From: JeffleXu <[email protected]>
To: Dave Chinner <[email protected]>, Hao Xu <[email protected]>
Cc: Alexander Viro <[email protected]>,
Christoph Hellwig <[email protected]>,
"Darrick J. Wong" <[email protected]>,
[email protected],
Konstantin Khlebnikov <[email protected]>,
Jens Axboe <[email protected]>,
[email protected], Joseph Qi <[email protected]>
Subject: Re: [PATCH v3 RESEND] iomap: set REQ_NOWAIT according to IOCB_NOWAIT in Direct IO
Date: Tue, 8 Dec 2020 13:46:47 +0800 [thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
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 <[email protected]>
>> Signed-off-by: Konstantin Khlebnikov <[email protected]>
>> Signed-off-by: Hao Xu <[email protected]>
>> ---
>>
>> 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
next prev parent reply other threads:[~2020-12-08 5:47 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-04 9:44 [PATCH v3 RESEND] iomap: set REQ_NOWAIT according to IOCB_NOWAIT in Direct IO Hao Xu
2020-12-04 11:44 ` Pavel Begunkov
2020-12-07 2:21 ` Dave Chinner
2020-12-07 23:40 ` Jens Axboe
2020-12-09 21:15 ` Dave Chinner
2020-12-10 2:33 ` JeffleXu
2020-12-08 5:46 ` JeffleXu [this message]
2020-12-09 21:23 ` Dave Chinner
2020-12-10 1:55 ` JeffleXu
2020-12-10 5:18 ` Dave Chinner
2020-12-11 2:50 ` JeffleXu
2020-12-14 2:56 ` Dave Chinner
2020-12-15 9:43 ` JeffleXu
2021-04-02 14:32 ` Pavel Begunkov
2021-04-02 16:26 ` Jens Axboe
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