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

[Eric Wheeler]

On Wed, 6 Apr 2016, 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)
> Reported-by: Sebastian Roesner <sroesner-kernelorg@xxxxxxxxxxxxxxxxx>
> Reported-by: Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx (4.2+)

Ming Lei,

get_max_io_size doesn't appear until 4.5 based on a quick LXR check so we 
won't see it in stable I don't think.

It would be nice to see this fixed in 4.1 (if affected there).  Is there 
another place this could be applied to be a bit more backward compatible?

-Eric


--
Eric Wheeler

	

> 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
> 
>  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
> 
>