Re: [PATCH v4] blk-mq: fix nr_requests wrong value when modify it from sysfs

[weiping zhang <zhangweiping@didichuxing.com>]

On Fri, Sep 22, 2017 at 11:36:28PM +0800, weiping zhang wrote:
> if blk-mq use "none" io scheduler, nr_request get a wrong value when
> input a number > tag_set->queue_depth. blk_mq_tag_update_depth will get
> the smaller one min(nr, set->queue_depth), and then q->nr_request get a
> wrong value.
> 
> Reproduce:
> 
> echo none > /sys/block/nvme0n1/queue/scheduler
> echo 1000000 > /sys/block/nvme0n1/queue/nr_requests
> cat /sys/block/nvme0n1/queue/nr_requests
> 1000000
> 
> Signed-off-by: weiping zhang <zhangweiping@didichuxing.com>
> ---
> 
> Changes since v4:
>  * fix typo in commit message(queue/ioscheduler => queue/scheduler)
> 
> Changes since v3:
>  * remove compare nr with tags->qdepth, pass nr to blk_mq_tag_update_depth
> directly
> 
>  * remove return EINVAL when user modify nr_request less than BLKDEV_MIN_RQ
> 
> Changes since v2:
>  * add return EINVAL when user modify nr_request less than BLKDEV_MIN_RQ
>  * remove pr_warn, and return EINVAL, if input number is too large
> 
>  block/blk-mq.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 98a1860..491e336 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2642,8 +2642,7 @@ int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
>  		 * queue depth. This is similar to what the old code would do.
>  		 */
>  		if (!hctx->sched_tags) {
> -			ret = blk_mq_tag_update_depth(hctx, &hctx->tags,
> -							min(nr, set->queue_depth),
> +			ret = blk_mq_tag_update_depth(hctx, &hctx->tags, nr,
>  							false);
>  		} else {
>  			ret = blk_mq_tag_update_depth(hctx, &hctx->sched_tags,
> -- 
> 2.9.4
> 

Hi Jens,

As you say before:
> blk_mq_tag_update_depth() should already return
> -EINVAL for the case where we can't grow the tags. Looks like this patch
> should simply remove the min(nr, set->queue_depth) and just pass in 'nr'.
I also find that hctx->tags->nr_tags equal to tag_set->queue_depth no matter use
hctx->sched_tags or not. So I think it's safe to verify 'nr' by
hctx->tags->nr_tags.

Thanks
weiping