Update bcachefs sources to 2f4e24d856 bcachefs: Split out dev_buckets_free()

1 files changed, 891 insertions, 0 deletions

diff --git a/libbcachefs/backpointers.c b/libbcachefs/backpointers.c
new file mode 100644
index 0000000..f3260bb
--- /dev/null
+++ b/libbcachefs/backpointers.c
@@ -0,0 +1,891 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "bcachefs.h"
+#include "alloc_background.h"
+#include "backpointers.h"
+#include "btree_cache.h"
+#include "btree_update.h"
+#include "error.h"
+
+#define MAX_EXTENT_COMPRESS_RATIO_SHIFT		10
+
+/*
+ * Convert from pos in backpointer btree to pos of corresponding bucket in alloc
+ * btree:
+ */
+static inline struct bpos bp_pos_to_bucket(const struct bch_fs *c,
+					   struct bpos bp_pos)
+{
+	struct bch_dev *ca = bch_dev_bkey_exists(c, bp_pos.inode);
+	u64 bucket_sector = bp_pos.offset >> MAX_EXTENT_COMPRESS_RATIO_SHIFT;
+
+	return POS(bp_pos.inode, sector_to_bucket(ca, bucket_sector));
+}
+
+/*
+ * Convert from pos in alloc btree + bucket offset to pos in backpointer btree:
+ */
+static inline struct bpos bucket_pos_to_bp(const struct bch_fs *c,
+					   struct bpos bucket,
+					   u64 bucket_offset)
+{
+	struct bch_dev *ca = bch_dev_bkey_exists(c, bucket.inode);
+
+	return POS(bucket.inode,
+		   (bucket_to_sector(ca, bucket.offset) <<
+		    MAX_EXTENT_COMPRESS_RATIO_SHIFT) + bucket_offset);
+}
+
+void bch2_extent_ptr_to_bp(struct bch_fs *c,
+			   enum btree_id btree_id, unsigned level,
+			   struct bkey_s_c k, struct extent_ptr_decoded p,
+			   struct bpos *bucket_pos, struct bch_backpointer *bp)
+{
+	enum bch_data_type data_type = level ? BCH_DATA_btree : BCH_DATA_user;
+	s64 sectors = level ? btree_sectors(c) : k.k->size;
+	u32 bucket_offset;
+
+	*bucket_pos = PTR_BUCKET_POS_OFFSET(c, &p.ptr, &bucket_offset);
+	*bp = (struct bch_backpointer) {
+		.btree_id	= btree_id,
+		.level		= level,
+		.data_type	= data_type,
+		.bucket_offset	= ((u64) bucket_offset << MAX_EXTENT_COMPRESS_RATIO_SHIFT) +
+			p.crc.offset,
+		.bucket_len	= ptr_disk_sectors(sectors, p),
+		.pos		= k.k->p,
+	};
+}
+
+static bool extent_matches_bp(struct bch_fs *c,
+			      enum btree_id btree_id, unsigned level,
+			      struct bkey_s_c k,
+			      struct bpos bucket,
+			      struct bch_backpointer bp)
+{
+	struct bkey_ptrs_c ptrs = bch2_bkey_ptrs_c(k);
+	const union bch_extent_entry *entry;
+	struct extent_ptr_decoded p;
+
+	bkey_for_each_ptr_decode(k.k, ptrs, p, entry) {
+		struct bpos bucket2;
+		struct bch_backpointer bp2;
+
+		if (p.ptr.cached)
+			continue;
+
+		bch2_extent_ptr_to_bp(c, btree_id, level, k, p,
+				      &bucket2, &bp2);
+		if (!bpos_cmp(bucket, bucket2) &&
+		    !memcmp(&bp, &bp2, sizeof(bp)))
+			return true;
+	}
+
+	return false;
+}
+
+int bch2_backpointer_invalid(const struct bch_fs *c, struct bkey_s_c k,
+			     int rw, struct printbuf *err)
+{
+	struct bkey_s_c_backpointer bp = bkey_s_c_to_backpointer(k);
+	struct bpos bucket = bp_pos_to_bucket(c, bp.k->p);
