Merge tag 'block-5.6-2020-02-05' of git://git.kernel.dk/linux-block

[mirror_ubuntu-jammy-kernel.git] / fs / ubifs / gc.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * This file is part of UBIFS.
 *
 * Copyright (C) 2006-2008 Nokia Corporation.
 *
 * 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 might
 * be unable to reclaim it. So, LEBs with free + dirty greater than dark
 * watermark are "good" LEBs from GC's point of view. The other LEBs are not so
 * good, and GC takes extra care when moving them.
 */

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

/*
 * 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(c, 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);
        return err;
}

/**
 * data_nodes_cmp - compare 2 data nodes.
 * @priv: UBIFS file-system description object
 * @a: first data node
 * @b: second data node
 *
 * This function compares data nodes @a and @b. Returns %1 if @a has greater
 * inode or block number, and %-1 otherwise.
 */
static int data_nodes_cmp(void *priv, struct list_head *a, struct list_head *b)
{
        ino_t inuma, inumb;
        struct ubifs_info *c = priv;
        struct ubifs_scan_node *sa, *sb;

        cond_resched();
        if (a == b)
                return 0;

        sa = list_entry(a, struct ubifs_scan_node, list);
        sb = list_entry(b, struct ubifs_scan_node, list);

        ubifs_assert(c, key_type(c, &sa->key) == UBIFS_DATA_KEY);
        ubifs_assert(c, key_type(c, &sb->key) == UBIFS_DATA_KEY);
        ubifs_assert(c, sa->type == UBIFS_DATA_NODE);
        ubifs_assert(c, sb->type == UBIFS_DATA_NODE);

        inuma = key_inum(c, &sa->key);
        inumb = key_inum(c, &sb->key);

        if (inuma == inumb) {
                unsigned int blka = key_block(c, &sa->key);
                unsigned int blkb = key_block(c, &sb->key);

                if (blka <= blkb)
                        return -1;
        } else if (inuma <= inumb)
                return -1;

        return 1;
}

/*
 * nondata_nodes_cmp - compare 2 non-data nodes.
 * @priv: UBIFS file-system description object
 * @a: first node
 * @a: second node
 *
 * This function compares nodes @a and @b. It makes sure that inode nodes go
 * first and sorted by length in descending order. Directory entry nodes go
 * after inode nodes and are sorted in ascending hash valuer order.
 */
static int nondata_nodes_cmp(void *priv, struct list_head *a,
                             struct list_head *b)
{
        ino_t inuma, inumb;
        struct ubifs_info *c = priv;
        struct ubifs_scan_node *sa, *sb;

        cond_resched();
        if (a == b)
                return 0;

        sa = list_entry(a, struct ubifs_scan_node, list);
        sb = list_entry(b, struct ubifs_scan_node, list);

        ubifs_assert(c, key_type(c, &sa->key) != UBIFS_DATA_KEY &&
                     key_type(c, &sb->key) != UBIFS_DATA_KEY);
        ubifs_assert(c, sa->type != UBIFS_DATA_NODE &&
                     sb->type != UBIFS_DATA_NODE);

        /* Inodes go before directory entries */
        if (sa->type == UBIFS_INO_NODE) {
                if (sb->type == UBIFS_INO_NODE)
                        return sb->len - sa->len;
                return -1;
        }
        if (sb->type == UBIFS_INO_NODE)
                return 1;

        ubifs_assert(c, key_type(c, &sa->key) == UBIFS_DENT_KEY ||
                     key_type(c, &sa->key) == UBIFS_XENT_KEY);
        ubifs_assert(c, key_type(c, &sb->key) == UBIFS_DENT_KEY ||
                     key_type(c, &sb->key) == UBIFS_XENT_KEY);
        ubifs_assert(c, sa->type == UBIFS_DENT_NODE ||
                     sa->type == UBIFS_XENT_NODE);
        ubifs_assert(c, sb->type == UBIFS_DENT_NODE ||
                     sb->type == UBIFS_XENT_NODE);

        inuma = key_inum(c, &sa->key);
        inumb = key_inum(c, &sb->key);

        if (inuma == inumb) {
                uint32_t hasha = key_hash(c, &sa->key);
                uint32_t hashb = key_hash(c, &sb->key);

                if (hasha <= hashb)
                        return -1;
        } else if (inuma <= inumb)
                return -1;

