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

[Qu Wenruo]

在 2026/8/21 20:08, Breno Leitao 写道:
> 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.

How much faster?

We are using write_trylock() already, meaning if it's not locked we 
should get the lock immediately, otherwise we skip the inode.

So the lock contention should be low already.

Furthermore, if there are some hidden concurrency bugs, it will be very 
hard to debug.

I strongly prefer to stick to the existing locking scheme, unless you 
have a very strong argument not to.

> Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
> ---
>   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 <leitao@xxxxxxxxxx>