+
+	if (bkey_val_bytes(bp.k) < sizeof(*bp.v)) {
+		prt_str(err, "incorrect value size");
+		return -EINVAL;
+	}
+
+	if (bpos_cmp(bp.k->p, bucket_pos_to_bp(c, bucket, bp.v->bucket_offset))) {
+		prt_str(err, "backpointer at wrong pos");
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+void bch2_backpointer_to_text(struct printbuf *out, const struct bch_backpointer *bp)
+{
+	prt_printf(out, "btree=%s l=%u offset=%llu:%u len=%u pos=",
+	       bch2_btree_ids[bp->btree_id],
+	       bp->level,
+	       (u64) (bp->bucket_offset >> MAX_EXTENT_COMPRESS_RATIO_SHIFT),
+	       (u32) bp->bucket_offset & ~(~0U << MAX_EXTENT_COMPRESS_RATIO_SHIFT),
+	       bp->bucket_len);
+	bch2_bpos_to_text(out, bp->pos);
+}
+
+void bch2_backpointer_k_to_text(struct printbuf *out, struct bch_fs *c, struct bkey_s_c k)
+{
+	bch2_backpointer_to_text(out, bkey_s_c_to_backpointer(k).v);
+}
+
+void bch2_backpointer_swab(struct bkey_s k)
+{
+	struct bkey_s_backpointer bp = bkey_s_to_backpointer(k);
+
+	bp.v->bucket_offset	= swab32(bp.v->bucket_offset);
+	bp.v->bucket_len	= swab32(bp.v->bucket_len);
+	bch2_bpos_swab(&bp.v->pos);
+}
+
+#define BACKPOINTER_OFFSET_MAX	((1ULL << 40) - 1)
+
+static inline int backpointer_cmp(struct bch_backpointer l, struct bch_backpointer r)
+{
+	return cmp_int(l.bucket_offset, r.bucket_offset);
+}
+
+static int bch2_backpointer_del_by_offset(struct btree_trans *trans,
+					  struct bpos bucket,
+					  u64 bp_offset,
+					  struct bch_backpointer bp)
+{
+	struct bch_fs *c = trans->c;
+	struct btree_iter iter;
+	struct bkey_s_c k;
+	int ret;
+
+	if (bp_offset < BACKPOINTER_OFFSET_MAX) {
+		struct bch_backpointer *bps;
+		struct bkey_i_alloc_v4 *a;
+		unsigned i, nr;
+
+		bch2_trans_iter_init(trans, &iter, BTREE_ID_alloc,
+				     bucket,
+				     BTREE_ITER_INTENT|
+				     BTREE_ITER_SLOTS|
+				     BTREE_ITER_WITH_UPDATES);
+		k = bch2_btree_iter_peek_slot(&iter);
+		ret = bkey_err(k);
+		if (ret)
+			goto err;
+
+		if (k.k->type != KEY_TYPE_alloc_v4) {
+			ret = -ENOENT;
+			goto err;
+		}
+
+		a = bch2_alloc_to_v4_mut(trans, k);
+		ret = PTR_ERR_OR_ZERO(a);
+		if (ret)
+			goto err;
+		bps = alloc_v4_backpointers(&a->v);
+		nr = BCH_ALLOC_V4_NR_BACKPOINTERS(&a->v);
+
+		for (i = 0; i < nr; i++) {
+			if (bps[i].bucket_offset == bp_offset)
+				goto found;
+			if (bps[i].bucket_offset > bp_offset)
+				break;
+		}
+
+		ret = -ENOENT;
+		goto err;
+found:
+		if (memcmp(&bps[i], &bp, sizeof(bp))) {
+			ret = -ENOENT;
+			goto err;
+		}
+		array_remove_item(bps, nr, i);
+		SET_BCH_ALLOC_V4_NR_BACKPOINTERS(&a->v, nr);
+		set_alloc_v4_u64s(a);
+		ret = bch2_trans_update(trans, &iter, &a->k_i, 0);
+	} else {
+		bp_offset -= BACKPOINTER_OFFSET_MAX;
+
+		bch2_trans_iter_init(trans, &iter, BTREE_ID_backpointers,
+				     bucket_pos_to_bp(c, bucket, bp_offset),
+				     BTREE_ITER_INTENT|
+				     BTREE_ITER_SLOTS|
+				     BTREE_ITER_WITH_UPDATES);
+		k = bch2_btree_iter_peek_slot(&iter);
