On 12/7/20 10:21 AM, Dave Chinner wrote:
> On Fri, Dec 04, 2020 at 05:44:56PM +0800, Hao Xu wrote:
>> Currently, IOCB_NOWAIT is ignored in Direct IO, REQ_NOWAIT is only set
>> when IOCB_HIPRI is set. But REQ_NOWAIT should be set as well when
>> IOCB_NOWAIT is set.
>>
>> Suggested-by: Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx>
>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
>> Signed-off-by: Hao Xu <haoxu@xxxxxxxxxxxxxxxxx>
>> ---
>>
>> Hi all,
>> I tested fio io_uring direct read for a file on ext4 filesystem on a
>> nvme ssd. I found that IOCB_NOWAIT is ignored in iomap layer, which
>> means REQ_NOWAIT is not set in bio->bi_opf.
>
> What iomap is doing is correct behaviour. IOCB_NOWAIT applies to the
> filesystem behaviour, not the block device.
>
> REQ_NOWAIT can result in partial IO failures because the error is
> only reported to the iomap layer via IO completions. Hence we can
> split a DIO into multiple bios and have random bios in that IO fail
> with EAGAIN because REQ_NOWAIT is set. This error will
> get reported to the submitter via completion, and it will override
> any of the partial IOs that actually completed.
>
> Hence, like the recently reported multi-mapping IOCB_NOWAIT bug
> reported by Jens and fixed in commit 883a790a8440 ("xfs: don't allow
> NOWAIT DIO across extent boundaries") we'll get silent partial
> writes occurring because the second submitted bio in an IO can
> trigger EAGAIN errors with partial IO completion having already
> occurred.
>
> Further, we don't allow partial IO completion for DIO on XFS at all.
> DIO must be completely submitted and completed or return an error
> without having issued any IO at all. Hence using REQ_NOWAIT for
> DIO bios is incorrect and not desirable.
Not familiar with xfs though, just in curiosity, how do you achive 'no
partial completion'? I mean you could avoid partial -EAGAIN by not
setting REQ_NOWAIT, but you could get other partial errors such as
-ENOMEM or something, as long as one DIO could be split to multiple bios.
--
Thanks,
Jeffle
