Merging upstream version 6.7.7.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 860 insertions, 0 deletions

diff --git a/fs/bcachefs/backpointers.c b/fs/bcachefs/backpointers.c
new file mode 100644
index 0000000000..23c0834a97
--- /dev/null
+++ b/fs/bcachefs/backpointers.c
@@ -0,0 +1,860 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "bcachefs.h"
+#include "bbpos.h"
+#include "alloc_background.h"
+#include "backpointers.h"
+#include "btree_cache.h"
+#include "btree_update.h"
+#include "btree_update_interior.h"
+#include "btree_write_buffer.h"
+#include "error.h"
+
+#include <linux/mm.h>
+
+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_eq(bucket, bucket2) &&
+		    !memcmp(&bp, &bp2, sizeof(bp)))
+			return true;
+	}
+
+	return false;
+}
+
+int bch2_backpointer_invalid(struct bch_fs *c, struct bkey_s_c k,
+			     enum bkey_invalid_flags flags,
+			     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);
+	int ret = 0;
+
+	bkey_fsck_err_on(!bpos_eq(bp.k->p, bucket_pos_to_bp(c, bucket, bp.v->bucket_offset)),
+			 c, err,
+			 backpointer_pos_wrong,
+			 "backpointer at wrong pos");
+fsck_err:
+	return ret;
+}
+
+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_id_str(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)
+{
+	prt_str(out, "bucket=");
+	bch2_bpos_to_text(out, bp_pos_to_bucket(c, k.k->p));
+	prt_str(out, " ");
+
+	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	= swab40(bp.v->bucket_offset);
+	bp.v->bucket_len	= swab32(bp.v->bucket_len);
+	bch2_bpos_swab(&bp.v->pos);
+}
+
+static noinline int backpointer_mod_err(struct btree_trans *trans,
+					struct bch_backpointer bp,
+					struct bkey_s_c bp_k,
+					struct bkey_s_c orig_k,
+					bool insert)
+{
+	struct bch_fs *c = trans->c;
+	struct printbuf buf = PRINTBUF;
+
+	if (insert) {
+		prt_printf(&buf, "existing backpointer 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, bp_k);
+		prt_newline(&buf);
+
+		prt_printf(&buf, "for ");
+		bch2_bkey_val_to_text(&buf, c, orig_k);
+
+		bch_err(c, "%s", buf.buf);
+	} else if (c->curr_recovery_pass > BCH_RECOVERY_PASS_check_extents_to_backpointers) {
+		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, bp_k);
+		prt_newline(&buf);
+
+		prt_printf(&buf, "for ");
+		bch2_bkey_val_to_text(&buf, c, orig_k);
+
+		bch_err(c, "%s", buf.buf);
+	}
+
+	printbuf_exit(&buf);
+
+	if (c->curr_recovery_pass > BCH_RECOVERY_PASS_check_extents_to_backpointers) {
+		bch2_inconsistent_error(c);
+		return -EIO;
+	} else {
+		return 0;
+	}
+}
+
+int bch2_bucket_backpointer_mod_nowritebuffer(struct btree_trans *trans,
+				struct bkey_i_backpointer *bp_k,
+				struct bch_backpointer bp,
+				struct bkey_s_c orig_k,
+				bool insert)
+{
+	struct btree_iter bp_iter;
+	struct bkey_s_c k;
+	int ret;
+
+	k = bch2_bkey_get_iter(trans, &bp_iter, BTREE_ID_backpointers,
+			       bp_k->k.p,
+			       BTREE_ITER_INTENT|
+			       BTREE_ITER_SLOTS|
+			       BTREE_ITER_WITH_UPDATES);
+	ret = bkey_err(k);
+	if (ret)
+		goto err;
+
+	if (insert
+	    ? k.k->type
+	    : (k.k->type != KEY_TYPE_backpointer ||
+	       memcmp(bkey_s_c_to_backpointer(k).v, &bp, sizeof(bp)))) {
+		ret = backpointer_mod_err(trans, bp, k, orig_k, insert);
