Re: [PATCH v2 1/6] btrfs: qgroup: fix swapped blocks existence check when tracing after COW

[Qu Wenruo <wqu@suse.com>]

在 2026/10/4 03:30, fdmanana@kernel.org 写道:
> From: Filipe Manana <fdmanana@suse.com>
> 
> In btrfs_qgroup_trace_subtree_after_cow(), after we erase the swapped
> block, iterate over all possible tree levels to check if we sill have
> other swapped blocks. However the check is wrong, it considers that there
> are other swapped blocks if any rbtree for any level is empty, instead of
> not empty. As a consequence we set blocks->swapped to true basically every
> time since we always find an empty rbtree at least at level 7, since in
> practice it's nearly impossible to find such a huge btree. This makes
> every future call to btrfs_qgroup_trace_subtree_after_cow() do an
> unnecessary rbtree search instead of returning immediately.
> 
> Fix this by updating the condition to set swapped to true if we find an
> rbtree that is not empty.
> 
> Fixes: f616f5cd9da7 ("btrfs: qgroup: Use delayed subtree rescan for balance")
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu

> ---
>   fs/btrfs/ctree.h  | 2 +-
>   fs/btrfs/qgroup.c | 2 +-
>   2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 22ba2b4505b3..00d4c4184e76 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -160,7 +160,7 @@ enum {
>    */
>   struct btrfs_qgroup_swapped_blocks {
>   	spinlock_t lock;
> -	/* RM_EMPTY_ROOT() of above blocks[] */
> +	/* True if any of the rbtrees in the blocks array below is not empty. */
>   	bool swapped;
>   	struct rb_root blocks[BTRFS_MAX_LEVEL];
>   };
> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> index 46de0a5e80d5..798f21608de5 100644
> --- a/fs/btrfs/qgroup.c
> +++ b/fs/btrfs/qgroup.c
> @@ -4913,7 +4913,7 @@ int btrfs_qgroup_trace_subtree_after_cow(struct btrfs_trans_handle *trans,
>   	/* Found one, remove it from @blocks first and update blocks->swapped */
>   	rb_erase(&block->node, &blocks->blocks[level]);
>   	for (i = 0; i < BTRFS_MAX_LEVEL; i++) {
> -		if (RB_EMPTY_ROOT(&blocks->blocks[i])) {
> +		if (!RB_EMPTY_ROOT(&blocks->blocks[i])) {
>   			swapped = true;
>   			break;
>   		}