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

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

On 8/23/24 2:19 PM, yangyun wrote:
> Sorry for the late reply.
> 
> On Wed, Aug 14, 2024 at 05:56:06PM +0800, Jingbo Xu wrote:
>>
>>
>> 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 Also remove the outdated comment since commit 6b2fb79963fb ("fuse: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.
> 
> The code can make sure that there are no race conditions because:
> 1. For O_DIRECT and FOPEN_DIRECT_IO with fc->direct_io_allow_mmap, the `filemap_write_and_wait_range` before can make the insert operation to be happend before the check operation.
> 2. For other cases, there are no pagecache operaions so the fi->writepages is always empty.
> 
> In my usercase, the fi->writepages is usually empty but the spin_lock associated with it contributes a great impact on the performace of my filesystem due to lock contention. So optimize it.

Thanks for the explanation.  There are multiple callsites of
fuse_wait_on_page_writeback(), and how do you ensure that there can't be
any concurrent fuse_writepage_add() with these callsites.

I'm not sure if the concurrency indeed makes a difference, since
fuse_range_is_writeback() releases fi->lock before it returns anyway.


-- 
Thanks,
Jingbo