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

[Ming Lei]

On Wed, Apr 6, 2016 at 1:44 AM, Ming Lei <ming.lei@xxxxxxxxxxxxx> 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)
> Reported-by: Sebastian Roesner <sroesner-kernelorg@xxxxxxxxxxxxxxxxx>
> Reported-by: Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx (4.2+)
> Cc: Shaohua Li <shli@xxxxxx>
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>
> ---
> 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

For anyone who is interested in issue/fix, forget to mention:

       The bucket size should be set as bigger than 1M during making bcache.
       In my test, the bucket size is 2M.

Thanks,
Ming

>
>  block/blk-merge.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 2613531..9a8651f 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -79,6 +79,18 @@ static inline unsigned get_max_io_size(struct request_queue *q,
>         /* aligned to logical block size */
>         sectors &= ~(mask >> 9);
>
> +       /*
> +        * With arbitrary bio size, 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().
> +        *
> +        * In the future, the limit might be converted into per-queue
> +        * flag.
> +        */
> +       sectors = min_t(unsigned, sectors, BIO_MAX_PAGES <<
> +                       (PAGE_CACHE_SHIFT - 9));
> +
>         return sectors;
>  }
>
> --
> 1.9.1
>