[PATCH 7/17] [LogFS] gc.c

[Joern Engel <joern@logfs.org>]

---
 fs/logfs/gc.c                       |  730 ++++++++++++

diff --git a/fs/logfs/gc.c b/fs/logfs/gc.c
new file mode 100644
index 0000000..b3656c4
--- /dev/null
+++ b/fs/logfs/gc.c
@@ -0,0 +1,730 @@
+/*
+ * fs/logfs/gc.c	- garbage collection code
+ *
+ * As should be obvious for Linux kernel code, license is GPLv2
+ *
+ * Copyright (c) 2005-2008 Joern Engel <joern@logfs.org>
+ */
+#include "logfs.h"
+#include <linux/sched.h>
+
+/*
+ * Wear leveling needs to kick in when the difference between low erase
+ * counts and high erase counts gets too big.  A good value for "too big"
+ * may be somewhat below 10% of maximum erase count for the device.
+ * Why not 397, to pick a nice round number with no specific meaning? :)
+ *
+ * WL_RATELIMIT is the minimum time between two wear level events.  A huge
+ * number of segments may fulfil the requirements for wear leveling at the
+ * same time.  If that happens we don't want to cause a latency from hell,
+ * but just gently pick one segment every so often and minimize overhead.
+ */
+#define WL_DELTA 397
+#define WL_RATELIMIT 100
+#define MAX_OBJ_ALIASES	2600
+#define SCAN_RATIO 512	/* number of scanned segments per gc'd segment */
+#define LIST_SIZE 64	/* base size of candidate lists */
+#define SCAN_ROUNDS 128	/* maximum number of complete medium scans */
+#define SCAN_ROUNDS_HIGH 4 /* maximum number of higher-level scans */
+
+static int no_free_segments(struct super_block *sb)
+{
+	struct logfs_super *super = logfs_super(sb);
+
+	return super->s_free_list.count;
+}
+
+/* journal has distance -1, top-most ifile layer distance 0 */
+static u8 root_distance(struct super_block *sb, gc_level_t __gc_level)
+{
+	struct logfs_super *super = logfs_super(sb);
+	u8 gc_level = (__force u8)__gc_level;
+
+	switch (gc_level) {
+	case 0: /* fall through */
+	case 1: /* fall through */
+	case 2: /* fall through */
+	case 3:
+		/* file data or indirect blocks */
+		return super->s_ifile_levels + super->s_iblock_levels - gc_level;
+	case 6: /* fall through */
+	case 7: /* fall through */
+	case 8: /* fall through */
+	case 9:
+		/* inode file data or indirect blocks */
+		return super->s_ifile_levels - (gc_level - 6);
+	default:
+		printk(KERN_ERR"LOGFS: segment of unknown level %x found\n",
+				gc_level);
+		WARN_ON(1);
+		return super->s_ifile_levels + super->s_iblock_levels;
+	}
+}
+
+static int segment_is_reserved(struct super_block *sb, u32 segno)
+{
+	struct logfs_super *super = logfs_super(sb);
+	struct logfs_area *area;
+	void *reserved;
+	int i;
+
+	/* Some segments are reserved.  Just pretend they were all valid */
+	reserved = btree_lookup32(&super->s_reserved_segments, segno);
+	if (reserved)
+		return 1;
+
+	/* Currently open segments */
+	for_each_area(i) {
+		area = super->s_area[i];
+		if (area->a_is_open && area->a_segno == segno)
+			return 1;
+	}
+
+	return 0;
+}
+
+static void logfs_mark_segment_bad(struct super_block *sb, u32 segno)
+{
+	BUG();
+}
+
+/*
+ * Returns the bytes consumed by valid objects in this segment.  Object headers
+ * are counted, the segment header is not.