+		if (ret)
+			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,
+			      struct bpos *bp_pos,
+			      struct bch_backpointer *bp,
+			      unsigned iter_flags)
+{
+	struct bch_fs *c = trans->c;
+	struct bpos bp_end_pos = bucket_pos_to_bp(c, bpos_nosnap_successor(bucket), 0);
+	struct btree_iter alloc_iter = { NULL }, bp_iter = { NULL };
+	struct bkey_s_c k;
+	int ret = 0;
+
+	if (bpos_ge(*bp_pos, bp_end_pos))
+		goto done;
+
+	if (gen >= 0) {
+		k = bch2_bkey_get_iter(trans, &alloc_iter, BTREE_ID_alloc,
+				       bucket, BTREE_ITER_CACHED|iter_flags);
+		ret = bkey_err(k);
+		if (ret)
+			goto out;
+
+		if (k.k->type != KEY_TYPE_alloc_v4 ||
+		    bkey_s_c_to_alloc_v4(k).v->gen != gen)
+			goto done;
+	}
+
+	*bp_pos = bpos_max(*bp_pos, bucket_pos_to_bp(c, bucket, 0));
+
+	for_each_btree_key_norestart(trans, bp_iter, BTREE_ID_backpointers,
+				     *bp_pos, iter_flags, k, ret) {
+		if (bpos_ge(k.k->p, bp_end_pos))
+			break;
+
+		*bp_pos = k.k->p;
+		*bp = *bkey_s_c_to_backpointer(k).v;
+		goto out;
+	}
+done:
+	*bp_pos = SPOS_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 bp_pos,
+				  struct bch_backpointer bp,
+				  struct bkey_s_c k)
+{
+	struct bch_fs *c = trans->c;
+	struct printbuf buf = PRINTBUF;
+	struct bpos bucket = bp_pos_to_bucket(c, bp_pos);
+
+	/*
+	 * If we're using the btree write buffer, the backpointer we were
+	 * looking at may have already been deleted - failure to find what it
+	 * pointed to is not an error:
+	 */
+	if (likely(!bch2_backpointers_no_use_write_buffer))
+		return;
+
+	prt_printf(&buf, "backpointer doesn't match %s it points to:\n  ",
+		   bp.level ? "btree node" : "extent");
+	prt_printf(&buf, "bucket: ");
+	bch2_bpos_to_text(&buf, bucket);
+	prt_printf(&buf, "\n  ");
+
+	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 (c->curr_recovery_pass >= BCH_RECOVERY_PASS_check_extents_to_backpointers)
+		bch_err_ratelimited(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 bp_pos,
+					 struct bch_backpointer bp,
+					 unsigned iter_flags)
+{
+	if (likely(!bp.level)) {
+		struct bch_fs *c = trans->c;
+		struct bpos bucket = bp_pos_to_bucket(c, bp_pos);
+		struct bkey_s_c k;
+
+		bch2_trans_node_iter_init(trans, iter,
+					  bp.btree_id,
+					  bp.pos,
+					  0, 0,
+					  iter_flags);
+		k = bch2_btree_iter_peek_slot(iter);
+		if (bkey_err(k)) {
+			bch2_trans_iter_exit(trans, iter);
+			return k;
+		}
+
+		if (k.k && extent_matches_bp(c, bp.btree_id, bp.level, k, bucket, bp))
+			return k;
+
+		bch2_trans_iter_exit(trans, iter);
+		backpointer_not_found(trans, bp_pos, bp, k);
+		return bkey_s_c_null;
+	} else {
+		struct btree *b = bch2_backpointer_get_node(trans, iter, bp_pos, bp);
+
+		if (IS_ERR_OR_NULL(b)) {
+			bch2_trans_iter_exit(trans, iter);
+			return IS_ERR(b) ? bkey_s_c_err(PTR_ERR(b)) : bkey_s_c_null;
+		}
+		return bkey_i_to_s_c(&b->key);
+	}
+}
+
+struct btree *bch2_backpointer_get_node(struct btree_trans *trans,
+					struct btree_iter *iter,
+					struct bpos bp_pos,
+					struct bch_backpointer bp)
+{
+	struct bch_fs *c = trans->c;
+	struct bpos bucket = bp_pos_to_bucket(c, bp_pos);
+	struct btree *b;
+