        return 1;
}

/**
 * sort_nodes - sort nodes for GC.
 * @c: UBIFS file-system description object
 * @sleb: describes nodes to sort and contains the result on exit
 * @nondata: contains non-data nodes on exit
 * @min: minimum node size is returned here
 *
 * This function sorts the list of inodes to garbage collect. First of all, it
 * kills obsolete nodes and separates data and non-data nodes to the
 * @sleb->nodes and @nondata lists correspondingly.
 *
 * Data nodes are then sorted in block number order - this is important for
 * bulk-read; data nodes with lower inode number go before data nodes with
 * higher inode number, and data nodes with lower block number go before data
 * nodes with higher block number;
 *
 * Non-data nodes are sorted as follows.
 *   o First go inode nodes - they are sorted in descending length order.
 *   o Then go directory entry nodes - they are sorted in hash order, which
 *     should supposedly optimize 'readdir()'. Direntry nodes with lower parent
 *     inode number go before direntry nodes with higher parent inode number,
 *     and direntry nodes with lower name hash values go before direntry nodes
 *     with higher name hash values.
 *
 * This function returns zero in case of success and a negative error code in
 * case of failure.
 */
static int sort_nodes(struct ubifs_info *c, struct ubifs_scan_leb *sleb,
                      struct list_head *nondata, int *min)
{
        int err;
        struct ubifs_scan_node *snod, *tmp;

        *min = INT_MAX;

        /* Separate data nodes and non-data nodes */
        list_for_each_entry_safe(snod, tmp, &sleb->nodes, list) {
                ubifs_assert(c, snod->type == UBIFS_INO_NODE  ||
                             snod->type == UBIFS_DATA_NODE ||
                             snod->type == UBIFS_DENT_NODE ||
                             snod->type == UBIFS_XENT_NODE ||
                             snod->type == UBIFS_TRUN_NODE ||
                             snod->type == UBIFS_AUTH_NODE);

                if (snod->type != UBIFS_INO_NODE  &&
                    snod->type != UBIFS_DATA_NODE &&
                    snod->type != UBIFS_DENT_NODE &&
                    snod->type != UBIFS_XENT_NODE) {
                        /* Probably truncation node, zap it */
                        list_del(&snod->list);
                        kfree(snod);
                        continue;
                }

                ubifs_assert(c, key_type(c, &snod->key) == UBIFS_DATA_KEY ||
                             key_type(c, &snod->key) == UBIFS_INO_KEY  ||
                             key_type(c, &snod->key) == UBIFS_DENT_KEY ||
                             key_type(c, &snod->key) == UBIFS_XENT_KEY);

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

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

                if (snod->len < *min)
                        *min = snod->len;

                if (key_type(c, &snod->key) != UBIFS_DATA_KEY)
                        list_move_tail(&snod->list, nondata);
        }

        /* Sort data and non-data nodes */
        list_sort(c, &sleb->nodes, &data_nodes_cmp);
        list_sort(c, nondata, &nondata_nodes_cmp);

        err = dbg_check_data_nodes_order(c, &sleb->nodes);
        if (err)
                return err;
        err = dbg_check_nondata_nodes_order(c, nondata);
        if (err)
                return err;
        return 0;
}

/**
 * move_node - move a node.
 * @c: UBIFS file-system description object
 * @sleb: describes the LEB to move nodes from
 * @snod: the mode to move
 * @wbuf: write-buffer to move node to
 *
 * This function moves node @snod to @wbuf, changes TNC correspondingly, and
 * destroys @snod. Returns zero in case of success and a negative error code in
 * case of failure.
 */
static int move_node(struct ubifs_info *c, struct ubifs_scan_leb *sleb,
                     struct ubifs_scan_node *snod, struct ubifs_wbuf *wbuf)
{
        int err, new_lnum = wbuf->lnum, new_offs = wbuf->offs + wbuf->used;

        cond_resched();
        err = ubifs_wbuf_write_nolock(wbuf, snod->node, snod->len);
        if (err)
                return err;

        err = ubifs_tnc_replace(c, &snod->key, sleb->lnum,
                                snod->offs, new_lnum, new_offs,
                                snod->len);
        list_del(&snod->list);
        kfree(snod);
        return err;
}

/**
 * move_nodes - move nodes.
 * @c: UBIFS file-system description object
 * @sleb: describes the LEB to move nodes from
 *
 * This function moves valid nodes from data LEB described by @sleb to the GC
 * journal head. 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)
{
        int err, min;
        LIST_HEAD(nondata);
        struct ubifs_wbuf *wbuf = &c->jheads[GCHD].wbuf;

        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)
                        return err;
        }

        err = sort_nodes(c, sleb, &nondata, &min);
        if (err)
                goto out;

        /* Write nodes to their new location. Use the first-fit strategy */
        while (1) {
                int avail, moved = 0;
                struct ubifs_scan_node *snod, *tmp;