+ */
+static u32 logfs_valid_bytes(struct super_block *sb, u32 segno, u32 *ec,
+		gc_level_t *gc_level)
+{
+	struct logfs_segment_entry se;
+	u32 ec_level;
+
+	logfs_get_segment_entry(sb, segno, &se);
+	if (se.ec_level == cpu_to_be32(BADSEG) ||
+			se.valid == cpu_to_be32(RESERVED))
+		return RESERVED;
+
+	ec_level = be32_to_cpu(se.ec_level);
+	*ec = ec_level >> 4;
+	*gc_level = GC_LEVEL(ec_level & 0xf);
+	return be32_to_cpu(se.valid);
+}
+
+static void logfs_cleanse_block(struct super_block *sb, u64 ofs, u64 ino,
+		u64 bix, gc_level_t gc_level)
+{
+	struct inode *inode;
+	int err, cookie;
+
+	inode = logfs_safe_iget(sb, ino, &cookie);
+	err = logfs_rewrite_block(inode, bix, ofs, gc_level, 0);
+	BUG_ON(err);
+	logfs_safe_iput(inode, cookie);
+}
+
+static u32 logfs_gc_segment(struct super_block *sb, u32 segno, u8 dist)
+{
+	struct logfs_super *super = logfs_super(sb);
+	struct logfs_segment_header sh;
+	struct logfs_object_header oh;
+	u64 ofs, ino, bix;
+	u32 seg_ofs, logical_segno, cleaned = 0;
+	int err, len, valid;
+	gc_level_t gc_level;
+
+	LOGFS_BUG_ON(segment_is_reserved(sb, segno), sb);
+
+	btree_insert32(&super->s_reserved_segments, segno, (void *)1, GFP_NOFS);
+	err = wbuf_read(sb, dev_ofs(sb, segno, 0), sizeof(sh), &sh);
+	BUG_ON(err);
+	gc_level = GC_LEVEL(sh.level);
+	logical_segno = be32_to_cpu(sh.segno);
+	if (sh.crc != logfs_crc32(&sh, sizeof(sh), 4)) {
+		logfs_mark_segment_bad(sb, segno);
+		cleaned = -1;
+		goto out;
+	}
+
+	for (seg_ofs = LOGFS_SEGMENT_HEADERSIZE;
+			seg_ofs + sizeof(oh) < super->s_segsize; ) {
+		ofs = dev_ofs(sb, logical_segno, seg_ofs);
+		err = wbuf_read(sb, dev_ofs(sb, segno, seg_ofs), sizeof(oh),
+				&oh);
+		BUG_ON(err);
+
+		if (!memchr_inv(&oh, 0xff, sizeof(oh)))
+			break;
+
+		if (oh.crc != logfs_crc32(&oh, sizeof(oh) - 4, 4)) {
+			logfs_mark_segment_bad(sb, segno);
+			cleaned = super->s_segsize - 1;
+			goto out;
+		}
+
+		ino = be64_to_cpu(oh.ino);
+		bix = be64_to_cpu(oh.bix);
+		len = sizeof(oh) + be16_to_cpu(oh.len);
+		valid = logfs_is_valid_block(sb, ofs, ino, bix, gc_level);
+		if (valid == 1) {
+			logfs_cleanse_block(sb, ofs, ino, bix, gc_level);
+			cleaned += len;
+		} else if (valid == 2) {
+			/* Will be invalid upon journal commit */
+			cleaned += len;
+		}
+		seg_ofs += len;
+	}
+out:
+	btree_remove32(&super->s_reserved_segments, segno);
+	return cleaned;
+}
+
+static struct gc_candidate *add_list(struct gc_candidate *cand,
+		struct candidate_list *list)
+{
+	struct rb_node **p = &list->rb_tree.rb_node;
+	struct rb_node *parent = NULL;
+	struct gc_candidate *cur;
+	int comp;
+
+	cand->list = list;
+	while (*p) {
+		parent = *p;
+		cur = rb_entry(parent, struct gc_candidate, rb_node);
+
+		if (list->sort_by_ec)
+			comp = cand->erase_count < cur->erase_count;
+		else
+			comp = cand->valid < cur->valid;
+
+		if (comp)
+			p = &parent->rb_left;
+		else
+			p = &parent->rb_right;
+	}
+	rb_link_node(&cand->rb_node, parent, p);
+	rb_insert_color(&cand->rb_node, &list->rb_tree);
+
+	if (list->count <= list->maxcount) {
+		list->count++;
+		return NULL;
+	}
+	cand = rb_entry(rb_last(&list->rb_tree), struct gc_candidate, rb_node);
+	rb_erase(&cand->rb_node, &list->rb_tree);
+	cand->list = NULL;
+	return cand;
+}
+
+static void remove_from_list(struct gc_candidate *cand)
+{
+	struct candidate_list *list = cand->list;
+
+	rb_erase(&cand->rb_node, &list->rb_tree);
+	list->count--;
+}
+
+static void free_candidate(struct super_block *sb, struct gc_candidate *cand)
+{
+	struct logfs_super *super = logfs_super(sb);
+
+	btree_remove32(&super->s_cand_tree, cand->segno);
+	kfree(cand);
+}
+
+u32 get_best_cand(struct super_block *sb, struct candidate_list *list, u32 *ec)
+{
+	struct gc_candidate *cand;
+	u32 segno;
+
+	BUG_ON(list->count == 0);
+
+	cand = rb_entry(rb_first(&list->rb_tree), struct gc_candidate, rb_node);
+	remove_from_list(cand);
+	segno = cand->segno;
+	if (ec)
+		*ec = cand->erase_count;
+	free_candidate(sb, cand);
+	return segno;
+}
+
+/*
+ * We have several lists to manage segments with.  The reserve_list is used to
+ * deal with bad blocks.  We try to keep the best (lowest ec) segments on this
+ * list.
