;
        struct ubifs_wbuf *wbuf = &c->jheads[GCHD].wbuf;
        int err = 0, lnum = lp->lnum;

        ubifs_assert(c->gc_lnum != -1 || wbuf->offs + wbuf->used == 0 ||
                     c->need_recovery);
        ubifs_assert(c->gc_lnum != lnum);
        ubifs_assert(wbuf->lnum != lnum);

        /*
         * We scan the entire LEB even though we only really need to scan up to
         * (c->leb_size - lp->free).
         */
        sleb = ubifs_scan(c, lnum, 0, c->sbuf);
        if (IS_ERR(sleb))
                return PTR_ERR(sleb);

        ubifs_assert(!list_empty(&sleb->nodes));
        snod = list_entry(sleb->nodes.next, struct ubifs_scan_node, list);

        if (snod->type == UBIFS_IDX_NODE) {
                struct ubifs_gced_idx_leb *idx_gc;

                dbg_gc("indexing LEB %d (free %d, dirty %d)",
                       lnum, lp->free, lp->dirty);
                list_for_each_entry(snod, &sleb->nodes, list) {
                        struct ubifs_idx_node *idx = snod->node;
                        int level = le16_to_cpu(idx->level);

                        ubifs_assert(snod->type == UBIFS_IDX_NODE);
                        key_read(c, ubifs_idx_key(c, idx), &snod->key);
                        err = ubifs_dirty_idx_node(c, &snod->key, level, lnum,
                                                   snod->offs);
                        if (err)
                                goto out;
                }

                idx_gc = kmalloc(sizeof(struct ubifs_gced_idx_leb), GFP_NOFS);
                if (!idx_gc) {
                        err = -ENOMEM;
                        goto out;
                }

                idx_gc->lnum = lnum;
                idx_gc->unmap = 0;
                list_add(&idx_gc->list, &c->idx_gc);

                /*
                 * Don't release the LEB until after the next commit, because
                 * it may contain data which is needed for recovery. So
                 * although we freed this LEB, it will become usable only after
                 * the commit.
                 */
                err = ubifs_change_one_lp(c, lnum, c->leb_size, 0, 0,
                                          LPROPS_INDEX, 1);
                if (err)
                        goto out;
                err = LEB_FREED_IDX;
        } else {
                dbg_gc("data LEB %d (free %d, dirty %d)",
                       lnum, lp->free, lp->dirty);

                err = move_nodes(c, sleb);
                if (err)
                        goto out_inc_seq;

                err = gc_sync_wbufs(c);
                if (err)
                        goto out_inc_seq;

                err = ubifs_change_one_lp(c, lnum, c->leb_size, 0, 0, 0, 0);
                if (err)
                        goto out_inc_seq;

                /* Allow for races with TNC */
                c->gced_lnum = lnum;
                smp_wmb();
                c->gc_seq += 1;
                smp_wmb();

                if (c->gc_lnum == -1) {
                        c->gc_lnum = lnum;
                        err = LEB_RETAINED;
                } else {
                        err = ubifs_wbuf_sync_nolock(wbuf);
                        if (err)
                                goto out;

                        err = ubifs_leb_unmap(c, lnum);
                        if (err)
                                goto out;

                        err = LEB_FREED;
                }
        }

out:
        ubifs_scan_destroy(sleb);
        return err;

out_inc_seq:
        /* We may have moved at least some nodes so allow for races with TNC */
        c->gced_lnum = lnum;
        smp_wmb();
        c->gc_seq += 1;
        smp_wmb();
        goto out;
}

