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 15:59:41 -0800
From:   Andres Freund <andres@...razel.de>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        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"

Hi,

On 2021-02-03 15:58:33 -0700, Jens Axboe wrote:
> On 2/3/21 3:06 PM, Greg Kroah-Hartman wrote:
> > On Wed, Feb 03, 2021 at 01:28:26PM -0800, Andres Freund wrote:
> >> 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.

Having looked a bit more through the history, I suspect that the reason
5.6 doesn't need c58c1f83436b501d45d4050fd1296d71a9760bcb - which I have
confirmed - is that ext4 was converted to the iomap infrastructure in
5.5, but not in 5.4.

I've confirmed that the repro I shared upthread triggers in
378f32bab3714f04c4e0c3aee4129f6703805550^ but not in
378f32bab3714f04c4e0c3aee4129f6703805550.


> > 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.

Thanks.

Greetings,

Andres Freund