linux-kernel - Re: [PATCH] bio: decrease bi_iter.bi

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Date:	Thu, 29 May 2014 12:13:16 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Jet Chen <jet.chen@...el.com>
Cc:	Dongsu Park <dongsu.park@...fitbricks.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Maurizio Lombardi <mlombard@...hat.com>
Subject: Re: [PATCH] bio: decrease bi_iter.bi_size by len in the fail path

On Thu, May 29, 2014 at 11:35 AM, Jet Chen <jet.chen@...el.com> wrote:
> On 05/29/2014 12:59 AM, Ming Lei wrote:
>> On Wed, May 28, 2014 at 11:42 PM, Ming Lei <tom.leiming@...il.com> wrote:
>>> Hi Dongsu,
>>>
>>> On Wed, May 28, 2014 at 11:09 PM, Dongsu Park
>>> <dongsu.park@...fitbricks.com> wrote:
>>>> From: Dongsu Park <dongsu.park@...fitbricks.com>
>>>>
>>>> Commit 3979ef4dcf3d1de55a560a3a4016c30a835df44d ("bio-modify-
>>>> __bio_add_page-to-accept-pages-that-dont-start-a-new-segment-v3")
>>>> introduced a regression as reported by Jet Chen.
>>>> That results in a kernel BUG at drivers/block/virtio_blk.c:166.
>>>>
>>>> To fix that, bi_iter.bi_size must be decreased by len, before
>>>> recounting the number of physical segments.
>>>>
>>>> Tested on with kernel 3.15.0-rc7-next-20140527 on qemu guest,
>>>> by running xfstests/ext4/271.
>>>>
>>>> Cc: Jens Axboe <axboe@...nel.dk>
>>>> Cc: Jet Chen <jet.chen@...el.com>
>>>> Cc: Maurizio Lombardi <mlombard@...hat.com>
>>>> Signed-off-by: Dongsu Park <dongsu.park@...fitbricks.com>
>>>> ---
>>>>  block/bio.c | 1 +
>>>>  1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/block/bio.c b/block/bio.c
>>>> index 0443694ccbb4..67d7cba1e5fd 100644
>>>> --- a/block/bio.c
>>>> +++ b/block/bio.c
>>>> @@ -810,6 +810,7 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
>>>>         bvec->bv_len = 0;
>>>>         bvec->bv_offset = 0;
>>>>         bio->bi_vcnt--;
>>>> +       bio->bi_iter.bi_size -= len;
>>>
>>> Would you mind explaining why bi_iter.bi_size need to be
>>> decreased by 'len'? In the failure path, it wasn't added by
>>> 'len', was it?
>>
>> Actually, the correct thing may be like what did in the
>> attached patch, as Maurizio discussed with me[1].
>>
>> Very interestingly, I have reproduced the problem one time
>> with ext4/271 ext4/301 ext4/305, but won't with the attached
>> patch after running it for 3 rounds.
>>
>> [tom@...alhost xfstests]$ sudo ./check ext4/271 ext4/301 ext4/305
>> FSTYP         -- ext4
>> PLATFORM      -- Linux/x86_64 localhost 3.15.0-rc7-next-20140527+
>> MKFS_OPTIONS  -- /dev/vdc
>> MOUNT_OPTIONS -- -o acl,user_xattr /dev/vdc /mnt/scratch
>>
>> ext4/271 1s ... 1s
>> ext4/301 31s ... 32s
>> ext4/305 181s ... 180s
>> Ran: ext4/271 ext4/301 ext4/305
>> Passed all 3 tests
>>
>> Jet, could you test the attached patch?
>
> sorry, could you specify which patch need me to test ?
> actually I got confused. I only find

Firstly, dongsu's patch is wrong, and it doesn't make sense to test
that.

Secondly, it is the patch attached in my last email, and the
name is 'fix_compute_segments.patch'.

Please let me know if you can find the patch, if you still can't, I
may resend to you.


Thanks,
-- 
Ming Lei
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