                /* Move data nodes */
                list_for_each_entry_safe(snod, tmp, &sleb->nodes, list) {
                        avail = c->leb_size - wbuf->offs - wbuf->used -
                                        ubifs_auth_node_sz(c);
                        if  (snod->len > avail)
                                /*
                                 * Do not skip data nodes in order to optimize
                                 * bulk-read.
                                 */
                                break;

                        err = ubifs_shash_update(c, c->jheads[GCHD].log_hash,
                                                 snod->node, snod->len);
                        if (err)
                                goto out;

                        err = move_node(c, sleb, snod, wbuf);
                        if (err)
                                goto out;
                        moved = 1;
                }

                /* Move non-data nodes */
                list_for_each_entry_safe(snod, tmp, &nondata, list) {
                        avail = c->leb_size - wbuf->offs - wbuf->used -
                                        ubifs_auth_node_sz(c);
                        if (avail < min)
                                break;

                        if  (snod->len > avail) {
                                /*
                                 * Keep going only if this is an inode with
                                 * some data. Otherwise stop and switch the GC
                                 * head. IOW, we assume that data-less inode
                                 * nodes and direntry nodes are roughly of the
                                 * same size.
                                 */
                                if (key_type(c, &snod->key) == UBIFS_DENT_KEY ||
                                    snod->len == UBIFS_INO_NODE_SZ)
                                        break;
                                continue;
                        }

                        err = ubifs_shash_update(c, c->jheads[GCHD].log_hash,
                                                 snod->node, snod->len);
                        if (err)
                                goto out;

                        err = move_node(c, sleb, snod, wbuf);
                        if (err)
                                goto out;
                        moved = 1;
                }

                if (ubifs_authenticated(c) && moved) {
                        struct ubifs_auth_node *auth;

                        auth = kmalloc(ubifs_auth_node_sz(c), GFP_NOFS);
                        if (!auth) {
                                err = -ENOMEM;
                                goto out;
                        }

                        err = ubifs_prepare_auth_node(c, auth,
                                                c->jheads[GCHD].log_hash);
                        if (err) {
                                kfree(auth);
                                goto out;
                        }

                        err = ubifs_wbuf_write_nolock(wbuf, auth,
                                                      ubifs_auth_node_sz(c));
                        if (err) {
                                kfree(auth);
                                goto out;
                        }

                        ubifs_add_dirt(c, wbuf->lnum, ubifs_auth_node_sz(c));
                }

                if (list_empty(&sleb->nodes) && list_empty(&nondata))
                        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_splice_tail(&nondata, &sleb->nodes);
        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, c->gc_lnum != -1 || wbuf->offs + wbuf->used == 0 ||
                     c->need_recovery);
        ubifs_assert(c, c->gc_lnum != lnum);
        ubifs_assert(c, wbuf->lnum != lnum);

        if (lp->free + lp->dirty == c->leb_size) {
                /* Special case - a free LEB  */
                dbg_gc("LEB %d is free, return it", lp->lnum);
                ubifs_assert(c, !(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->lnum'.
                         */
                        err = gc_sync_wbufs(c);
                        if (err)
                                return err;
                        err = ubifs_change_one_lp(c, lp->lnum, c->leb_size,
                                                  0, 0, 0, 0);
                        if (err)
                                return err;
                }
                err = ubifs_leb_unmap(c, lp->lnum);
                if (err)
                        return err;

                if (c->gc_lnum == -1) {
                        c->gc_lnum = lnum;
                        return LEB_RETAINED;
                }

                return LEB_FREED;
        }

        /*
         * 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, 0);
        if (IS_ERR(sleb))
                return PTR_ERR(sleb);

        ubifs_assert(c, !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(c, 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);
        ubifs_assert(c, !c->ro_media && !c->ro_mount);

        if (ubifs_gc_should_commit(c))
                return -EAGAIN;

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

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

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

        for (i = 0; ; i++) {
                int space_before, 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);

                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) {
                                /*
                                 * This is not error, so we have to return the
                                 * LEB to lprops. But if 'ubifs_return_leb()'
                                 * fails, its failure code is propagated to the
                                 * caller instead of the original '-EAGAIN'.
                                 */
                                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(c, 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(c, ret < 0);
        ubifs_assert(c, ret != -ENOSPC && ret != -EAGAIN);
        ubifs_wbuf_sync_nolock(wbuf);
        ubifs_ro_mode(c, ret);
        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 (!lp)
                        break;
                ubifs_assert(c, !(lp->flags & LPROPS_TAKEN));
                ubifs_assert(c, !(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(c, !(lp->flags & LPROPS_TAKEN));
                ubifs_assert(c, !(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(c, !(lp->flags & LPROPS_TAKEN));
                ubifs_assert(c, 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(c, lp->flags & LPROPS_TAKEN);
                ubifs_assert(c, !(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;
}