Re: [PATCH v2] block: make sure big bio is splitted into at most 256 bvecs

[Ming Lei]

On Fri, Jun 10, 2016 at 7:37 PM, Hannes Reinecke <hare@xxxxxxx> wrote:
> On 06/10/2016 01:07 PM, 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]
>>
>> The issue can be reproduced by the following steps:
>>       - create one raid1 over two virtio-blk
>>       - build bcache device over the above raid1 and another cache device
>>       and bucket size is set as 2Mbytes
>>       - set cache mode as writeback
>>       - run random write over ext4 on the bcache device
>>
>> Fixes: 54efd50(block: make generic_make_request handle arbitrarily sized bios)
>> Reported-by: Sebastian Roesner <sroesner-kernelorg@xxxxxxxxxxxxxxxxx>
>> Reported-by: Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx>
>> Cc: stable@xxxxxxxxxxxxxxx (4.3+)
>> Cc: Shaohua Li <shli@xxxxxx>
>> Acked-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
>> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>
>> ---
>> V2:
>>       - don't mark as REQ_NOMERGE in case the bio is splitted
>>       for reaching the limit of bvecs count
>> V1:
>>         - Kent pointed out that using max io size can't cover
>>         the case of non-full bvecs/pages
>>  block/blk-merge.c | 35 ++++++++++++++++++++++++++++++++---
>>  1 file changed, 32 insertions(+), 3 deletions(-)
>>
> Hmm. So everybody is suffering because someone _might_ be using bio_clone?

I believe most of usages are involved with <= 256 bvecs in one bio, so
only few(such as bcache) will 'suffer', not everybody, :-)

> Why can't we fixup bio_clone() (or the callers of which) to correctly
> set the queue limits?

IMO there isn't a good solution to fix the issue in bio_clone.

Firstly one page can held at most 256 bvecs, and not safe to allocate
multi-pages in I/O path.

Secondaly as said in the comment of the patch it can't be a queue limit
now because bio_clone() is used inside bio bounce.

But it should be possible to use bio splitting to deal with bio bounce, and
it can be a following up job, and of course that change can be a bit too big
for backporting.

That is why I suggest to fix the issue with this patch. Or other ideas?


Thanks,
Ming

>
> Cheers,
>
> Hannes
> --
> Dr. Hannes Reinecke                Teamlead Storage & Networking
> hare@xxxxxxx                                   +49 911 74053 688
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