+		ret = bkey_err(k);
+		if (ret)
+			goto err;
+
+		if (k.k->type != KEY_TYPE_backpointer ||
+		    memcmp(bkey_s_c_to_backpointer(k).v, &bp, sizeof(bp))) {
+			ret = -ENOENT;
+			goto err;
+		}
+
+		ret = bch2_btree_delete_at(trans, &iter, 0);
+	}
+err:
+	bch2_trans_iter_exit(trans, &iter);
+	return ret;
+}
+
+int bch2_bucket_backpointer_del(struct btree_trans *trans,
+				struct bkey_i_alloc_v4 *a,
+				struct bch_backpointer bp,
+				struct bkey_s_c orig_k)
+{
+	struct bch_fs *c = trans->c;
+	struct bch_backpointer *bps = alloc_v4_backpointers(&a->v);
+	unsigned i, nr = BCH_ALLOC_V4_NR_BACKPOINTERS(&a->v);
+	struct btree_iter bp_iter;
+	struct bkey_s_c k;
+	int ret;
+
+	for (i = 0; i < nr; i++) {
+		int cmp = backpointer_cmp(bps[i], bp) ?:
+			memcmp(&bps[i], &bp, sizeof(bp));
+		if (!cmp)
+			goto found;
+		if (cmp >= 0)
+			break;
+	}
+
+	goto btree;
+found:
+	array_remove_item(bps, nr, i);
+	SET_BCH_ALLOC_V4_NR_BACKPOINTERS(&a->v, nr);
+	set_alloc_v4_u64s(a);
+	return 0;
+btree:
+	bch2_trans_iter_init(trans, &bp_iter, BTREE_ID_backpointers,
+			     bucket_pos_to_bp(c, a->k.p, bp.bucket_offset),
+			     BTREE_ITER_INTENT|
+			     BTREE_ITER_SLOTS|
+			     BTREE_ITER_WITH_UPDATES);
+	k = bch2_btree_iter_peek_slot(&bp_iter);
+	ret = bkey_err(k);
+	if (ret)
+		goto err;
+
+	if (k.k->type != KEY_TYPE_backpointer ||
+	    memcmp(bkey_s_c_to_backpointer(k).v, &bp, sizeof(bp))) {
+		struct printbuf buf = PRINTBUF;
+
+		prt_printf(&buf, "backpointer not found when deleting");
+		prt_newline(&buf);
+		printbuf_indent_add(&buf, 2);
+
+		prt_printf(&buf, "searching for ");
+		bch2_backpointer_to_text(&buf, &bp);
+		prt_newline(&buf);
+
+		prt_printf(&buf, "got ");
+		bch2_bkey_val_to_text(&buf, c, k);
+		prt_newline(&buf);
+
+		prt_str(&buf, "alloc ");
+		bch2_bkey_val_to_text(&buf, c, bkey_i_to_s_c(&a->k_i));
+		prt_newline(&buf);
+
+		prt_printf(&buf, "for ");
+		bch2_bkey_val_to_text(&buf, c, orig_k);
+
+		if (!test_bit(BCH_FS_CHECK_BACKPOINTERS_DONE, &c->flags)) {
+			bch_err(c, "%s", buf.buf);
+		} else {
+			ret = -EIO;
+			bch2_trans_inconsistent(trans, "%s", buf.buf);
+		}
+		printbuf_exit(&buf);
+		goto err;
+	}
+
+	ret = bch2_btree_delete_at(trans, &bp_iter, 0);
+err:
+	bch2_trans_iter_exit(trans, &bp_iter);
+	return ret;
+}
+
+int bch2_bucket_backpointer_add(struct btree_trans *trans,
+				struct bkey_i_alloc_v4 *a,
+				struct bch_backpointer bp,
+				struct bkey_s_c orig_k)
+{
+	struct bch_fs *c = trans->c;
+	struct bch_dev *ca;
+	struct bch_backpointer *bps = alloc_v4_backpointers(&a->v);
+	unsigned i, nr = BCH_ALLOC_V4_NR_BACKPOINTERS(&a->v);
+	struct bkey_i_backpointer *bp_k;
+	struct btree_iter bp_iter;
+	struct bkey_s_c k;
+	int ret;
+
+	/* Check for duplicates: */
+	for (i = 0; i < nr; i++) {
+		int cmp = backpointer_cmp(bps[i], bp);
+		if (cmp >= 0)
+			break;
+	}
+
+	if ((i &&
+	     (bps[i - 1].bucket_offset +
+	      bps[i - 1].bucket_len > bp.bucket_offset)) ||
