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

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