+	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_OR_NULL(b))
+		goto err;
+
+	BUG_ON(b->c.level != bp.level - 1);
+
+	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)) {
+		b = ERR_PTR(-BCH_ERR_backpointer_to_overwritten_btree_node);
+	} else {
+		backpointer_not_found(trans, bp_pos, bp, bkey_i_to_s_c(&b->key));
+		b = NULL;
+	}
+err:
+	bch2_trans_iter_exit(trans, iter);
+	return b;
+}
+
+static int bch2_check_btree_backpointer(struct btree_trans *trans, struct btree_iter *bp_iter,
+					struct bkey_s_c k)
+{
+	struct bch_fs *c = trans->c;
+	struct btree_iter alloc_iter = { NULL };
+	struct bkey_s_c alloc_k;
+	struct printbuf buf = PRINTBUF;
+	int ret = 0;
+
+	if (fsck_err_on(!bch2_dev_exists2(c, k.k->p.inode), c,
+			backpointer_to_missing_device,
+			"backpointer for missing device:\n%s",
+			(bch2_bkey_val_to_text(&buf, c, k), buf.buf))) {
+		ret = bch2_btree_delete_at(trans, bp_iter, 0);
+		goto out;
+	}
+
+	alloc_k = bch2_bkey_get_iter(trans, &alloc_iter, BTREE_ID_alloc,
+				     bp_pos_to_bucket(c, k.k->p), 0);
+	ret = bkey_err(alloc_k);
+	if (ret)
+		goto out;
+
+	if (fsck_err_on(alloc_k.k->type != KEY_TYPE_alloc_v4, c,
+			backpointer_to_missing_alloc,
+			"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_iter iter;
+	struct bkey_s_c k;
+	int ret;
+
+	ret = bch2_trans_run(c,
+		for_each_btree_key_commit(trans, iter,
+			BTREE_ID_backpointers, POS_MIN, 0, k,
+			NULL, NULL, BTREE_INSERT_LAZY_RW|BTREE_INSERT_NOFAIL,
+		  bch2_check_btree_backpointer(trans, &iter, k)));
+	if (ret)
+		bch_err_fn(c, ret);
+	return ret;
+}
+
+struct bpos_level {
+	unsigned	level;
+	struct bpos	pos;
+};
+
+static int check_bp_exists(struct btree_trans *trans,
+			   struct bpos bucket,
+			   struct bch_backpointer bp,
+			   struct bkey_s_c orig_k,
+			   struct bpos bucket_start,
+			   struct bpos bucket_end,
+			   struct bpos_level *last_flushed)
+{
+	struct bch_fs *c = trans->c;
+	struct btree_iter bp_iter = { NULL };
+	struct printbuf buf = PRINTBUF;
+	struct bkey_s_c bp_k;
+	int ret;
+
+	if (bpos_lt(bucket, bucket_start) ||
+	    bpos_gt(bucket, bucket_end))
+		return 0;
+
+	if (!bch2_dev_bucket_exists(c, bucket))
+		goto missing;
+
+	bp_k = bch2_bkey_get_iter(trans, &bp_iter, BTREE_ID_backpointers,
+				  bucket_pos_to_bp(c, bucket, bp.bucket_offset),
+				  0);
+	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))) {
+		if (last_flushed->level != bp.level ||
+		    !bpos_eq(last_flushed->pos, orig_k.k->p)) {
+			last_flushed->level = bp.level;
+			last_flushed->pos = orig_k.k->p;
+
+			ret = bch2_btree_write_buffer_flush_sync(trans) ?:
+				-BCH_ERR_transaction_restart_write_buffer_flush;
+			goto out;
+		}
+		goto missing;
+	}
+out:
+err:
+fsck_err:
+	bch2_trans_iter_exit(trans, &bp_iter);
+	printbuf_exit(&buf);
+	return ret;
+missing:
+	prt_printf(&buf, "missing backpointer for btree=%s l=%u ",
+	       bch2_btree_id_str(bp.btree_id), bp.level);
+	bch2_bkey_val_to_text(&buf, c, orig_k);
+	prt_printf(&buf, "\nbp pos ");
+	bch2_bpos_to_text(&buf, bp_iter.pos);
+
+	if (c->sb.version_upgrade_complete < bcachefs_metadata_version_backpointers ||
+	    c->opts.reconstruct_alloc ||
+	    fsck_err(c, ptr_to_missing_backpointer, "%s", buf.buf))
