linux-kernel - Re: 32-bit bio regression with 4.3 [was: Re: cgroup/loop Bad page state oops in Linux v4.2-rc3-136-g45b4b782e848]

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Date:	Tue, 15 Sep 2015 08:14:55 -0400
From:	Josh Boyer <jwboyer@...oraproject.org>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Mike Snitzer <snitzer@...hat.com>, ejt@...hat.com,
	Johannes Weiner <hannes@...xchg.org>,
	Tejun Heo <tj@...nel.org>, Jens Axboe <axboe@...com>,
	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
	mlin@...nel.org, Adam Williamson <awilliam@...hat.com>,
	tom.leiming@...il.com
Subject: Re: 32-bit bio regression with 4.3 [was: Re: cgroup/loop Bad page
 state oops in Linux v4.2-rc3-136-g45b4b782e848]

On Sat, Sep 12, 2015 at 9:19 AM, Ming Lei <ming.lei@...onical.com> wrote:
> On Fri, 11 Sep 2015 17:43:15 -0400
> Mike Snitzer <snitzer@...hat.com> wrote:
>
>> Ming, Jens, others:
>>
>> Please see this BZ comment that speaks to a 4.3 regression due to the
>> late bio splitting changes:
>> https://bugzilla.redhat.com/show_bug.cgi?id=1247382#c41
>
> I think it is a bug of bounce_end_io, and the following patch may
> fix it.
>
> ----
> From 08df0db0be41e6bea306bcf5b4d325f5a79dc7a1 Mon Sep 17 00:00:00 2001
> From: Ming Lei <ming.lei@...onical.com>
> Date: Sat, 12 Sep 2015 20:48:42 +0800
> Subject: [PATCH] block: fix bounce_end_io
>
> When bio bounce is involved, one new bio and its io vector are
> cloned from the coming bio, which can be one fast-cloned bio
> and its io vector can be shared with another bio too, especially
> after bio_split() is introduced.
>
> So it is obviously wrong to assume the start index of the original
> bio's io vector is zero, which can be any value between 0 and
> (bi_max_vecs - 1), especially in case of bio split.
>
> Signed-off-by: Ming Lei <ming.lei@...onical.com>
> ---
>  block/bounce.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/block/bounce.c b/block/bounce.c
> index 0611aea..1cb5dd3 100644
> --- a/block/bounce.c
> +++ b/block/bounce.c
> @@ -128,12 +128,14 @@ static void bounce_end_io(struct bio *bio, mempool_t *pool)
>         struct bio *bio_orig = bio->bi_private;
>         struct bio_vec *bvec, *org_vec;
>         int i;
> +       int start = bio_orig->bi_iter.bi_idx;
>
>         /*
>          * free up bounce indirect pages used
>          */
>         bio_for_each_segment_all(bvec, bio, i) {
> -               org_vec = bio_orig->bi_io_vec + i;
> +               org_vec = bio_orig->bi_io_vec + i + start;
> +
>                 if (bvec->bv_page == org_vec->bv_page)
>                         continue;
>
> --
> 1.9.1
>
>> But inlined here so we can continue on list:
>> (In reply to Josh Boyer from comment #40)
>> > The function that was fixed in 4.2 doesn't exist any longer in
>> > 4.3.0-0.rc0.git6.1.fc24.  That kernel corresponds to Linux
>> > v4.2-6105-gdd5cdb48edfd which contains commit
>> > 8ae126660fddbeebb9251a174e6fa45b6ad8f932, which removed it completely.  So
>> > whatever fix was made in dm_merge_bvec doesn't seem to have made it to
>> > whatever replaced it.
>>
>> The dm core fix to dm_merge_bvec was commit bd4aaf8f9b ("dm: fix
>> dm_merge_bvec regression on 32 bit systems").  But I'm not sure there is
>> a clear equivalent in the late bio splitting code that replaced block
>> core's merge_bvec logic.
>>
>> merge_bvec was all about limiting bios (by asking "can/should this page
>> be added to this bio?") whereas the late bio splitting is more "build
>> the bios as large as possible and worry about splitting later".
>
> IMO, given one vector can only point to one page, there shouldn't
> have difference between the two.
>
>>
>> Regardless, this regression needs to be reported to Ming Lin
>> <ming.l@....samsung.com>, Jens Axboe and the others involved in
>> maintaining the late bio splitting changes in block core.
>>
>> Josh and/or Adam: it would _really_ help if the regression test you guys
>> are using could be handed-over and/or explained to us.  Is it as simple
>> as loading a 32bit with a particular config?  Can you share the guest
>> image if it is small enough?
>
> Josh, Adam, would you mind testing the above patch to see if it can fix
> your issue?

Sorry for the delay in reply.  I'll try and work with Adam today to
get this tested.

josh
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