linux-kernel - Re: [PATCH 2/2] block: return error if too many reserved tags are requested

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Message-ID: <541095C1.2040500@kernel.dk>
Date:	Wed, 10 Sep 2014 12:17:37 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Robert Elliott <relliott@...rdog.cce.hp.com>, elliott@...com,
	hch@....de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] block: return error if too many reserved tags are
 requested

On 09/09/2014 06:18 PM, Robert Elliott wrote:
> From: Robert Elliott <elliott@...com>
> 
> Make blk_mq_alloc_tag_set return an error if set->reserved_tags
> is greater than BLK_MQ_MAX_DEPTH minus the minimum number of
> tags, since:
> * set->queue_depth is truncated to that value
> * set->reserved_tags needs to be less than set->queue_depth
> 
> Signed-off-by: Robert Elliott <elliott@...com>
> ---
>  block/blk-mq.c |    8 +++++---
>  1 files changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index c49fe00..dc2970d 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1936,16 +1936,18 @@ int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
>  		return -EINVAL;
>  	if (!set->queue_depth)
>  		return -EINVAL;
> +	if (set->reserved_tags > BLK_MQ_MAX_DEPTH - BLK_MQ_TAG_MIN)
> +		return -EINVAL;
>  	if (set->queue_depth < set->reserved_tags + BLK_MQ_TAG_MIN)
>  		return -EINVAL;
>  
>  	if (!set->nr_hw_queues || !set->ops->queue_rq || !set->ops->map_queue)
>  		return -EINVAL;
>  
> -	if (set->queue_depth > BLK_MQ_MAX_DEPTH) {
> +	if (set->queue_depth > BLK_MQ_MAX_DEPTH - set->reserved_tags) {

This is harder to read than:

	if (set->queue_depth + set->reserved_tags > BLK_MQ_MAX_DEPTH) {

which more clearly expresses the same thing. But set->queue_depth is the
total pool, reserved tags come out of that pool. So I don't think this
is correct.

-- 
Jens Axboe

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