+		ret = bch2_bucket_backpointer_mod(trans, bucket, bp, orig_k, true);
+
+	goto out;
+}
+
+static int check_extent_to_backpointers(struct btree_trans *trans,
+					struct btree_iter *iter,
+					struct bpos bucket_start,
+					struct bpos bucket_end,
+					struct bpos_level *last_flushed)
+{
+	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,
+				      bucket_start, bucket_end,
+				      last_flushed);
+		if (ret)
+			return ret;
+	}
+
+	return 0;
+}
+
+static int check_btree_root_to_backpointers(struct btree_trans *trans,
+					    enum btree_id btree_id,
+					    struct bpos bucket_start,
+					    struct bpos bucket_end,
+					    struct bpos_level *last_flushed)
+{
+	struct bch_fs *c = trans->c;
+	struct btree_root *r = bch2_btree_id_root(c, btree_id);
+	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, r->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, b->c.level + 1,
+				      k, p, &bucket_pos, &bp);
+
+		ret = check_bp_exists(trans, bucket_pos, bp, k,
+				      bucket_start, bucket_end,
+				      last_flushed);
+		if (ret)
+			goto err;
+	}
+err:
+	bch2_trans_iter_exit(trans, &iter);
+	return ret;
+}
+
+static inline struct bbpos bp_to_bbpos(struct bch_backpointer bp)
+{
+	return (struct bbpos) {
+		.btree	= bp.btree_id,
+		.pos	= bp.pos,
+	};
+}
+
+static size_t btree_nodes_fit_in_ram(struct bch_fs *c)
+{
+	struct sysinfo i;
+	u64 mem_bytes;
+
+	si_meminfo(&i);
+	mem_bytes = i.totalram * i.mem_unit;
+	return div_u64(mem_bytes >> 1, btree_bytes(c));
+}
+
+static int bch2_get_btree_in_memory_pos(struct btree_trans *trans,
+					unsigned btree_leaf_mask,
+					unsigned btree_interior_mask,
+					struct bbpos start, struct bbpos *end)
+{
+	struct btree_iter iter;
+	struct bkey_s_c k;
+	size_t btree_nodes = btree_nodes_fit_in_ram(trans->c);
+	enum btree_id btree;
+	int ret = 0;
+
+	for (btree = start.btree; btree < BTREE_ID_NR && !ret; btree++) {
+		unsigned depth = ((1U << btree) & btree_leaf_mask) ? 1 : 2;
+
+		if (!((1U << btree) & btree_leaf_mask) &&
+		    !((1U << btree) & btree_interior_mask))
+			continue;
+
+		bch2_trans_node_iter_init(trans, &iter, btree,
+					  btree == start.btree ? start.pos : POS_MIN,
+					  0, depth, 0);
+		/*
+		 * for_each_btree_key_contineu() doesn't check the return value
+		 * from bch2_btree_iter_advance(), which is needed when
+		 * iterating over interior nodes where we'll see keys at
+		 * SPOS_MAX:
+		 */
+		do {
+			k = __bch2_btree_iter_peek_and_restart(trans, &iter, 0);
+			ret = bkey_err(k);
+			if (!k.k || ret)
+				break;
+
+			--btree_nodes;
+			if (!btree_nodes) {
+				*end = BBPOS(btree, k.k->p);
+				bch2_trans_iter_exit(trans, &iter);
+				return 0;
+			}
+		} while (bch2_btree_iter_advance(&iter));
+		bch2_trans_iter_exit(trans, &iter);
+	}
+
+	*end = BBPOS_MAX;
+	return ret;
+}
+
+static int bch2_check_extents_to_backpointers_pass(struct btree_trans *trans,
+						   struct bpos bucket_start,
+						   struct bpos bucket_end)
+{
+	struct bch_fs *c = trans->c;
+	struct btree_iter iter;
+	enum btree_id btree_id;
+	struct bpos_level last_flushed = { UINT_MAX, POS_MIN };
+	int ret = 0;
+
+	for (btree_id = 0; btree_id < btree_id_nr_alive(c); btree_id++) {
