Message-ID: <CACVXFVPWBnm8H4BgQ_=V-Z-NLrkt+TE7T_RpEctNrjEwpRpJhw@mail.gmail.com>
Date:	Mon, 23 Nov 2015 10:46:20 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Mark Salter <msalter@...hat.com>
Cc:	Laurent Dufour <ldufour@...ux.vnet.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Christoph Hellwig <hch@...radead.org>,
	"James E. J. Bottomley" <JBottomley@...n.com>,
	brking <brking@...ibm.com>,
	Linux SCSI List <linux-scsi@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linuxppc-dev@...ts.ozlabs.org, linux-block@...r.kernel.org
Subject: Re: kernel BUG at drivers/scsi/scsi_lib.c:1096!

Hi Mark,

On Mon, Nov 23, 2015 at 9:50 AM, Mark Salter <msalter@...hat.com> wrote:
> On Mon, 2015-11-23 at 08:36 +0800, Ming Lei wrote:
>> On Mon, Nov 23, 2015 at 7:20 AM, Mark Salter <msalter@...hat.com> wrote:
>> > On Sun, 2015-11-22 at 00:56 +0800, Ming Lei wrote:
>> > > On Sat, 21 Nov 2015 12:30:14 +0100
>> > > Laurent Dufour <ldufour@...ux.vnet.ibm.com> wrote:
>> > >
>> > > > On 20/11/2015 13:10, Michael Ellerman wrote:
>> > > > > On Thu, 2015-11-19 at 00:23 -0800, Christoph Hellwig wrote:
>> > > > >
>> > > > > > It's pretty much guaranteed a block layer bug, most likely in the
>> > > > > > merge bios to request infrastucture where we don't obey the merging
>> > > > > > limits properly.
>> > > > > >
>> > > > > > Does either of you have a known good and first known bad kernel?
>> > > > >
>> > > > > Not me, I've only hit it one or two times. All I can say is I have hit it in
>> > > > > 4.4-rc1.
>> > > > >
>> > > > > Laurent, can you narrow it down at all?
>> > > >
>> > > > It seems that the panic is triggered by the commit bdced438acd8 ("block:
>> > > > setup bi_phys_segments after splitting") which has been pulled by the
>> > > > merge d9734e0d1ccf ("Merge branch 'for-4.4/core' of
>> > > > git://git.kernel.dk/linux-block").
>> > > >
>> > > > My system is panicing promptly when running a kernel built at
>> > > > d9734e0d1ccf, while reverting the commit bdced438acd8, it can run hours
>> > > > without panicing.
>> > > >
>> > > > This being said, I can't explain what's going wrong.
>> > > >
>> > > > May Ming shed some light here ?
>> > >
>> > > Laurent, looks there is one bug in blk_bio_segment_split(), would you
>> > > mind testing the following patch to see if it fixes your issue?
>> > >
>> > > ---
>> > > From 6fc701231dcc000bc8bc4b9105583380d9aa31f4 Mon Sep 17 00:00:00 2001
>> > > From: Ming Lei <ming.lei@...onical.com>
>> > > Date: Sun, 22 Nov 2015 00:47:13 +0800
>> > > Subject: [PATCH] block: fix segment split
>> > >
>> > > Inside blk_bio_segment_split(), previous bvec pointer('bvprvp')
>> > > always points to the iterator local variable, which is obviously
>> > > wrong, so fix it by pointing to the local variable of 'bvprv'.
>> > >
>> > > Signed-off-by: Ming Lei <ming.lei@...onical.com>
>> > > ---
>> > >  block/blk-merge.c | 4 ++--
>> > >  1 file changed, 2 insertions(+), 2 deletions(-)
>> > >
>> > > diff --git a/block/blk-merge.c b/block/blk-merge.c
>> > > index de5716d8..f2efe8a 100644
>> > > --- a/block/blk-merge.c
>> > > +++ b/block/blk-merge.c
>> > > @@ -98,7 +98,7 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
>> > >
>> > >                       seg_size += bv.bv_len;
>> > >                       bvprv = bv;
>> > > -                     bvprvp = &bv;
>> > > +                     bvprvp = &bvprv;
>> > >                       sectors += bv.bv_len >> 9;
>> > >                       continue;
>> > >               }
>> > > @@ -108,7 +108,7 @@ new_segment:
>> > >
>> > >               nsegs++;
>> > >               bvprv = bv;
>> > > -             bvprvp = &bv;
>> > > +             bvprvp = &bvprv;
>> > >               seg_size = bv.bv_len;
>> > >               sectors += bv.bv_len >> 9;
>> > >       }
>> >
>> > I'm still hitting the BUG even with this patch applied on top of 4.4-rc1.
>>
>> OK, looks there are still other bugs, care to share us how to reproduce
>> it on arm64?
>>
>> thanks,
>> Ming
>
> Unfortunately, the best reproducer I have is to boot the platform. I have seen the
> BUG a few times post-boot, but I don't have a consistant reproducer. I am using
> upstream 4.4-rc1 with this config:
>
>   http://people.redhat.com/msalter/fh_defconfig
>
> With 4.4-rc1 on an APM Mustang platform, I see the BUG about once every 6-7 boots.
> On an AMD Seattle platform, about every 9 boots.

Thanks for the input, and I will try to reproduce the issue on mustang with
your kernel config.

>
> I have a script that loops through an ssh command to reboot the platform under test.
> I manually install test kernels and then run the script and wait for failure. While
> debugging, I have tried more minimal configs with which I have been unable to
> reproduce the problem even after several hours of reboots. With the above mentioned
> fh_defconfig, I have been able to get a failure within 20 or so boots with most
> kernel builds but at certain kernel commits, the failure has taken a longer time to
> reproduce.
>
> From my POV, I can't say which commit causes the problem. So far, I have not been
> able to reproduce at all before commit d9734e0d1ccf but I am currently trying to
> reproduce with commit 0d51ce9ca1116 (one merge earlier than d9734e0d1ccf).

The patch for fixing 'bvprvp' is better to be included for test,
because that issue
may have a big effect on computing physical seg count.

Also I appreciate if you, Laurent or anyone may provide debug log which
can be captured with the attached debug patch when this issue is trigered .


Thanks,

View attachment "dbg_scsi_blk_merge.patch" of type "text/x-patch" (1429 bytes)

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