Merge tag 'bcachefs-2023-10-30' of https://evilpiepirate.org/git/bcachefs

Pull initial bcachefs updates from Kent Overstreet:
 "Here's the bcachefs filesystem pull request.

  One new patch since last week: the exportfs constants ended up
  conflicting with other filesystems that are also getting added to the
  global enum, so switched to new constants picked by Amir.

  The only new non fs/bcachefs/ patch is the objtool patch that adds
  bcachefs functions to the list of noreturns. The patch that exports
  osq_lock() has been dropped for now, per Ingo"

* tag 'bcachefs-2023-10-30' of https://evilpiepirate.org/git/bcachefs: (2781 commits)
  exportfs: Change bcachefs fid_type enum to avoid conflicts
  bcachefs: Refactor memcpy into direct assignment
  bcachefs: Fix drop_alloc_keys()
  bcachefs: snapshot_create_lock
  bcachefs: Fix snapshot skiplists during snapshot deletion
  bcachefs: bch2_sb_field_get() refactoring
  bcachefs: KEY_TYPE_error now counts towards i_sectors
  bcachefs: Fix handling of unknown bkey types
  bcachefs: Switch to unsafe_memcpy() in a few places
  bcachefs: Use struct_size()
  bcachefs: Correctly initialize new buckets on device resize
  bcachefs: Fix another smatch complaint
  bcachefs: Use strsep() in split_devs()
  bcachefs: Add iops fields to bch_member
  bcachefs: Rename bch_sb_field_members -> bch_sb_field_members_v1
  bcachefs: New superblock section members_v2
  bcachefs: Add new helper to retrieve bch_member from sb
  bcachefs: bucket_lock() is now a sleepable lock
  bcachefs: fix crc32c checksum merge byte order problem
  bcachefs: Fix bch2_inode_delete_keys()
  ...

1 files changed, 868 insertions, 0 deletions

diff --git a/fs/bcachefs/backpointers.c b/fs/bcachefs/backpointers.c
new file mode 100644
index 000000000000..cc856150a948
--- /dev/null
+++ b/fs/bcachefs/backpointers.c
@@ -0,0 +1,868 @@
+// 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_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(const 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);
+
+	if (!bpos_eq(bp.k->p, bucket_pos_to_bp(c, bucket, bp.v->bucket_offset))) {
+		prt_str(err, "backpointer at wrong pos");
+		return -BCH_ERR_invalid_bkey;
+	}
+
+	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)
+{
+	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	= swab32(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,
+				  const char *thing_it_points_to)
+{
+	struct bch_fs *c = trans->c;
+	struct printbuf buf = PRINTBUF;
+	struct bpos bucket = bp_pos_to_bucket(c, bp_pos);
+
+	if (likely(!bch2_backpointers_no_use_write_buffer))
+		return;
+
+	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  ");
+
+	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)
+{
+	struct bch_fs *c = trans->c;
+	struct btree_root *r = bch2_btree_id_root(c, bp.btree_id);
+	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,
+				  min(bp.level, r->level),
+				  iter_flags);
+	k = bch2_btree_iter_peek_slot(iter);
+	if (bkey_err(k)) {
+		bch2_trans_iter_exit(trans, iter);
+		return k;
+	}
+
+	if (bp.level == r->level + 1)
+		k = bkey_i_to_s_c(&r->key);
+
+	if (k.k && extent_matches_bp(c, bp.btree_id, bp.level, k, bucket, bp))
+		return k;
+
+	bch2_trans_iter_exit(trans, iter);
+
+	if (unlikely(bch2_backpointers_no_use_write_buffer)) {
+		if (bp.level) {
+			struct btree *b;
+
+			/*
+			 * If a backpointer for a btree node wasn't found, it may be
+			 * because it was overwritten by a new btree node that hasn't
+			 * been written out yet - backpointer_get_node() checks for
+			 * this:
+			 */
+			b = bch2_backpointer_get_node(trans, iter, bp_pos, bp);
+			if (!IS_ERR_OR_NULL(b))
+				return bkey_i_to_s_c(&b->key);
+
+			bch2_trans_iter_exit(trans, iter);
+
+			if (IS_ERR(b))
+				return bkey_s_c_err(PTR_ERR(b));
+			return bkey_s_c_null;
+		}
+
+		backpointer_not_found(trans, bp_pos, bp, k, "extent");
+	}
+
+	return bkey_s_c_null;
+}
+
+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(b))
+		goto err;
+
+	if (b && extent_matches_bp(c, bp.btree_id, bp.level,
+				   bkey_i_to_s_c(&b->key),
+				   bucket, bp))
+		return b;
+
+	if (b && 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), "btree node");
+		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 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 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_ids[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, "%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 for missing extent\n  %s",
+			(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;
+}