+		unsigned depth = btree_type_has_ptrs(btree_id) ? 0 : 1;
+
+		bch2_trans_node_iter_init(trans, &iter, btree_id, POS_MIN, 0,
+					  depth,
+					  BTREE_ITER_ALL_LEVELS|
+					  BTREE_ITER_PREFETCH);
+
+		do {
+			ret = commit_do(trans, NULL, NULL,
+					BTREE_INSERT_LAZY_RW|
+					BTREE_INSERT_NOFAIL,
+					check_extent_to_backpointers(trans, &iter,
+								bucket_start, bucket_end,
+								&last_flushed));
+			if (ret)
+				break;
+		} while (!bch2_btree_iter_advance(&iter));
+
+		bch2_trans_iter_exit(trans, &iter);
+
+		if (ret)
+			break;
+
+		ret = commit_do(trans, NULL, NULL,
+				BTREE_INSERT_LAZY_RW|
+				BTREE_INSERT_NOFAIL,
+				check_btree_root_to_backpointers(trans, btree_id,
+							bucket_start, bucket_end,
+							&last_flushed));
+		if (ret)
+			break;
+	}
+	return ret;
+}
+
+static struct bpos bucket_pos_to_bp_safe(const struct bch_fs *c,
+					 struct bpos bucket)
+{
+	return bch2_dev_exists2(c, bucket.inode)
+		? bucket_pos_to_bp(c, bucket, 0)
+		: bucket;
+}
+
+static int bch2_get_alloc_in_memory_pos(struct btree_trans *trans,
+					struct bpos start, struct bpos *end)
+{
+	struct btree_iter alloc_iter;
+	struct btree_iter bp_iter;
+	struct bkey_s_c alloc_k, bp_k;
+	size_t btree_nodes = btree_nodes_fit_in_ram(trans->c);
+	bool alloc_end = false, bp_end = false;
+	int ret = 0;
+
+	bch2_trans_node_iter_init(trans, &alloc_iter, BTREE_ID_alloc,
+				  start, 0, 1, 0);
+	bch2_trans_node_iter_init(trans, &bp_iter, BTREE_ID_backpointers,
+				  bucket_pos_to_bp_safe(trans->c, start), 0, 1, 0);
+	while (1) {
+		alloc_k = !alloc_end
+			? __bch2_btree_iter_peek_and_restart(trans, &alloc_iter, 0)
+			: bkey_s_c_null;
+		bp_k = !bp_end
+			? __bch2_btree_iter_peek_and_restart(trans, &bp_iter, 0)
+			: bkey_s_c_null;
+
+		ret = bkey_err(alloc_k) ?: bkey_err(bp_k);
+		if ((!alloc_k.k && !bp_k.k) || ret) {
+			*end = SPOS_MAX;
+			break;
+		}
+
+		--btree_nodes;
+		if (!btree_nodes) {
+			*end = alloc_k.k ? alloc_k.k->p : SPOS_MAX;
+			break;
+		}
+
+		if (bpos_lt(alloc_iter.pos, SPOS_MAX) &&
+		    bpos_lt(bucket_pos_to_bp_safe(trans->c, alloc_iter.pos), bp_iter.pos)) {
+			if (!bch2_btree_iter_advance(&alloc_iter))
+				alloc_end = true;
+		} else {
+			if (!bch2_btree_iter_advance(&bp_iter))
+				bp_end = true;
+		}
+	}
+	bch2_trans_iter_exit(trans, &bp_iter);
+	bch2_trans_iter_exit(trans, &alloc_iter);
+	return ret;
+}
+
+int bch2_check_extents_to_backpointers(struct bch_fs *c)
+{
+	struct btree_trans *trans = bch2_trans_get(c);
+	struct bpos start = POS_MIN, end;
+	int ret;
+
+	while (1) {
+		ret = bch2_get_alloc_in_memory_pos(trans, start, &end);
+		if (ret)
+			break;
+
+		if (bpos_eq(start, POS_MIN) && !bpos_eq(end, SPOS_MAX))
+			bch_verbose(c, "%s(): alloc info does not fit in ram, running in multiple passes with %zu nodes per pass",
+				    __func__, btree_nodes_fit_in_ram(c));
+
+		if (!bpos_eq(start, POS_MIN) || !bpos_eq(end, SPOS_MAX)) {
+			struct printbuf buf = PRINTBUF;
+
+			prt_str(&buf, "check_extents_to_backpointers(): ");
+			bch2_bpos_to_text(&buf, start);
+			prt_str(&buf, "-");
+			bch2_bpos_to_text(&buf, end);
+
+			bch_verbose(c, "%s", buf.buf);
+			printbuf_exit(&buf);
+		}
