On Fri, Nov 30, 2018 at 9:20 PM Darrick J. Wong <darrick.wong@xxxxxxxxxx> wrote:
>
> From: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
>
> In commit 4721a601099, we tried to fix a problem wherein directio reads
> into a splice pipe will bounce EFAULT/EAGAIN all the way out to
> userspace by simulating a zero-byte short read. This happens because
> some directio read implementations (xfs) will call
> bio_iov_iter_get_pages to grab pipe buffer pages and issue asynchronous
> reads, but as soon as we run out of pipe buffers that _get_pages call
> returns EFAULT, which the splice code translates to EAGAIN and bounces
> out to userspace.
>
> In that commit, the iomap code catches the EFAULT and simulates a
> zero-byte read, but that causes assertion errors on regular splice reads
> because xfs doesn't allow short directio reads.
>
> The brokenness is compounded by splice_direct_to_actor immediately
> bailing on do_splice_to returning <= 0 without ever calling ->actor
> (which empties out the pipe), so if userspace calls back we'll EFAULT
> again on the full pipe, and nothing ever gets copied.
>
> Therefore, teach splice_direct_to_actor to clamp its requests to the
> amount of free space in the pipe and remove the simulated short read
> from the iomap directio code.
>
> Fixes: 4721a601099 ("iomap: dio data corruption and spurious errors when pipes fill")
> Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> ---
Problem is the regressing commit does two things and you here revert one
of them and re-fix the bug.
IMO, it would be nicer to:
- Revert the regression commit
- Attribute Reported-by and specify regression details in revert commit
- Re-apply Dave's zero_tail fix with its own commit instead of a
"To make matters worse..." clause.
- Apply your re-fix with description of original problem and without the
story about the regression
Your call.
Miklos,
Can you please review the suggested re-fix?
Thanks,
Amir.
