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

[alexjlzheng]

On Fri, 16 Aug 2024 20:21:22 +0800, yangerkun@xxxxxxxxxxxxxxx wrote:
> 在 2024/8/15 19:27, alexjlzheng@xxxxxxxxx 写道:
> > From: Jinliang Zheng <alexjlzheng@xxxxxxxxxxx>
> > 
> > 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@xxxxxxxxxxx>
> > ---
> >   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.

> 
> 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
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()?

Thanks,
Jinliang Zheng

> >   }
> >   
> >   static const struct iomap_dio_ops ext4_dio_write_ops = {