+
+		ret = bch2_check_extents_to_backpointers_pass(trans, start, end);
+		if (ret || bpos_eq(end, SPOS_MAX))
+			break;
+
+		start = bpos_successor(end);
+	}
+	bch2_trans_put(trans);
+
+	if (ret)
+		bch_err_fn(c, ret);
+	return ret;
+}
+
+static int check_one_backpointer(struct btree_trans *trans,
+				 struct bbpos start,
+				 struct bbpos end,
+				 struct bkey_s_c_backpointer bp,
+				 struct bpos *last_flushed_pos)
+{
+	struct bch_fs *c = trans->c;
+	struct btree_iter iter;
+	struct bbpos pos = bp_to_bbpos(*bp.v);
+	struct bkey_s_c k;
+	struct printbuf buf = PRINTBUF;
+	int ret;
+
+	if (bbpos_cmp(pos, start) < 0 ||
+	    bbpos_cmp(pos, end) > 0)
+		return 0;
+
+	k = bch2_backpointer_get_key(trans, &iter, bp.k->p, *bp.v, 0);
+	ret = bkey_err(k);
+	if (ret == -BCH_ERR_backpointer_to_overwritten_btree_node)
+		return 0;
+	if (ret)
+		return ret;
+
+	if (!k.k && !bpos_eq(*last_flushed_pos, bp.k->p)) {
+		*last_flushed_pos = bp.k->p;
+		ret = bch2_btree_write_buffer_flush_sync(trans) ?:
+			-BCH_ERR_transaction_restart_write_buffer_flush;
+		goto out;
+	}
+
+	if (fsck_err_on(!k.k, c,
+			backpointer_to_missing_ptr,
+			"backpointer for missing %s\n  %s",
+			bp.v->level ? "btree node" : "extent",
+			(bch2_bkey_val_to_text(&buf, c, bp.s_c), buf.buf))) {
+		ret = bch2_btree_delete_at_buffered(trans, BTREE_ID_backpointers, bp.k->p);
+		goto out;
+	}
+out:
+fsck_err:
+	bch2_trans_iter_exit(trans, &iter);
+	printbuf_exit(&buf);
+	return ret;
+}
+
+static int bch2_check_backpointers_to_extents_pass(struct btree_trans *trans,
+						   struct bbpos start,
+						   struct bbpos end)
+{
+	struct btree_iter iter;
+	struct bkey_s_c k;
+	struct bpos last_flushed_pos = SPOS_MAX;
+
+	return for_each_btree_key_commit(trans, iter, BTREE_ID_backpointers,
+				  POS_MIN, BTREE_ITER_PREFETCH, k,
+				  NULL, NULL, BTREE_INSERT_LAZY_RW|BTREE_INSERT_NOFAIL,
+		check_one_backpointer(trans, start, end,
+				      bkey_s_c_to_backpointer(k),
+				      &last_flushed_pos));
+}
+
+int bch2_check_backpointers_to_extents(struct bch_fs *c)
+{
+	struct btree_trans *trans = bch2_trans_get(c);
+	struct bbpos start = (struct bbpos) { .btree = 0, .pos = POS_MIN, }, end;
+	int ret;
+
+	while (1) {
+		ret = bch2_get_btree_in_memory_pos(trans,
+						   (1U << BTREE_ID_extents)|
+						   (1U << BTREE_ID_reflink),
+						   ~0,
+						   start, &end);
+		if (ret)
+			break;
+
+		if (!bbpos_cmp(start, BBPOS_MIN) &&
+		    bbpos_cmp(end, BBPOS_MAX))
+			bch_verbose(c, "%s(): extents do not fit in ram, running in multiple passes with %zu nodes per pass",
+				    __func__, btree_nodes_fit_in_ram(c));
+
+		if (bbpos_cmp(start, BBPOS_MIN) ||
+		    bbpos_cmp(end, BBPOS_MAX)) {
+			struct printbuf buf = PRINTBUF;
+
+			prt_str(&buf, "check_backpointers_to_extents(): ");
+			bch2_bbpos_to_text(&buf, start);
+			prt_str(&buf, "-");
+			bch2_bbpos_to_text(&buf, end);
+
+			bch_verbose(c, "%s", buf.buf);
+			printbuf_exit(&buf);
+		}
+
+		ret = bch2_check_backpointers_to_extents_pass(trans, start, end);
+		if (ret || !bbpos_cmp(end, BBPOS_MAX))
+			break;
+
+		start = bbpos_successor(end);
+	}
+	bch2_trans_put(trans);
+
+	if (ret)
+		bch_err_fn(c, ret);
+	return ret;
+}