+	    (i < nr &&
+	     (bp.bucket_offset + bp.bucket_len > bps[i].bucket_offset))) {
+		struct printbuf buf = PRINTBUF;
+
+		prt_printf(&buf, "overlapping backpointer found when inserting ");
+		bch2_backpointer_to_text(&buf, &bp);
+		prt_newline(&buf);
+		printbuf_indent_add(&buf, 2);
+
+		prt_printf(&buf, "into ");
+		bch2_bkey_val_to_text(&buf, c, bkey_i_to_s_c(&a->k_i));
+		prt_newline(&buf);
+
+		prt_printf(&buf, "for ");
+		bch2_bkey_val_to_text(&buf, c, orig_k);
+
+		if (!test_bit(BCH_FS_CHECK_BACKPOINTERS_DONE, &c->flags))
+			bch_err(c, "%s", buf.buf);
+		else {
+			bch2_trans_inconsistent(trans, "%s", buf.buf);
+			printbuf_exit(&buf);
+			return -EIO;
+		}
+	}
+
+	if (nr < BCH_ALLOC_V4_NR_BACKPOINTERS_MAX) {
+		array_insert_item(bps, nr, i, bp);
+		SET_BCH_ALLOC_V4_NR_BACKPOINTERS(&a->v, nr);
+		set_alloc_v4_u64s(a);
+		return 0;
+	}
+
+	/* Overflow: use backpointer btree */
+	bp_k = bch2_trans_kmalloc(trans, sizeof(*bp_k));
+	ret = PTR_ERR_OR_ZERO(bp_k);
+	if (ret)
+		return ret;
+
+	ca = bch_dev_bkey_exists(c, a->k.p.inode);
+
+	bkey_backpointer_init(&bp_k->k_i);
+	bp_k->k.p = bucket_pos_to_bp(c, a->k.p, bp.bucket_offset);
+	bp_k->v = bp;
+
+	bch2_trans_iter_init(trans, &bp_iter, BTREE_ID_backpointers, bp_k->k.p,
+			     BTREE_ITER_INTENT|
+			     BTREE_ITER_SLOTS|
+			     BTREE_ITER_WITH_UPDATES);
+	k = bch2_btree_iter_peek_slot(&bp_iter);
+	ret = bkey_err(k);
+	if (ret)
+		goto err;
+
+	if (k.k->type) {
+		struct printbuf buf = PRINTBUF;
+
+		prt_printf(&buf, "existing btree backpointer key found when inserting ");
+		bch2_backpointer_to_text(&buf, &bp);
+		prt_newline(&buf);
+		printbuf_indent_add(&buf, 2);
+
+		prt_printf(&buf, "found ");
+		bch2_bkey_val_to_text(&buf, c, k);
+		prt_newline(&buf);
+
+		prt_printf(&buf, "for ");
+		bch2_bkey_val_to_text(&buf, c, orig_k);
+
+		if (!test_bit(BCH_FS_CHECK_BACKPOINTERS_DONE, &c->flags))
+			bch_err(c, "%s", buf.buf);
+		else {
+			bch2_trans_inconsistent(trans, "%s", buf.buf);
+			printbuf_exit(&buf);
+			ret = -EIO;
+			goto err;
+		}
+	}
+
+	ret = bch2_trans_update(trans, &bp_iter, &bp_k->k_i, 0);
+err:
+	bch2_trans_iter_exit(trans, &bp_iter);
+	return ret;
+}
+
+/*
+ * Find the next backpointer >= *bp_offset:
+ */
+int bch2_get_next_backpointer(struct btree_trans *trans,
+			      struct bpos bucket, int gen,
+			      u64 *bp_offset,
+			      struct bch_backpointer *dst)
+{
+	struct bch_fs *c = trans->c;
+	struct bpos bp_pos =
+		bucket_pos_to_bp(c, bucket,
+				max(*bp_offset, BACKPOINTER_OFFSET_MAX) - BACKPOINTER_OFFSET_MAX);
+	struct bpos bp_end_pos =
+		bucket_pos_to_bp(c, bpos_nosnap_successor(bucket), 0);
+	struct btree_iter alloc_iter, bp_iter = { NULL };
+	struct bkey_s_c k;
+	struct bkey_s_c_alloc_v4 a;
+	size_t i;
+	int ret;
+
+	bch2_trans_iter_init(trans, &alloc_iter, BTREE_ID_alloc,
+			     bucket, BTREE_ITER_CACHED);
+	k = bch2_btree_iter_peek_slot(&alloc_iter);
+	ret = bkey_err(k);
+	if (ret)
+		goto out;
