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Date: Tue, 5 Apr 2016 18:04:33 -0700
From: Shaohua Li <shli@fb.com>
To: Ming Lei <ming.lei@canonical.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
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On Wed, Apr 06, 2016 at 08:47:56AM +0800, Ming Lei wrote:
> 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

I don't see why, really, except it declares you are right :)

why it's 2560 instead of 2048?