On Sat, Aug 17, 2024 at 12:57:27AM +0800, alexjlzheng@xxxxxxxxx wrote:
> 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()?
iomap_dio_ops.end_io() implemenations should return one of:
- negative errno
- 0 if @size bytes were written
I'm not sure why ext4_dio_write_end_io returns size explicitly or why
ext4_handle_inode_extension returns count. Neither function need to do
that.
--D
> Thanks,
> Jinliang Zheng
>
> > > }
> > >
> > > static const struct iomap_dio_ops ext4_dio_write_ops = {
>