+
+	if (k.k->type != KEY_TYPE_alloc_v4)
+		goto done;
+
+	a = bkey_s_c_to_alloc_v4(k);
+	if (gen >= 0 && a.v->gen != gen)
+		goto done;
+
+	for (i = 0; i < BCH_ALLOC_V4_NR_BACKPOINTERS(a.v); i++) {
+		if (alloc_v4_backpointers_c(a.v)[i].bucket_offset < *bp_offset)
+			continue;
+
+		*dst = alloc_v4_backpointers_c(a.v)[i];
+		*bp_offset = dst->bucket_offset;
+		goto out;
+	}
+
+	for_each_btree_key(trans, bp_iter, BTREE_ID_backpointers,
+			   bp_pos, 0, k, ret) {
+		if (bpos_cmp(k.k->p, bp_end_pos) >= 0)
+			break;
+
+		if (k.k->type != KEY_TYPE_backpointer)
+			continue;
+
+		*dst = *bkey_s_c_to_backpointer(k).v;
+		*bp_offset = dst->bucket_offset + BACKPOINTER_OFFSET_MAX;
+		goto out;
+	}
+done:
+	*bp_offset = U64_MAX;
+out:
+	bch2_trans_iter_exit(trans, &bp_iter);
+	bch2_trans_iter_exit(trans, &alloc_iter);
+	return ret;
+}
+
+static void backpointer_not_found(struct btree_trans *trans,
+				  struct bpos bucket,
+				  u64 bp_offset,
+				  struct bch_backpointer bp,
+				  struct bkey_s_c k,
+				  const char *thing_it_points_to)
+{
+	struct bch_fs *c = trans->c;
+	struct printbuf buf = PRINTBUF;
+
+	prt_printf(&buf, "backpointer doesn't match %s it points to:\n  ",
+		   thing_it_points_to);
+	prt_printf(&buf, "bucket: ");
+	bch2_bpos_to_text(&buf, bucket);
+	prt_printf(&buf, "\n  ");
+
+	if (bp_offset >= BACKPOINTER_OFFSET_MAX) {
+		struct bpos bp_pos =
+			bucket_pos_to_bp(c, bucket,
+					bp_offset - BACKPOINTER_OFFSET_MAX);
+		prt_printf(&buf, "backpointer pos: ");
+		bch2_bpos_to_text(&buf, bp_pos);
+		prt_printf(&buf, "\n  ");
+	}
+
+	bch2_backpointer_to_text(&buf, &bp);
+	prt_printf(&buf, "\n  ");
+	bch2_bkey_val_to_text(&buf, c, k);
+	if (!test_bit(BCH_FS_CHECK_BACKPOINTERS_DONE, &c->flags))
+		bch_err(c, "%s", buf.buf);
+	else
+		bch2_trans_inconsistent(trans, "%s", buf.buf);
+
+	printbuf_exit(&buf);
+}
+
+struct bkey_s_c bch2_backpointer_get_key(struct btree_trans *trans,
+					 struct btree_iter *iter,
+					 struct bpos bucket,
+					 u64 bp_offset,
+					 struct bch_backpointer bp)
+{
+	struct bch_fs *c = trans->c;
+	struct bkey_s_c k;
+
+	bch2_trans_node_iter_init(trans, iter,
+				  bp.btree_id,
+				  bp.pos,
+				  0,
+				  min(bp.level, c->btree_roots[bp.btree_id].level),
+				  0);
+	k = bch2_btree_iter_peek_slot(iter);
+	if (bkey_err(k)) {
+		bch2_trans_iter_exit(trans, iter);
+		return k;
+	}
+
+	if (bp.level == c->btree_roots[bp.btree_id].level + 1)
+		k = bkey_i_to_s_c(&c->btree_roots[bp.btree_id].key);
+
+	if (extent_matches_bp(c, bp.btree_id, bp.level, k, bucket, bp))
+		return k;
+
+	backpointer_not_found(trans, bucket, bp_offset, bp, k, "extent");
+
+	bch2_trans_iter_exit(trans, iter);
+	return bkey_s_c_null;
+}
+
+struct btree *bch2_backpointer_get_node(struct btree_trans *trans,
+					struct btree_iter *iter,
+					struct bpos bucket,
+					u64 bp_offset,
+					struct bch_backpointer bp)
+{
+	struct bch_fs *c = trans->c;
+	struct btree *b;
+	struct bkey_s_c k;