/**
 * ubifs_garbage_collect - UBIFS garbage collector.
 * @c: UBIFS file-system description object
 * @anyway: do GC even if there are free LEBs
 *
 * This function does out-of-place garbage collection. The return codes are:
 *   o positive LEB number if the LEB has been freed and may be used;
 *   o %-EAGAIN if the caller has to run commit;
 *   o %-ENOSPC if GC failed to make any progress;
 *   o other negative error codes in case of other errors.
 *
 * Garbage collector writes data to the journal when GC'ing data LEBs, and just
 * marking indexing nodes dirty when GC'ing indexing LEBs. Thus, at some point
 * commit may be required. But commit cannot be run from inside GC, because the
 * caller might be holding the commit lock, so %-EAGAIN is returned instead;
 * And this error code means that the caller has to run commit, and re-run GC
 * if there is still no free space.
 *
 * There are many reasons why this function may return %-EAGAIN:
 * o the log is full and there is no space to write an LEB reference for
 *   @c->gc_lnum;
 * o the journal is too large and exceeds size limitations;
 * o GC moved indexing LEBs, but they can be used only after the commit;
 * o the shrinker fails to find clean znodes to free and requests the commit;
 * o etc.
 *
 * Note, if the file-system is close to be full, this function may return
 * %-EAGAIN infinitely, so the caller has to limit amount of re-invocations of
 * the function. E.g., this happens if the limits on the journal size are too
 * tough and GC writes too much to the journal before an LEB is freed. This
 * might also mean that the journal is too large, and the TNC becomes to big,
 * so that the shrinker is constantly called, finds not clean znodes to free,
 * and requests commit. Well, this may also happen if the journal is all right,
 * but another kernel process consumes too much memory. Anyway, infinite
 * %-EAGAIN may happen, but in some extreme/misconfiguration cases.
 */
int ubifs_garbage_collect(struct ubifs_info *c, int anyway)
{
        int i, err, ret, min_space = c->dead_wm;
        struct ubifs_lprops lp;
        struct ubifs_wbuf *wbuf = &c->jheads[GCHD].wbuf;

        ubifs_assert_cmt_locked(c);

        if (ubifs_gc_should_commit(c))
                return -EAGAIN;

        mutex_lock_nested(&wbuf->io_mutex, wbuf->jhead);

        if (c->ro_media) {
                ret = -EROFS;
                goto out_unlock;
        }

        /* We expect the write-buffer to be empty on entry */
        ubifs_assert(!wbuf->used);

        for (i = 0; ; i++) {
                int space_before = c->leb_size - wbuf->offs - wbuf->used;
                int space_after;

                cond_resched();

                /* Give the commit an opportunity to run */
                if (ubifs_gc_should_commit(c)) {
                        ret = -EAGAIN;
                        break;
                }

                if (i > SOFT_LEBS_LIMIT && !list_empty(&c->idx_gc)) {
                        /*
                         * We've done enough iterations. Indexing LEBs were
                         * moved and will be available after the commit.
                         */
                        dbg_gc("soft limit, some index LEBs GC'ed, -EAGAIN");
                        ubifs_commit_required(c);
                        ret = -EAGAIN;
                        break;
                }

                if (i > HARD_LEBS_LIMIT) {
                        /*
                         * We've moved too many LEBs and have not made
                         * progress, give up.
                         */
                        dbg_gc("hard limit, -ENOSPC");
                        ret = -ENOSPC;
                        break;
                }

                /*
                 * Empty and freeable LEBs can turn up while we waited for
                 * the wbuf lock, or while we have been running GC. In that
                 * case, we should just return one of those instead of
                 * continuing to GC dirty LEBs. Hence we request
                 * 'ubifs_find_dirty_leb()' to return an empty LEB if it can.
                 */
                ret = ubifs_find_dirty_leb(c, &lp, min_space, anyway ? 0 : 1);
                if (ret) {
                        if (ret == -ENOSPC)
                                dbg_gc("no more dirty LEBs");
                        break;
                }

                dbg_gc("found LEB %d: free %d, dirty %d, sum %d "
                       "(min. space %d)", lp.lnum, lp.free, lp.dirty,
                       lp.free + lp.dirty, min_space);

                if (lp.free + lp.dirty == c->leb_size) {
                        /* An empty LEB was returned */
                        dbg_gc("LEB %d is free, return it", lp.lnum);
                        /*
                         * ubifs_find_dirty_leb() doesn't return freeable index
                         * LEBs.
                         */
                        ubifs_assert(!(lp.flags & LPROPS_INDEX));
                        if (lp.free != c->leb_size) {
                                /*
                                 * Write buffers must be sync'd before
                                 * unmapping freeable LEBs, because one of them
                                 * may contain data which obsoletes something
                                 * in 'lp.pnum'.
                                 */
                                ret = gc_sync_wbufs(c);
                                if (ret)
                                        goto out;
                                ret = ubifs_change_one_lp(c, lp.lnum,
                                                          c->leb_size, 0, 0, 0,
                                                          0);
                                if (ret)
                                        goto out;
                        }
                        ret = ubifs_leb_unmap(c, lp.lnum);
                        if (ret)
                                goto out;
                        ret = lp.lnum;
                        break;
                }

                space_before = c->leb_size - wbuf->offs - wbuf->used;
                if (wbuf->lnum == -1)
                        space_before = 0;

