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

[Hannes Reinecke]

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

Cheers,

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