+
+	BUG_ON(!bp.level);
+
+	bch2_trans_node_iter_init(trans, iter,
+				  bp.btree_id,
+				  bp.pos,
+				  0,
+				  bp.level - 1,
+				  0);
+	b = bch2_btree_iter_peek_node(iter);
+	if (IS_ERR(b)) {
+		bch2_trans_iter_exit(trans, iter);
+		return b;
+	}
+
+	if (extent_matches_bp(c, bp.btree_id, bp.level,
+			      bkey_i_to_s_c(&b->key),
+			      bucket, bp))
+		return b;
+
+	if (!btree_node_will_make_reachable(b))
+		backpointer_not_found(trans, bucket, bp_offset,
+				      bp, k, "btree node");
+
+	bch2_trans_iter_exit(trans, iter);
+	return NULL;
+}
+
+static int bch2_check_btree_backpointer(struct btree_trans *trans, struct btree_iter *bp_iter)
+{
+	struct bch_fs *c = trans->c;
+	struct btree_iter alloc_iter = { NULL };
+	struct bch_dev *ca;
+	struct bkey_s_c k, alloc_k;
+	struct printbuf buf = PRINTBUF;
+	int ret = 0;
+
+	k = bch2_btree_iter_peek(bp_iter);
+	ret = bkey_err(k);
+	if (ret)
+		return ret;
+	if (!k.k)
+		return 0;
+
+	if (fsck_err_on(!bch2_dev_exists2(c, k.k->p.inode), c,
+			"backpointer for mising device:\n%s",
+			(bch2_bkey_val_to_text(&buf, c, k), buf.buf))) {
+		ret = bch2_btree_delete_at(trans, bp_iter, 0);
+		goto out;
+	}
+
+	ca = bch_dev_bkey_exists(c, k.k->p.inode);
+
+	bch2_trans_iter_init(trans, &alloc_iter, BTREE_ID_alloc,
+			     bp_pos_to_bucket(c, k.k->p), 0);
+
+	alloc_k = bch2_btree_iter_peek_slot(&alloc_iter);
+	ret = bkey_err(alloc_k);
+	if (ret)
+		goto out;
+
+	if (fsck_err_on(alloc_k.k->type != KEY_TYPE_alloc_v4, c,
+			"backpointer for nonexistent alloc key: %llu:%llu:0\n%s",
+			alloc_iter.pos.inode, alloc_iter.pos.offset,
+			(bch2_bkey_val_to_text(&buf, c, alloc_k), buf.buf))) {
+		ret = bch2_btree_delete_at(trans, bp_iter, 0);
+		goto out;
+	}
+out:
+fsck_err:
+	bch2_trans_iter_exit(trans, &alloc_iter);
+	printbuf_exit(&buf);
+	return ret;
+}
+
+/* verify that every backpointer has a corresponding alloc key */
+int bch2_check_btree_backpointers(struct bch_fs *c)
+{
+	struct btree_trans trans;
+	struct btree_iter iter;
+	int ret = 0;
+
+	bch2_trans_init(&trans, c, 0, 0);
+	bch2_trans_iter_init(&trans, &iter, BTREE_ID_backpointers, POS_MIN, 0);
+
+	do {
+		ret = __bch2_trans_do(&trans, NULL, NULL,
+				      BTREE_INSERT_LAZY_RW|
+				      BTREE_INSERT_NOFAIL,
+				      bch2_check_btree_backpointer(&trans, &iter));
+		if (ret)
+			break;
+	} while (bch2_btree_iter_advance(&iter));
+
+	bch2_trans_iter_exit(&trans, &iter);
+	bch2_trans_exit(&trans);
+	return ret;
+}
+
+static int check_bp_exists(struct btree_trans *trans,
+			   struct bpos bucket_pos,
+			   struct bch_backpointer bp,
+			   struct bkey_s_c orig_k)
+{
+	struct bch_fs *c = trans->c;
+	struct btree_iter alloc_iter, bp_iter = { NULL };
+	struct printbuf buf = PRINTBUF;
+	struct bkey_s_c alloc_k, bp_k;
+	int ret;
+
+	bch2_trans_iter_init(trans, &alloc_iter, BTREE_ID_alloc, bucket_pos, 0);
+	alloc_k = bch2_btree_iter_peek_slot(&alloc_iter);
+	ret = bkey_err(alloc_k);
+	if (ret)
