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

Jens Axboe
2020-11-18T21:00:25Z

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;

> 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!

--
Jens Axboe