Re: [PATCH] btrfs: preallocate extent changeset before acquiring extent_io_tree lock

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2026/4/17 16:11, tchou 写道:
> In btrfs_clear_extent_bit_changeset(), the extent changeset ulist may need
> to allocate new nodes. Currently, this can happen while holding the
> extent_io_tree->lock spinlock. Although ulist_prealloc() uses GFP_NOFS to
> avoid deadlock with filesystem reclaim, it's better to preallocate before
> acquiring the spinlock to:
> 
> 1. Avoid potential allocation failures while holding the lock
> 2. Be consistent with set_extent_bit() which already preallocates both
>     extent state and changeset before the spinlock
> 3. Reduce lock contention by not doing allocations under the lock
> 
> Preallocate the changeset ulist node before acquiring the spinlock,
> mirroring the pattern used for extent state preallocation.

Missing the SoB line.

Otherwise looks good to me.

Thanks,
Qu
> ---
>   fs/btrfs/extent-io-tree.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/fs/btrfs/extent-io-tree.c b/fs/btrfs/extent-io-tree.c
> index 626702244809..b5ba650cdb55 100644
> --- a/fs/btrfs/extent-io-tree.c
> +++ b/fs/btrfs/extent-io-tree.c
> @@ -663,6 +663,9 @@ int btrfs_clear_extent_bit_changeset(struct extent_io_tree *tree, u64 start, u64
>   		 */
>   		prealloc = alloc_extent_state(mask);
>   	}
> +	/* Preallocate the extent changeset ulist node before acquiring spinlock. */
> +	if (changeset)
> +		extent_changeset_prealloc(changeset, mask);
>   
>   	spin_lock(&tree->lock);
>   	if (cached_state) {