                ret = ubifs_garbage_collect_leb(c, &lp);
                if (ret < 0) {
                        if (ret == -EAGAIN || ret == -ENOSPC) {
                                /*
                                 * These codes are not errors, so we have to
                                 * return the LEB to lprops. But if the
                                 * 'ubifs_return_leb()' function fails, its
                                 * failure code is propagated to the caller
                                 * instead of the original '-EAGAIN' or
                                 * '-ENOSPC'.
                                 */
                                err = ubifs_return_leb(c, lp.lnum);
                                if (err)
                                        ret = err;
                                break;
                        }
                        goto out;
                }

                if (ret == LEB_FREED) {
                        /* An LEB has been freed and is ready for use */
                        dbg_gc("LEB %d freed, return", lp.lnum);
                        ret = lp.lnum;
                        break;
                }

                if (ret == LEB_FREED_IDX) {
                        /*
                         * This was an indexing LEB and it cannot be
                         * immediately used. And instead of requesting the
                         * commit straight away, we try to garbage collect some
                         * more.
                         */
                        dbg_gc("indexing LEB %d freed, continue", lp.lnum);
                        continue;
                }

                ubifs_assert(ret == LEB_RETAINED);
                space_after = c->leb_size - wbuf->offs - wbuf->used;
                dbg_gc("LEB %d retained, freed %d bytes", lp.lnum,
                       space_after - space_before);

                if (space_after > space_before) {
                        /* GC makes progress, keep working */
                        min_space >>= 1;
                        if (min_space < c->dead_wm)
                                min_space = c->dead_wm;
                        continue;
                }

                dbg_gc("did not make progress");

                /*
                 * GC moved an LEB bud have not done any progress. This means
                 * that the previous GC head LEB contained too few free space
                 * and the LEB which was GC'ed contained only large nodes which
                 * did not fit that space.
                 *
                 * We can do 2 things:
                 * 1. pick another LEB in a hope it'll contain a small node
                 *    which will fit the space we have at the end of current GC
                 *    head LEB, but there is no guarantee, so we try this out
                 *    unless we have already been working for too long;
                 * 2. request an LEB with more dirty space, which will force
                 *    'ubifs_find_dirty_leb()' to start scanning the lprops
                 *    table, instead of just picking one from the heap
                 *    (previously it already picked the dirtiest LEB).
                 */
                if (i < SOFT_LEBS_LIMIT) {
                        dbg_gc("try again");
                        continue;
                }

                min_space <<= 1;
                if (min_space > c->dark_wm)
                        min_space = c->dark_wm;
                dbg_gc("set min. space to %d", min_space);
        }

        if (ret == -ENOSPC && !list_empty(&c->idx_gc)) {
                dbg_gc("no space, some index LEBs GC'ed, -EAGAIN");
                ubifs_commit_required(c);
                ret = -EAGAIN;
        }

        err = ubifs_wbuf_sync_nolock(wbuf);
        if (!err)
                err = ubifs_leb_unmap(c, c->gc_lnum);
        if (err) {
                ret = err;
                goto out;
        }
out_unlock:
        mutex_unlock(&wbuf->io_mutex);
        return ret;

out:
        ubifs_assert(ret < 0);
        ubifs_assert(ret != -ENOSPC && ret != -EAGAIN);
        ubifs_ro_mode(c, ret);
        ubifs_wbuf_sync_nolock(wbuf);
        mutex_unlock(&wbuf->io_mutex);
        ubifs_return_leb(c, lp.lnum);
        return ret;
}

/**
 * ubifs_gc_start_commit - garbage collection at start of commit.
 * @c: UBIFS file-system description object
 *
 * If a LEB has only dirty and free space, then we may safely unmap it and make
 * it free.  Note, we cannot do this with indexing LEBs because dirty space may
 * correspond index nodes that are required for recovery.  In that case, the
 * LEB cannot be unmapped until after the next commit.
 *
 * This function returns %0 upon success and a negative error code upon failure.
 */
int ubifs_gc_start_commit(struct ubifs_info *c)
{
        struct ubifs_gced_idx_leb *idx_gc;
        const struct ubifs_lprops *lp;
        int err = 0, flags;

        ubifs_get_lprops(c);

        /*
         * Unmap (non-index) freeable LEBs. Note that recovery requires that all
         * wbufs are sync'd before this, which is done in 'do_commit()'.
         */
        while (1) {
                lp = ubifs_fast_find_freeable(c);
                if (IS_ERR(lp)) {
                        err = PTR_ERR(lp);
                        goto out;
                }
                if (!lp)
                        break;
                ubifs_assert(!(lp->flags & LPROPS_TAKEN));
                ubifs_assert(!(lp->flags & LPROPS_INDEX));
                err = ubifs_leb_unmap(c, lp->lnum);
                if (err)
                        goto out;
                lp = ubifs_change_lp(c, lp, c->leb_size, 0, lp->flags, 0);
                if (IS_ERR(lp)) {
                        err = PTR_ERR(lp);
                        goto out;
                }
                ubifs_assert(!(lp->flags & LPROPS_TAKEN));
                ubifs_assert(!(lp->flags & LPROPS_INDEX));
        }

