Subject: Re: kernel BUG at drivers/scsi/scsi_lib.c:1096!
To: Mark Salter <msalter@redhat.com>, Ming Lei <ming.lei@canonical.com>
References: <1447838334.1564.2.camel@ellerman.id.au>
 <1447855399.3974.24.camel@redhat.com>
 <1447894964.15206.0.camel@ellerman.id.au>
 <20151119082325.GA11419@infradead.org>
 <1448021448.14769.7.camel@ellerman.id.au>
 <565055C6.5040801@linux.vnet.ibm.com> <20151122005635.1b9ffbe1@tom-T450>
 <1448234410.8209.3.camel@redhat.com>
Cc: linux-block@vger.kernel.org, linux-scsi@vger.kernel.org,
        "James E. J. Bottomley" <JBottomley@odin.com>,
        linux-kernel@vger.kernel.org, Christoph Hellwig <hch@infradead.org>,
        brking <brking@us.ibm.com>, tom.leiming@gmail.com,
        linuxppc-dev@lists.ozlabs.org
From: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Message-ID: <56531B3F.60503@linux.vnet.ibm.com>
Date: Mon, 23 Nov 2015 14:57:19 +0100
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On 23/11/2015 00:20, Mark Salter 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@linux.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@canonical.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@canonical.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.

On my side, with the patch applied on top of 4.4-rc1, I can't get the
panic anymore.


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