Re: [PATCH] ext4: disambiguate the return value of ext4_dio_write_end_io()

[yangerkun <yangerkun@huaweicloud.com>]

在 2024/8/17 0:57, alexjlzheng@gmail.com 写道:
> On Fri, 16 Aug 2024 20:21:22 +0800, yangerkun@huaweicloud.com wrote:
>> 在 2024/8/15 19:27, alexjlzheng@gmail.com 写道:
>>> From: Jinliang Zheng <alexjlzheng@tencent.com>
>>>
>>> The commit 91562895f803 ("ext4: properly sync file size update after O_SYNC
>>> direct IO") causes confusion about the meaning of the return value of
>>> ext4_dio_write_end_io().
>>>
>>> Specifically, when the ext4_handle_inode_extension() operation succeeds,
>>> ext4_dio_write_end_io() directly returns count instead of 0.
>>>
>>> This does not cause a bug in the current kernel, but the semantics of the
>>> return value of the ext4_dio_write_end_io() function are wrong, which is
>>> likely to introduce bugs in the future code evolution.
>>>
>>> Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>
>>> ---
>>>    fs/ext4/file.c | 5 +++--
>>>    1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
>>> index c89e434db6b7..6df5a92cec2b 100644
>>> --- a/fs/ext4/file.c
>>> +++ b/fs/ext4/file.c
>>> @@ -392,8 +392,9 @@ static int ext4_dio_write_end_io(struct kiocb *iocb, ssize_t size,
>>>    	 */
>>>    	if (pos + size <= READ_ONCE(EXT4_I(inode)->i_disksize) &&
>>>    	    pos + size <= i_size_read(inode))
>>> -		return size;
>>> -	return ext4_handle_inode_extension(inode, pos, size);
>>> +		return 0;
>>> +	error = ext4_handle_inode_extension(inode, pos, size);
>>> +	return error < 0 ? error : 0;
>>
>> Why?
> 
> Before commit 91562895f803 ("ext4: properly sync file size update after O_SYNC
> direct IO"), all filesystems' iomap_dio_ops.end_io() return 0 on success and
> negative value on failure.
> 
> Moreover, this confusion of return value semantics caused data corruption when
> this above patch was merged to the stable branch. See
> https://lwn.net/Articles/954285/ for details.

Yeah, I know this problem, you should backport 936e114a245b("iomap:
update ki_pos a little later in iomap_dio_complete") too to help update
iocb->ki_pos since ext4_dio_write_end_io now return > 0.

> 
>>
>> iomap_dio_complete can use the return value directly without any bug.
>> And I think the code now seems more clearly...
>>
> 
> In my opinion, clean code should be clearly defined code, especially the

Agree.

> interface functions connecting various modules. So, what is the return value
> definition of iomap_dio_ops.end_io()? What is the return value definition of
> ext4_dio_write_end_io()?

I have not seen the definition of return value for
iomap_dio_ops.end_io(), so I think the code is ok now. If we give a
definition for the return value like Darrick describe, this patch looks
good to me.

> 
> Thanks,
> Jinliang Zheng
> 
>>>    }
>>>    
>>>    static const struct iomap_dio_ops ext4_dio_write_ops = {
>