Re: [PATCH] md/raid5: limit request size according to implementation limits

[Coly Li <colyli@suse.de>]

On 2016/11/28 上午12:32, Konstantin Khlebnikov wrote:
> Current implementation employ 16bit counter of active stripes in lower
> bits of bio->bi_phys_segments. If request is big enough to overflow
> this counter bio will be completed and freed too early.
> 
> Fortunately this not happens in default configuration because several
> other limits prevent that: stripe_cache_size * nr_disks effectively
> limits count of active stripes. And small max_sectors_kb at lower
> disks prevent that during normal read/write operations.
> 
> Overflow easily happens in discard if it's enabled by module parameter
> "devices_handle_discard_safely" and stripe_cache_size is set big enough.
> 
> This patch limits requests size with 256Mb - 8Kb to prevent overflows.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> Cc: Shaohua Li <shli@kernel.org>
> Cc: Neil Brown <neilb@suse.com>
> Cc: stable@vger.kernel.org
> ---
>  drivers/md/raid5.c |    9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 92ac251e91e6..cce6057b9aca 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -6984,6 +6984,15 @@ static int raid5_run(struct mddev *mddev)
>  			stripe = (stripe | (stripe-1)) + 1;
>  		mddev->queue->limits.discard_alignment = stripe;
>  		mddev->queue->limits.discard_granularity = stripe;
> +
> +		/*
> +		 * We use 16-bit counter of active stripes in bi_phys_segments
> +		 * (minus one for over-loaded initialization)
> +		 */
> +		blk_queue_max_hw_sectors(mddev->queue, 0xfffe * STRIPE_SECTORS);
> +		blk_queue_max_discard_sectors(mddev->queue,
> +					      0xfffe * STRIPE_SECTORS);
> +

Could you please to explain why use 0xfffe * STRIPE_SECTORS here ?

Thanks.

Coly