Re: [PATCH] block: make sure big bio is splitted into at most 256 bvecs

From: Ming Lei <ming.lei@canonical.com>
To: Shaohua Li <shli@fb.com>
Cc: Jens Axboe <axboe@fb.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-block@vger.kernel.org,
	Kent Overstreet <kent.overstreet@gmail.com>,
	Christoph Hellwig <hch@infradead.org>,
	Eric Wheeler <bcache@lists.ewheeler.net>,
	Sebastian Roesner <sroesner-kernelorg@roesner-online.de>,
	"4.2+" <stable@vger.kernel.org>
Subject: Re: [PATCH] block: make sure big bio is splitted into at most 256 bvecs
Date: Wed, 6 Apr 2016 08:47:56 +0800	[thread overview]
Message-ID: <CACVXFVPveQZmbT0yQ3NMJK-W-9JJy5S4dqfv1e7TTDAVgC56pg@mail.gmail.com> (raw)
In-Reply-To: <20160405182720.GA2375685@devbig084.prn1.facebook.com>

On Wed, Apr 6, 2016 at 2:27 AM, Shaohua Li <shli@fb.com> wrote:
> On Wed, Apr 06, 2016 at 01:44:06AM +0800, Ming Lei wrote:
>> After arbitrary bio size is supported, the incoming bio may
>> be very big. We have to split the bio into small bios so that
>> each holds at most BIO_MAX_PAGES bvecs for safety reason, such
>> as bio_clone().
>>
>> This patch fixes the following kernel crash:
>>
>> > [  172.660142] BUG: unable to handle kernel NULL pointer dereference at
>> > 0000000000000028
>> > [  172.660229] IP: [<ffffffff811e53b4>] bio_trim+0xf/0x2a
>> > [  172.660289] PGD 7faf3e067 PUD 7f9279067 PMD 0
>> > [  172.660399] Oops: 0000 [#1] SMP
>> > [...]
>> > [  172.664780] Call Trace:
>> > [  172.664813]  [<ffffffffa007f3be>] ? raid1_make_request+0x2e8/0xad7 [raid1]
>> > [  172.664846]  [<ffffffff811f07da>] ? blk_queue_split+0x377/0x3d4
>> > [  172.664880]  [<ffffffffa005fb5f>] ? md_make_request+0xf6/0x1e9 [md_mod]
>> > [  172.664912]  [<ffffffff811eb860>] ? generic_make_request+0xb5/0x155
>> > [  172.664947]  [<ffffffffa0445c89>] ? prio_io+0x85/0x95 [bcache]
>> > [  172.664981]  [<ffffffffa0448252>] ? register_cache_set+0x355/0x8d0 [bcache]
>> > [  172.665016]  [<ffffffffa04497d3>] ? register_bcache+0x1006/0x1174 [bcache]
>>
>> Fixes: 54efd50(block: make generic_make_request handle arbitrarily sized bios)
>
> this bug is introduced by d2be537c3ba
>> Reported-by: Sebastian Roesner <sroesner-kernelorg@roesner-online.de>
>> Reported-by: Eric Wheeler <bcache@lists.ewheeler.net>
>> Cc: stable@vger.kernel.org (4.2+)
>> Cc: Shaohua Li <shli@fb.com>
>> Signed-off-by: Ming Lei <ming.lei@canonical.com>
>> ---
>> I can reproduce the issue and verify the fix by the following approach:
>>       - create one raid1 over two virtio-blk
>>       - build bcache device over the above raid1 and another cache device.
>>       - set cache mode as writeback
>>       - run random write over ext4 on the bcache device
>>       - then the crash can be triggered
>
> can you explain why this is better than my original patch?

Shaohua, your patch is wrong, please see the following link:

    https://lkml.org/lkml/2016/3/30/893

Thanks,

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