Re: [PATCH/RFC] pass dio_complete proper offset from finished_one_bio

[Felix Blyakher <felixb@sgi.com>]

Good find, Eric.
Looks good to me. I guess, that can explain some problems I saw
on SLES10, and never had time to dig in it.
Seems to me, it's been that way for quite a while, though. This code was
introduced in 2.6.12. Prior to that, the code in finished_one_bio() did
following:

                        dio_complete(dio, dio->block_in_file << 
dio->blkbits,
                                        dio->result);

where block_in_file is "current offset into the underlying file in
dio_block units", which is still the post-IO position, not the IO start
offset, isn't it?

- Felix

Eric Sandeen wrote:

> We saw problems w/ xfs doing AIO+DIO into a sparse file.
> https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=217098
>
> It seemed that xfs was doing "extent conversion" at the wrong offsets, so
> written regions came up as unwritten (zeros) and stale data was exposed
> in the region after the write.  Thanks to Peter Backes for the very
> nice testcase.
>
> This also broke xen with blktap over xfs.
>
> Here's what I found:
> finished_one_bio calls dio_complete calls xfs_end_io_direct with an 
> offset, but:
>
>     offset = dio->iocb->ki_pos;
>
> so this is the -current- post-IO position, not the IO start point that 
> dio_complete expects.  So, xfs converts the the wrong region.  Ouch!
>
> XFS seems to be the only filesystem that uses the offset passed to the 
> end_io function, so only it is affected by this as near as I can tell.
>
> However, the "short read" case is probably also wrong, as it is checking:
>
>                        if ((dio->rw == READ) &&
>                            ((offset + transferred) > dio->i_size))
>                                transferred = dio->i_size - offset;
>
> but offset is the ending IO position in this case too.
>
> This patch seems to fix it up.
>
> Comments?
>
> Thanks,
> -Eric
>
> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
>
> Index: linux-2.6.18/fs/direct-io.c
> ===================================================================
> --- linux-2.6.18.orig/fs/direct-io.c
> +++ linux-2.6.18/fs/direct-io.c
> @@ -244,12 +244,12 @@ static void finished_one_bio(struct dio 
>              */
>             spin_unlock_irqrestore(&dio->bio_lock, flags);
>
> -            /* Check for short read case */
>             transferred = dio->result;
> -            offset = dio->iocb->ki_pos;
> +            offset = dio->iocb->ki_pos - transferred;
>
> +            /* Check for short read case */
>             if ((dio->rw == READ) &&
> -                ((offset + transferred) > dio->i_size))
> +                (dio->iocb->ki_pos > dio->i_size))
>                 transferred = dio->i_size - offset;
>
>             /* check for error in completion path */
>
>
>