+		goto err;
+
+	if (alloc_k.k->type == KEY_TYPE_alloc_v4) {
+		struct bkey_s_c_alloc_v4 a = bkey_s_c_to_alloc_v4(alloc_k);
+		const struct bch_backpointer *bps = alloc_v4_backpointers_c(a.v);
+		unsigned i, nr = BCH_ALLOC_V4_NR_BACKPOINTERS(a.v);
+
+		for (i = 0; i < nr; i++) {
+			int cmp = backpointer_cmp(bps[i], bp) ?:
+				memcmp(&bps[i], &bp, sizeof(bp));
+			if (!cmp)
+				goto out;
+			if (cmp >= 0)
+				break;
+		}
+	} else {
+		goto missing;
+	}
+
+	bch2_trans_iter_init(trans, &bp_iter, BTREE_ID_backpointers,
+			     bucket_pos_to_bp(c, bucket_pos, bp.bucket_offset),
+			     0);
+	bp_k = bch2_btree_iter_peek_slot(&bp_iter);
+	ret = bkey_err(bp_k);
+	if (ret)
+		goto err;
+
+	if (bp_k.k->type != KEY_TYPE_backpointer ||
+	    memcmp(bkey_s_c_to_backpointer(bp_k).v, &bp, sizeof(bp)))
+		goto missing;
+out:
+err:
+fsck_err:
+	bch2_trans_iter_exit(trans, &bp_iter);
+	bch2_trans_iter_exit(trans, &alloc_iter);
+	printbuf_exit(&buf);
+	return ret;
+missing:
+	prt_printf(&buf, "missing backpointer for btree=%s l=%u ",
+	       bch2_btree_ids[bp.btree_id], bp.level);
+	bch2_bkey_val_to_text(&buf, c, orig_k);
+	prt_printf(&buf, "\nin alloc key ");
+	bch2_bkey_val_to_text(&buf, c, alloc_k);
+
+	if (c->sb.version < bcachefs_metadata_version_backpointers ||
+	    fsck_err(c, "%s", buf.buf)) {
+		struct bkey_i_alloc_v4 *a = bch2_alloc_to_v4_mut(trans, alloc_k);
+
+		ret   = PTR_ERR_OR_ZERO(a) ?:
+			bch2_bucket_backpointer_add(trans, a, bp, orig_k) ?:
+			bch2_trans_update(trans, &alloc_iter, &a->k_i, 0);
+	}
+
+	goto out;
+}
+
+static int check_extent_to_backpointers(struct btree_trans *trans,
+					struct btree_iter *iter)
+{
+	struct bch_fs *c = trans->c;
+	struct bkey_ptrs_c ptrs;
+	const union bch_extent_entry *entry;
+	struct extent_ptr_decoded p;
+	struct bkey_s_c k;
+	int ret;
+
+	k = bch2_btree_iter_peek_all_levels(iter);
+	ret = bkey_err(k);
+	if (ret)
+		return ret;
+	if (!k.k)
+		return 0;
+
+	ptrs = bch2_bkey_ptrs_c(k);
+	bkey_for_each_ptr_decode(k.k, ptrs, p, entry) {
+		struct bpos bucket_pos;
+		struct bch_backpointer bp;
+
+		if (p.ptr.cached)
+			continue;
+
+		bch2_extent_ptr_to_bp(c, iter->btree_id, iter->path->level,
+				      k, p, &bucket_pos, &bp);
+
+		ret = check_bp_exists(trans, bucket_pos, bp, k);
+		if (ret)
+			return ret;
+	}
+
+	return 0;
+}
+
+static int check_btree_root_to_backpointers(struct btree_trans *trans,
+					    enum btree_id btree_id)
+{
+	struct bch_fs *c = trans->c;
+	struct btree_iter iter;
+	struct btree *b;
+	struct bkey_s_c k;
+	struct bkey_ptrs_c ptrs;
+	struct extent_ptr_decoded p;
+	const union bch_extent_entry *entry;
+	int ret;
+
+	bch2_trans_node_iter_init(trans, &iter, btree_id, POS_MIN, 0,
+				  c->btree_roots[btree_id].level, 0);
+	b = bch2_btree_iter_peek_node(&iter);
+	ret = PTR_ERR_OR_ZERO(b);
+	if (ret)
+		goto err;
+
+	BUG_ON(b != btree_node_root(c, b));
+
+	k = bkey_i_to_s_c(&b->key);
+	ptrs = bch2_bkey_ptrs_c(k);