+ * The free_list contains free segments for normal usage.  It usually gets the
+ * second pick after the reserve_list.  But when the free_list is running short
+ * it is more important to keep the free_list full than to keep a reserve.
+ *
+ * Segments that are not free are put onto a per-level low_list.  If we have
+ * to run garbage collection, we pick a candidate from there.  All segments on
+ * those lists should have at least some free space so GC will make progress.
+ *
+ * And last we have the ec_list, which is used to pick segments for wear
+ * leveling.
+ *
+ * If all appropriate lists are full, we simply free the candidate and forget
+ * about that segment for a while.  We have better candidates for each purpose.
+ */
+static void __add_candidate(struct super_block *sb, struct gc_candidate *cand)
+{
+	struct logfs_super *super = logfs_super(sb);
+	u32 full = super->s_segsize - LOGFS_SEGMENT_RESERVE;
+
+	if (cand->valid == 0) {
+		/* 100% free segments */
+		log_gc_noisy("add reserve segment %x (ec %x) at %llx\n",
+				cand->segno, cand->erase_count,
+				dev_ofs(sb, cand->segno, 0));
+		cand = add_list(cand, &super->s_reserve_list);
+		if (cand) {
+			log_gc_noisy("add free segment %x (ec %x) at %llx\n",
+					cand->segno, cand->erase_count,
+					dev_ofs(sb, cand->segno, 0));
+			cand = add_list(cand, &super->s_free_list);
+		}
+	} else {
+		/* good candidates for Garbage Collection */
+		if (cand->valid < full)
+			cand = add_list(cand, &super->s_low_list[cand->dist]);
+		/* good candidates for wear leveling,
+		 * segments that were recently written get ignored */
+		if (cand)
+			cand = add_list(cand, &super->s_ec_list);
+	}
+	if (cand)
+		free_candidate(sb, cand);
+}
+
+static int add_candidate(struct super_block *sb, u32 segno, u32 valid, u32 ec,
+		u8 dist)
+{
+	struct logfs_super *super = logfs_super(sb);
+	struct gc_candidate *cand;
+
+	cand = kmalloc(sizeof(*cand), GFP_NOFS);
+	if (!cand)
+		return -ENOMEM;
+
+	cand->segno = segno;
+	cand->valid = valid;
+	cand->erase_count = ec;
+	cand->dist = dist;
+
+	btree_insert32(&super->s_cand_tree, segno, cand, GFP_NOFS);
+	__add_candidate(sb, cand);
+	return 0;
+}
+
+static void remove_segment_from_lists(struct super_block *sb, u32 segno)
+{
+	struct logfs_super *super = logfs_super(sb);
+	struct gc_candidate *cand;
+
+	cand = btree_lookup32(&super->s_cand_tree, segno);
+	if (cand) {
+		remove_from_list(cand);
+		free_candidate(sb, cand);
+	}
+}
+
+static void scan_segment(struct super_block *sb, u32 segno)
+{
+	u32 valid, ec = 0;
+	gc_level_t gc_level = 0;
+	u8 dist;
+
+	if (segment_is_reserved(sb, segno))
+		return;
+
+	remove_segment_from_lists(sb, segno);
+	valid = logfs_valid_bytes(sb, segno, &ec, &gc_level);
+	if (valid == RESERVED)
+		return;
+
+	dist = root_distance(sb, gc_level);
+	add_candidate(sb, segno, valid, ec, dist);
+}
+
+static struct gc_candidate *first_in_list(struct candidate_list *list)
+{
+	if (list->count == 0)
+		return NULL;
+	return rb_entry(rb_first(&list->rb_tree), struct gc_candidate, rb_node);
+}
+
+/*
+ * Find the best segment for garbage collection.  Main criterion is
+ * the segment requiring the least effort to clean.  Secondary
+ * criterion is to GC on the lowest level available.
