linux-kernel - Re: [PATCH 5.4 103/142] Revert "block: end bio with BLK_STS_AGAIN in case of non-mq devs and REQ

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Date:   Wed, 3 Feb 2021 15:58:33 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andres Freund <andres@...razel.de>
Cc:     io-uring@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org,
        Bijan Mottahedeh <bijan.mottahedeh@...cle.com>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 5.4 103/142] Revert "block: end bio with BLK_STS_AGAIN in
 case of non-mq devs and REQ_NOWAIT"

On 2/3/21 3:06 PM, Greg Kroah-Hartman wrote:
> On Wed, Feb 03, 2021 at 01:28:26PM -0800, Andres Freund wrote:
>> Hi,
>>
>> On 2021-02-03 14:03:09 +0100, Greg Kroah-Hartman wrote:
>>>> On v5.4.43-101-gbba91cdba612 this fails with
>>>> fio: io_u error on file /mnt/t2/test.0.0: Input/output error: write offset=0, buflen=4096
>>>> fio: pid=734, err=5/file:io_u.c:1834, func=io_u error, error=Input/output error
>>>>
>>>> whereas previously it worked. libaio still works...
>>>>
>>>> I haven't checked which major kernel version fixed this again, but I did
>>>> verify that it's still broken in 5.4.94 and that 5.10.9 works.
>>>>
>>>> I would suspect it's
>>>>
>>>> commit 4503b7676a2e0abe69c2f2c0d8b03aec53f2f048
>>>> Author: Jens Axboe <axboe@...nel.dk>
>>>> Date:   2020-06-01 10:00:27 -0600
>>>>
>>>>     io_uring: catch -EIO from buffered issue request failure
>>>>
>>>>     -EIO bubbles up like -EAGAIN if we fail to allocate a request at the
>>>>     lower level. Play it safe and treat it like -EAGAIN in terms of sync
>>>>     retry, to avoid passing back an errant -EIO.
>>>>
>>>>     Catch some of these early for block based file, as non-mq devices
>>>>     generally do not support NOWAIT. That saves us some overhead by
>>>>     not first trying, then retrying from async context. We can go straight
>>>>     to async punt instead.
>>>>
>>>>     Signed-off-by: Jens Axboe <axboe@...nel.dk>
>>>>
>>>>
>>>> which isn't in stable/linux-5.4.y
>>>
>>> Can you test that if the above commit is added, all works well again?
>>
>> It doesn't apply cleanly, I'll try to resolve the conflict. However, I
>> assume that the revert was for a concrete reason - but I can't quite
>> figure out what b0beb28097fa04177b3769f4bb7a0d0d9c4ae76e was concretely
>> solving, and whether reverting the revert in 5.4 would re-introduce a
>> different problem.
>>
>> commit b0beb28097fa04177b3769f4bb7a0d0d9c4ae76e (tag: block-5.7-2020-05-29, linux-block/block-5.7)
>> Author: Jens Axboe <axboe@...nel.dk>
>> Date:   2020-05-28 13:19:29 -0600
>>
>>     Revert "block: end bio with BLK_STS_AGAIN in case of non-mq devs and REQ_NOWAIT"
>>
>>     This reverts commit c58c1f83436b501d45d4050fd1296d71a9760bcb.
>>
>>     io_uring does do the right thing for this case, and we're still returning
>>     -EAGAIN to userspace for the cases we don't support. Revert this change
>>     to avoid doing endless spins of resubmits.
>>
>>     Cc: stable@...r.kernel.org # v5.6
>>     Reported-by: Bijan Mottahedeh <bijan.mottahedeh@...cle.com>
>>     Signed-off-by: Jens Axboe <axboe@...nel.dk>
>>
>> I suspect it just wasn't aimed at 5.4, and that's that, but I'm not
>> sure. In which case presumably reverting
>> bba91cdba612fbce4f8575c5d94d2b146fb83ea3 would be the right fix, not
>> backporting 4503b7676a2e0abe69c2f2c0d8b03aec53f2f048 et al.
> 
> Ok, can you send a revert patch for this?
> 
> But it would be good to get Jens to weigh in on this...

I'll take a look at this.

-- 
Jens Axboe