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Date:   Wed, 3 Feb 2021 13:28:26 -0800
From:   Andres Freund <andres@anarazel.de>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     io-uring@vger.kernel.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org,
        Bijan Mottahedeh <bijan.mottahedeh@oracle.com>,
        Sasha Levin <sashal@kernel.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"
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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@kernel.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@kernel.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@kernel.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@vger.kernel.org # v5.6
    Reported-by: Bijan Mottahedeh <bijan.mottahedeh@oracle.com>
    Signed-off-by: Jens Axboe <axboe@kernel.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.

Greetings,

Andres Freund