+ *
+ * So we search the least effort segment on the lowest level first,
+ * then move up and pick another segment iff is requires significantly
+ * less effort.  Hence the LOGFS_MAX_OBJECTSIZE in the comparison.
+ */
+static struct gc_candidate *get_candidate(struct super_block *sb)
+{
+	struct logfs_super *super = logfs_super(sb);
+	int i, max_dist;
+	struct gc_candidate *cand = NULL, *this;
+
+	max_dist = min(no_free_segments(sb), LOGFS_NO_AREAS);
+
+	for (i = max_dist; i >= 0; i--) {
+		this = first_in_list(&super->s_low_list[i]);
+		if (!this)
+			continue;
+		if (!cand)
+			cand = this;
+		if (this->valid + LOGFS_MAX_OBJECTSIZE <= cand->valid)
+			cand = this;
+	}
+	return cand;
+}
+
+static int __logfs_gc_once(struct super_block *sb, struct gc_candidate *cand)
+{
+	struct logfs_super *super = logfs_super(sb);
+	gc_level_t gc_level;
+	u32 cleaned, valid, segno, ec;
+	u8 dist;
+
+	if (!cand) {
+		log_gc("GC attempted, but no candidate found\n");
+		return 0;
+	}
+
+	segno = cand->segno;
+	dist = cand->dist;
+	valid = logfs_valid_bytes(sb, segno, &ec, &gc_level);
+	free_candidate(sb, cand);
+	log_gc("GC segment #%02x at %llx, %x required, %x free, %x valid, %llx free\n",
+			segno, (u64)segno << super->s_segshift,
+			dist, no_free_segments(sb), valid,
+			super->s_free_bytes);
+	cleaned = logfs_gc_segment(sb, segno, dist);
+	log_gc("GC segment #%02x complete - now %x valid\n", segno,
+			valid - cleaned);
+	BUG_ON(cleaned != valid);
+	return 1;
+}
+
+static int logfs_gc_once(struct super_block *sb)
+{
+	struct gc_candidate *cand;
+
+	cand = get_candidate(sb);
+	if (cand)
+		remove_from_list(cand);
+	return __logfs_gc_once(sb, cand);
+}
+
+/* returns 1 if a wrap occurs, 0 otherwise */
+static int logfs_scan_some(struct super_block *sb)
+{
+	struct logfs_super *super = logfs_super(sb);
+	u32 segno;
+	int i, ret = 0;
+
+	segno = super->s_sweeper;
+	for (i = SCAN_RATIO; i > 0; i--) {
+		segno++;
+		if (segno >= super->s_no_segs) {
+			segno = 0;
+			ret = 1;
+			/* Break out of the loop.  We want to read a single
+			 * block from the segment size on next invocation if
+			 * SCAN_RATIO is set to match block size
+			 */
+			break;
+		}
+
+		scan_segment(sb, segno);
+	}
+	super->s_sweeper = segno;
+	return ret;
+}
+
+/*
+ * In principle, this function should loop forever, looking for GC candidates
+ * and moving data.  LogFS is designed in such a way that this loop is
+ * guaranteed to terminate.
+ *
+ * Limiting the loop to some iterations serves purely to catch cases when
+ * these guarantees have failed.  An actual endless loop is an obvious bug
+ * and should be reported as such.
+ */
+static void __logfs_gc_pass(struct super_block *sb, int target)
+{
+	struct logfs_super *super = logfs_super(sb);
+	struct logfs_block *block;
+	int round, progress, last_progress = 0;
+
+	if (no_free_segments(sb) >= target &&
+			super->s_no_object_aliases < MAX_OBJ_ALIASES)
+		return;
+
+	log_gc("__logfs_gc_pass(%x)\n", target);
+	for (round = 0; round < SCAN_ROUNDS; ) {
+		if (no_free_segments(sb) >= target)
+			goto write_alias;
+
+		/* Sync in-memory state with on-medium state in case they
+		 * diverged */
+		logfs_write_anchor(super->s_master_inode);
+		round += logfs_scan_some(sb);
+		if (no_free_segments(sb) >= target)
+			goto write_alias;
+		progress = logfs_gc_once(sb);
+		if (progress)
+			last_progress = round;
+		else if (round - last_progress > 2)
+			break;
+		continue;
+
+		/*
+		 * The goto logic is nasty, I just don't know a better way to
+		 * code it.  GC is supposed to ensure two things:
+		 * 1. Enough free segments are available.
+		 * 2. The number of aliases is bounded.
