Re: [PATCH RFC] iomap: only return IO error if no data has been transferred

Dave Chinner
2020-11-18T21:33:45Z

On Wed, Nov 18, 2020 at 02:19:30PM -0700, Jens Axboe wrote:
> On 11/18/20 2:15 PM, Dave Chinner wrote:
> > On Wed, Nov 18, 2020 at 02:00:06PM -0700, Jens Axboe wrote:
> >> On 11/18/20 1:37 PM, Dave Chinner wrote:
> >>> On Wed, Nov 18, 2020 at 08:26:50AM -0700, Jens Axboe wrote:
> >>>> On 11/18/20 12:19 AM, Dave Chinner wrote:
> >>>>> On Tue, Nov 17, 2020 at 03:17:18PM -0700, Jens Axboe wrote:
> >>>>>> If we've successfully transferred some data in __iomap_dio_rw(),
> >>>>>> don't mark an error for a latter segment in the dio.
> >>>>>>
> >>>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> >>>>>>
> >>>>>> ---
> >>>>>>
> >>>>>> Debugging an issue with io_uring, which uses IOCB_NOWAIT for the
> >>>>>> IO. If we do parts of an IO, then once that completes, we still
> >>>>>> return -EAGAIN if we ran into a problem later on. That seems wrong,
> >>>>>> normal convention would be to return the short IO instead. For the
> >>>>>> -EAGAIN case, io_uring will retry later parts without IOCB_NOWAIT
> >>>>>> and complete it successfully.
> >>>>>
> >>>>> So you are getting a write IO that is split across an allocated
> >>>>> extent and a hole, and the second mapping is returning EAGAIN
> >>>>> because allocation would be required? This sort of split extent IO
> >>>>> is fairly common, so I'm not sure that splitting them into two
> >>>>> separate IOs may not be the best approach.
> >>>>
> >>>> The case I seem to be hitting is this one:
> >>>>
> >>>> if (iocb->ki_flags & IOCB_NOWAIT) {
> >>>> if (filemap_range_has_page(mapping, pos, end)) {
> >>>> ret = -EAGAIN;
> >>>> goto out_free_dio;
> >>>> }
> >>>> flags |= IOMAP_NOWAIT;
> >>>> }
> >>>>
> >>>> in __iomap_dio_rw(), which isn't something we can detect upfront like IO
> >>>> over a multiple extents...
> >>>
> >>> This specific situation cannot result in the partial IO behaviour
> >>> you described. It is an -upfront check- that is done before any IO
> >>> is mapped or issued so results in the entire IO being skipped and we
> >>> don't get anywhere near the code you changed.
> >>>
> >>> IOWs, this doesn't explain why you saw a partial IO, or why changing
> >>> partial IO return values avoids -EAGAIN from a range we apparently
> >>> just did a partial IO over and -didn't have page cache pages-
> >>> sitting over it.
> >>
> >> You are right, I double checked and recreated my debugging. What's
> >> triggering is that we're hitting this in xfs_direct_write_iomap_begin()
> >> after we've already done some IO:
> >>
> >> allocate_blocks:
> >> error = -EAGAIN;
> >> if (flags & IOMAP_NOWAIT)
> >> goto out_unlock;
> >
> > Ok, that's exactly the case the reproducer I wrote triggers.
>
> OK good, then we're on the same page :-)
>
> >>> Can you provide an actual event trace of the IOs in question that
> >>> are failing in your tests (e.g. from something like `trace-cmd
> >>> record -e xfs_file\* -e xfs_i\* -e xfs_\*write -e iomap\*` over the
> >>> sequential that reproduces the issue) so that there's no ambiguity
> >>> over how this problem is occurring in your systems?
> >>
> >> Let me know if you still want this!
> >
> > No, it makes sense now :)
>
> What's the next step here? Are you working on an XFS fix for this?

I'm just building the patch now for testing.

> Was looking at other potential -EAGAIN during the loop, and seems like
> we'd be able to hit this if we fail xfs_ilock_for_iomap() as well. And
> not sure how that would be solvable, without accepting that IOCB_NOWAIT
> reads/writes can be short.

The change I'm making should solves that, too. i.e. NOWAIT IO must
map entirely within a single extent, so there is no scope for a
short IO and re-entering the mapping code under NOWAIT conditions
that could then fail.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com