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Date: Tue, 5 Apr 2016 11:27:21 -0700
From: Shaohua Li <shli@fb.com>
To: Ming Lei <ming.lei@canonical.com>
CC: Jens Axboe <axboe@fb.com>, <linux-kernel@vger.kernel.org>,
        <linux-block@vger.kernel.org>, <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
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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?