+		 * When 1. is achieved, we take a look at 2. and write back
+		 * some alias-containing blocks, if necessary.  However, after
+		 * each such write we need to go back to 1., as writes can
+		 * consume free segments.
+		 */
+write_alias:
+		if (super->s_no_object_aliases < MAX_OBJ_ALIASES)
+			return;
+		if (list_empty(&super->s_object_alias)) {
+			/* All aliases are still in btree */
+			return;
+		}
+		log_gc("Write back one alias\n");
+		block = list_entry(super->s_object_alias.next,
+				struct logfs_block, alias_list);
+		block->ops->write_block(block);
+		/*
+		 * To round off the nasty goto logic, we reset round here.  It
+		 * is a safety-net for GC not making any progress and limited
+		 * to something reasonably small.  If incremented it for every
+		 * single alias, the loop could terminate rather quickly.
+		 */
+		round = 0;
+	}
+	LOGFS_BUG(sb);
+}
+
+static int wl_ratelimit(struct super_block *sb, u64 *next_event)
+{
+	struct logfs_super *super = logfs_super(sb);
+
+	if (*next_event < super->s_gec) {
+		*next_event = super->s_gec + WL_RATELIMIT;
+		return 0;
+	}
+	return 1;
+}
+
+static void logfs_wl_pass(struct super_block *sb)
+{
+	struct logfs_super *super = logfs_super(sb);
+	struct gc_candidate *wl_cand, *free_cand;
+
+	if (wl_ratelimit(sb, &super->s_wl_gec_ostore))
+		return;
+
+	wl_cand = first_in_list(&super->s_ec_list);
+	if (!wl_cand)
+		return;
+	free_cand = first_in_list(&super->s_free_list);
+	if (!free_cand)
+		return;
+
+	if (wl_cand->erase_count < free_cand->erase_count + WL_DELTA) {
+		remove_from_list(wl_cand);
+		__logfs_gc_once(sb, wl_cand);
+	}
+}
+
+/*
+ * The journal needs wear leveling as well.  But moving the journal is an
+ * expensive operation so we try to avoid it as much as possible.  And if we
+ * have to do it, we move the whole journal, not individual segments.
+ *
+ * Ratelimiting is not strictly necessary here, it mainly serves to avoid the
+ * calculations.  First we check whether moving the journal would be a
+ * significant improvement.  That means that a) the current journal segments
+ * have more wear than the future journal segments and b) the current journal
+ * segments have more wear than normal ostore segments.
+ * Rationale for b) is that we don't have to move the journal if it is aging
+ * less than the ostore, even if the reserve segments age even less (they are
+ * excluded from wear leveling, after all).
+ * Next we check that the superblocks have less wear than the journal.  Since
+ * moving the journal requires writing the superblocks, we have to protect the
+ * superblocks even more than the journal.
+ *
+ * Also we double the acceptable wear difference, compared to ostore wear
+ * leveling.  Journal data is read and rewritten rapidly, comparatively.  So
+ * soft errors have much less time to accumulate and we allow the journal to
+ * be a bit worse than the ostore.
+ */
+static void logfs_journal_wl_pass(struct super_block *sb)
+{
+	struct logfs_super *super = logfs_super(sb);
+	struct gc_candidate *cand;
+	u32 min_journal_ec = -1, max_reserve_ec = 0;
+	int i;
+
+	if (wl_ratelimit(sb, &super->s_wl_gec_journal))
+		return;
+
+	if (super->s_reserve_list.count < super->s_no_journal_segs) {
+		/* Reserve is not full enough to move complete journal */
+		return;
+	}
+
+	journal_for_each(i)
+		if (super->s_journal_seg[i])
+			min_journal_ec = min(min_journal_ec,
+					super->s_journal_ec[i]);
+	cand = rb_entry(rb_first(&super->s_free_list.rb_tree),
+			struct gc_candidate, rb_node);
+	max_reserve_ec = cand->erase_count;
+	for (i = 0; i < 2; i++) {
+		struct logfs_segment_entry se;
+		u32 segno = seg_no(sb, super->s_sb_ofs[i]);
+		u32 ec;
+
+		logfs_get_segment_entry(sb, segno, &se);
+		ec = be32_to_cpu(se.ec_level) >> 4;
+		max_reserve_ec = max(max_reserve_ec, ec);
+	}
+
+	if (min_journal_ec > max_reserve_ec + 2 * WL_DELTA) {
+		do_logfs_journal_wl_pass(sb);
+	}
+}
+
+void logfs_gc_pass(struct super_block *sb)
+{
+	struct logfs_super *super = logfs_super(sb);
+
+	//BUG_ON(mutex_trylock(&logfs_super(sb)->s_w_mutex));
+	/* Write journal before free space is getting saturated with dirty
+	 * objects.