        /* Mark GC'd index LEBs OK to unmap after this commit finishes */
        list_for_each_entry(idx_gc, &c->idx_gc, list)
                idx_gc->unmap = 1;

        /* Record index freeable LEBs for unmapping after commit */
        while (1) {
                lp = ubifs_fast_find_frdi_idx(c);
                if (IS_ERR(lp)) {
                        err = PTR_ERR(lp);
                        goto out;
                }
                if (!lp)
                        break;
                idx_gc = kmalloc(sizeof(struct ubifs_gced_idx_leb), GFP_NOFS);
                if (!idx_gc) {
                        err = -ENOMEM;
                        goto out;
                }
                ubifs_assert(!(lp->flags & LPROPS_TAKEN));
                ubifs_assert(lp->flags & LPROPS_INDEX);
                /* Don't release the LEB until after the next commit */
                flags = (lp->flags | LPROPS_TAKEN) ^ LPROPS_INDEX;
                lp = ubifs_change_lp(c, lp, c->leb_size, 0, flags, 1);
                if (IS_ERR(lp)) {
                        err = PTR_ERR(lp);
                        kfree(idx_gc);
                        goto out;
                }
                ubifs_assert(lp->flags & LPROPS_TAKEN);
                ubifs_assert(!(lp->flags & LPROPS_INDEX));
                idx_gc->lnum = lp->lnum;
                idx_gc->unmap = 1;
                list_add(&idx_gc->list, &c->idx_gc);
        }
out:
        ubifs_release_lprops(c);
        return err;
}

/**
 * ubifs_gc_end_commit - garbage collection at end of commit.
 * @c: UBIFS file-system description object
 *
 * This function completes out-of-place garbage collection of index LEBs.
 */
int ubifs_gc_end_commit(struct ubifs_info *c)
{
        struct ubifs_gced_idx_leb *idx_gc, *tmp;
        struct ubifs_wbuf *wbuf;
        int err = 0;

        wbuf = &c->jheads[GCHD].wbuf;
        mutex_lock_nested(&wbuf->io_mutex, wbuf->jhead);
        list_for_each_entry_safe(idx_gc, tmp, &c->idx_gc, list)
                if (idx_gc->unmap) {
                        dbg_gc("LEB %d", idx_gc->lnum);
                        err = ubifs_leb_unmap(c, idx_gc->lnum);
                        if (err)
                                goto out;
                        err = ubifs_change_one_lp(c, idx_gc->lnum, LPROPS_NC,
                                          LPROPS_NC, 0, LPROPS_TAKEN, -1);
                        if (err)
                                goto out;
                        list_del(&idx_gc->list);
                        kfree(idx_gc);
                }
out:
        mutex_unlock(&wbuf->io_mutex);
        return err;
}

/**
 * ubifs_destroy_idx_gc - destroy idx_gc list.
 * @c: UBIFS file-system description object
 *
 * This function destroys the @c->idx_gc list. It is called when unmounting
 * so locks are not needed. Returns zero in case of success and a negative
 * error code in case of failure.
 */
void ubifs_destroy_idx_gc(struct ubifs_info *c)
{
        while (!list_empty(&c->idx_gc)) {
                struct ubifs_gced_idx_leb *idx_gc;

                idx_gc = list_entry(c->idx_gc.next, struct ubifs_gced_idx_leb,
                                    list);
                c->idx_gc_cnt -= 1;
                list_del(&idx_gc->list);
                kfree(idx_gc);
        }
}

/**
 * ubifs_get_idx_gc_leb - get a LEB from GC'd index LEB list.
 * @c: UBIFS file-system description object
 *
 * Called during start commit so locks are not needed.
 */
int ubifs_get_idx_gc_leb(struct ubifs_info *c)
{
        struct ubifs_gced_idx_leb *idx_gc;
        int lnum;

        if (list_empty(&c->idx_gc))
                return -ENOSPC;
        idx_gc = list_entry(c->idx_gc.next, struct ubifs_gced_idx_leb, list);
        lnum = idx_gc->lnum;
        /* c->idx_gc_cnt is updated by the caller when lprops are updated */
        list_del(&idx_gc->list);
        kfree(idx_gc);
        return lnum;
}