btrfs: backref: distinguish reloc and non-reloc use of indirect resolution

For relocation tree detection, relocation backref cache uses
btrfs_should_ignore_reloc_root() which uses relocation-specific checks
like checking the DEAD_RELOC_ROOT bit.

However for general purpose backref cache, we can rely on that check, as
it's possible that relocation is also running.

For generic purposed backref cache, we detect reloc root by
SHARED_BLOCK_REF item.  Only reloc root node has its parent bytenr
pointing back to itself.

And in that case, backref cache will mark the reloc root node useless,
dropping any child orphan nodes.

So only call btrfs_should_ignore_reloc_root() if the backref cache is
for relocation.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/backref.c

@@ -2708,7 +2708,17 @@ static int handle_indirect_tree_backref(struct btrfs_backref_cache *cache,
 	if (btrfs_root_level(&root->root_item) == cur->level) {
 		/* Tree root */
 		ASSERT(btrfs_root_bytenr(&root->root_item) == cur->bytenr);
-		if (btrfs_should_ignore_reloc_root(root)) {
+		/*
+		 * For reloc backref cache, we may ignore reloc root.  But for
+		 * general purpose backref cache, we can't rely on
+		 * btrfs_should_ignore_reloc_root() as it may conflict with
+		 * current running relocation and lead to missing root.
+		 *
+		 * For general purpose backref cache, reloc root detection is
+		 * completely relying on direct backref (key->offset is parent
+		 * bytenr), thus only do such check for reloc cache.
+		 */
+		if (btrfs_should_ignore_reloc_root(root) && cache->is_reloc) {
 			btrfs_put_root(root);
 			list_add(&cur->list, &cache->useless_node);
 		} else {
@@ -2749,7 +2759,9 @@ static int handle_indirect_tree_backref(struct btrfs_backref_cache *cache,
 		if (!path->nodes[level]) {
 			ASSERT(btrfs_root_bytenr(&root->root_item) ==
 			       lower->bytenr);
-			if (btrfs_should_ignore_reloc_root(root)) {
+			/* Same as previous should_ignore_reloc_root() call */
+			if (btrfs_should_ignore_reloc_root(root) &&
+			    cache->is_reloc) {
 				btrfs_put_root(root);
 				list_add(&lower->list, &cache->useless_node);
 			} else {