+	 */
+	if (super->s_dirty_used_bytes + super->s_dirty_free_bytes
+			+ LOGFS_MAX_OBJECTSIZE >= super->s_free_bytes)
+		logfs_write_anchor(super->s_master_inode);
+	__logfs_gc_pass(sb, logfs_super(sb)->s_total_levels);
+	logfs_wl_pass(sb);
+	logfs_journal_wl_pass(sb);
+}
+
+static int check_area(struct super_block *sb, int i)
+{
+	struct logfs_super *super = logfs_super(sb);
+	struct logfs_area *area = super->s_area[i];
+	struct logfs_object_header oh;
+	u32 segno = area->a_segno;
+	u32 ofs = area->a_used_bytes;
+	__be32 crc;
+	int err;
+
+	if (!area->a_is_open)
+		return 0;
+
+	for (ofs = area->a_used_bytes;
+	     ofs <= super->s_segsize - sizeof(oh);
+	     ofs += (u32)be16_to_cpu(oh.len) + sizeof(oh)) {
+		err = wbuf_read(sb, dev_ofs(sb, segno, ofs), sizeof(oh), &oh);
+		if (err)
+			return err;
+
+		if (!memchr_inv(&oh, 0xff, sizeof(oh)))
+			break;
+
+		crc = logfs_crc32(&oh, sizeof(oh) - 4, 4);
+		if (crc != oh.crc) {
+			printk(KERN_INFO "interrupted header at %llx\n",
+					dev_ofs(sb, segno, ofs));
+			return 0;
+		}
+	}
+	if (ofs != area->a_used_bytes) {
+		printk(KERN_INFO "%x bytes unaccounted data found at %llx\n",
+				ofs - area->a_used_bytes,
+				dev_ofs(sb, segno, area->a_used_bytes));
+		area->a_used_bytes = ofs;
+	}
+	return 0;
+}
+
+int logfs_check_areas(struct super_block *sb)
+{
+	int i, err;
+
+	for_each_area(i) {
+		err = check_area(sb, i);
+		if (err)
+			return err;
+	}
+	return 0;
+}
+
+static void logfs_init_candlist(struct candidate_list *list, int maxcount,
+		int sort_by_ec)
+{
+	list->count = 0;
+	list->maxcount = maxcount;
+	list->sort_by_ec = sort_by_ec;
+	list->rb_tree = RB_ROOT;
+}
+
+int logfs_init_gc(struct super_block *sb)
+{
+	struct logfs_super *super = logfs_super(sb);
+	int i;
+
+	btree_init_mempool32(&super->s_cand_tree, super->s_btree_pool);
+	logfs_init_candlist(&super->s_free_list, LIST_SIZE + SCAN_RATIO, 1);
+	logfs_init_candlist(&super->s_reserve_list,
+			super->s_bad_seg_reserve, 1);
+	for_each_area(i)
+		logfs_init_candlist(&super->s_low_list[i], LIST_SIZE, 0);
+	logfs_init_candlist(&super->s_ec_list, LIST_SIZE, 1);
+	return 0;
+}
+
+static void logfs_cleanup_list(struct super_block *sb,
+		struct candidate_list *list)
+{
+	struct gc_candidate *cand;
+
+	while (list->count) {
+		cand = rb_entry(list->rb_tree.rb_node, struct gc_candidate,
+				rb_node);
+		remove_from_list(cand);
+		free_candidate(sb, cand);
+	}
+	BUG_ON(list->rb_tree.rb_node);
+}
+
+void logfs_cleanup_gc(struct super_block *sb)
+{
+	struct logfs_super *super = logfs_super(sb);
+	int i;
+
+	if (!super->s_free_list.count)
+		return;
+
+	/*
+	 * FIXME: The btree may still contain a single empty node.  So we
+	 * call the grim visitor to clean up that mess.  Btree code should
+	 * do it for us, really.
+	 */
+	btree_grim_visitor32(&super->s_cand_tree, 0, NULL);
+	logfs_cleanup_list(sb, &super->s_free_list);
+	logfs_cleanup_list(sb, &super->s_reserve_list);
+	for_each_area(i)
+		logfs_cleanup_list(sb, &super->s_low_list[i]);
+	logfs_cleanup_list(sb, &super->s_ec_list);
+}