+	bkey_for_each_ptr_decode(k.k, ptrs, p, entry) {
+		struct bpos bucket_pos;
+		struct bch_backpointer bp;
+
+		if (p.ptr.cached)
+			continue;
+
+		bch2_extent_ptr_to_bp(c, iter.btree_id, iter.path->level + 1,
+				      k, p, &bucket_pos, &bp);
+
+		ret = check_bp_exists(trans, bucket_pos, bp, k);
+		if (ret)
+			goto err;
+	}
+err:
+	bch2_trans_iter_exit(trans, &iter);
+	return ret;
+}
+
+int bch2_check_extents_to_backpointers(struct bch_fs *c)
+{
+	struct btree_trans trans;
+	struct btree_iter iter;
+	enum btree_id btree_id;
+	int ret = 0;
+
+	bch2_trans_init(&trans, c, 0, 0);
+	for (btree_id = 0; btree_id < BTREE_ID_NR; btree_id++) {
+		bch2_trans_node_iter_init(&trans, &iter, btree_id, POS_MIN, 0,
+					  0,
+					  BTREE_ITER_ALL_LEVELS|
+					  BTREE_ITER_PREFETCH);
+
+		do {
+			ret = __bch2_trans_do(&trans, NULL, NULL,
+					      BTREE_INSERT_LAZY_RW|
+					      BTREE_INSERT_NOFAIL,
+					      check_extent_to_backpointers(&trans, &iter));
+			if (ret)
+				break;
+		} while (!bch2_btree_iter_advance(&iter));
+
+		bch2_trans_iter_exit(&trans, &iter);
+
+		if (ret)
+			break;
+
+		ret = __bch2_trans_do(&trans, NULL, NULL,
+				      BTREE_INSERT_LAZY_RW|
+				      BTREE_INSERT_NOFAIL,
+				      check_btree_root_to_backpointers(&trans, btree_id));
+		if (ret)
+			break;
+	}
+	bch2_trans_exit(&trans);
+	return ret;
+}
+
+static int check_one_backpointer(struct btree_trans *trans,
+				 struct bpos bucket,
+				 u64 *bp_offset)
+{
+	struct btree_iter iter;
+	struct bch_backpointer bp;
+	struct bkey_s_c k;
+	struct printbuf buf = PRINTBUF;
+	int ret;
+
+	ret = bch2_get_next_backpointer(trans, bucket, -1,
+					bp_offset, &bp);
+	if (ret || *bp_offset == U64_MAX)
+		return ret;
+
+	k = bch2_backpointer_get_key(trans, &iter, bucket, *bp_offset, bp);
+	ret = bkey_err(k);
+	if (ret)
+		return ret;
+
+	if (fsck_err_on(!k.k, trans->c,
+			"%s backpointer points to missing extent\n%s",
+			*bp_offset < BACKPOINTER_OFFSET_MAX ? "alloc" : "btree",
+			(bch2_backpointer_to_text(&buf, &bp), buf.buf))) {
+		ret = bch2_backpointer_del_by_offset(trans, bucket, *bp_offset, bp);
+		if (ret == -ENOENT)
+			bch_err(trans->c, "backpointer at %llu not found", *bp_offset);
+	}
+
+	bch2_trans_iter_exit(trans, &iter);
+fsck_err:
+	printbuf_exit(&buf);
+	return ret;
+}
+
+int bch2_check_backpointers_to_extents(struct bch_fs *c)
+{
+	struct btree_trans trans;
+	struct btree_iter iter;
+	struct bkey_s_c k;
+	int ret = 0;
+
+	bch2_trans_init(&trans, c, 0, 0);
+	for_each_btree_key(&trans, iter, BTREE_ID_alloc, POS_MIN,
+			   BTREE_ITER_PREFETCH, k, ret) {
+		u64 bp_offset = 0;
+
+		while (!(ret = __bch2_trans_do(&trans, NULL, NULL,
+					       BTREE_INSERT_LAZY_RW|
+					       BTREE_INSERT_NOFAIL,
+				check_one_backpointer(&trans, iter.pos, &bp_offset))) &&
+		       bp_offset < U64_MAX)
+			bp_offset++;
+
+		if (ret)
+			break;
+	}
+	bch2_trans_iter_exit(&trans, &iter);
+	bch2_trans_exit(&trans);
+	return ret < 0 ? ret : 0;
+}