[PATCH] btrfs: skip the extent map tree lock for inodes without extent maps

The shrinker (find_first_inode_to_shrink()) takes inode->extent_tree.lock in write
mode on every inode it walks, only to find out whether that inode has
any extent maps.

Most have none, from what I understand, so, avoid the lock  by testing the
tree with a plain read before taking the lock. tree->root is only
modified with the tree lock held for write, so the unlocked read is
a benign race: a false negative just defers the inode to a later scan.

On my tests, find_first_inode_to_shrink() was a bit faster, so, if this
patch is correct, I think it is worth having to reduce lock contention.

Signed-off-by: Breno Leitao <[email protected]>
---
 fs/btrfs/extent_map.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c
index 6ad7b39ae358b..cdcd2b779050d 100644
--- a/fs/btrfs/extent_map.c
+++ b/fs/btrfs/extent_map.c
@@ -1219,6 +1219,9 @@ static struct btrfs_inode *find_first_inode_to_shrink(struct btrfs_root *root,
 
 		tree = &inode->extent_tree;
 
+		if (!READ_ONCE(tree->root.rb_node))
+			goto next;
+
 		/*
 		 * We want to be fast so if the lock is busy we don't want to
 		 * spend time waiting for it (some task is about to do IO for

---
base-commit: 6a746cd265aed59107ebdaa9ce039bb832922969
change-id: 20260820-b4-btrfs-em-shrinker-7382d7f0dd05

Best regards,
--  
Breno Leitao <[email protected]>