Re: [PATCH] fuse: add fast path for fuse_range_is_writeback

[Jingbo Xu <jefflexu@linux.alibaba.com>]

On 8/14/24 5:36 PM, yangyun wrote:
> In some cases, the fi->writepages may be empty. And there is no need
> to check fi->writepages with spin_lock, which may have an impact on
> performance due to lock contention. For example, in scenarios where
> multiple readers read the same file without any writers, or where
> the page cache is not enabled.
> 
> Also remove the outdated comment since commit 6b2fb79963fb ("fuse:
> optimize writepages search") has optimize the situation by replacing
> list with rb-tree.
> 
> Signed-off-by: yangyun <yangyun50@huawei.com>
> ---
>  fs/fuse/file.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index f39456c65ed7..59c911b61000 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -448,9 +448,6 @@ static struct fuse_writepage_args *fuse_find_writeback(struct fuse_inode *fi,
>  
>  /*
>   * Check if any page in a range is under writeback
> - *
> - * This is currently done by walking the list of writepage requests
> - * for the inode, which can be pretty inefficient.
>   */
>  static bool fuse_range_is_writeback(struct inode *inode, pgoff_t idx_from,
>  				   pgoff_t idx_to)
> @@ -458,6 +455,9 @@ static bool fuse_range_is_writeback(struct inode *inode, pgoff_t idx_from,
>  	struct fuse_inode *fi = get_fuse_inode(inode);
>  	bool found;
>  
> +	if (RB_EMPTY_ROOT(&fi->writepages))
> +		return false;

fi->lock is held when inserting wpa into fi->writepages rbtree (see
fuse_writepage_add()).  I doubt if there is race condition when checking
if fi->writepages rbtree is empty without fi->lock held.


-- 
Thanks,
Jingbo