/*
   * 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 might
   * 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/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(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_wbuf_sync_nolock(wbuf);
  	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
   * @a: 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(key_type(c, &sa->key) == UBIFS_DATA_KEY);
  	ubifs_assert(key_type(c, &sb->key) == UBIFS_DATA_KEY);
  	ubifs_assert(sa->type == UBIFS_DATA_NODE);
  	ubifs_assert(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(key_type(c, &sa->key) != UBIFS_DATA_KEY &&
  		     key_type(c, &sb->key) != UBIFS_DATA_KEY);
  	ubifs_assert(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(key_type(c, &sa->key) == UBIFS_DENT_KEY ||
  		     key_type(c, &sa->key) == UBIFS_XENT_KEY);
  	ubifs_assert(key_type(c, &sb->key) == UBIFS_DENT_KEY ||
  		     key_type(c, &sb->key) == UBIFS_XENT_KEY);
  	ubifs_assert(sa->type == UBIFS_DENT_NODE ||
  		     sa->type == UBIFS_XENT_NODE);
  	ubifs_assert(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(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);
  
  		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(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;
  		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;
  			if  (snod->len > avail)
  				/*
  				 * Do not skip data nodes in order to optimize
  				 * bulk-read.
  				 */
  				break;
  
  			err = move_node(c, sleb, snod, wbuf);
  			if (err)
  				goto out;
  		}
  
  		/* Move non-data nodes */
  		list_for_each_entry_safe(snod, tmp, &nondata, list) {
  			avail = c->leb_size - wbuf->offs - wbuf->used;
  			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 = move_node(c, sleb, snod, wbuf);
  			if (err)
  				goto out;
  		}
  
  		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->gc_lnum != -1 || wbuf->offs + wbuf->used == 0 ||
  		     c->need_recovery);
  	ubifs_assert(c->gc_lnum != lnum);
  	ubifs_assert(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(!(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'.
  			 */
  			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(!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);
  	ubifs_assert(!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(!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(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_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 (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;
  }