linux-kernel - Re: [PATCH] virtio_blk: allow 0 as num_request

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Date:   Sun, 24 Oct 2021 17:19:33 +0300
From:   Max Gurtovoy <mgurtovoy@...dia.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        <linux-kernel@...r.kernel.org>
CC:     Jason Wang <jasowang@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Jens Axboe <axboe@...nel.dk>,
        <virtualization@...ts.linux-foundation.org>,
        <linux-block@...r.kernel.org>
Subject: Re: [PATCH] virtio_blk: allow 0 as num_request_queues


On 10/24/2021 4:54 PM, Michael S. Tsirkin wrote:
> The default value is 0 meaning "no limit". However if 0
> is specified on the command line it is instead silently
> converted to 1. Further, the value is already validated
> at point of use, there's no point in duplicating code
> validating the value when it is set.
>
> Simplify the code while making the behaviour more consistent
> by using plain module_param.
>
> Fixes: 1a662cf6cb9a ("virtio-blk: add num_request_queues module parameter")
> Cc: Max Gurtovoy <mgurtovoy@...dia.com>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>   drivers/block/virtio_blk.c | 14 +-------------
>   1 file changed, 1 insertion(+), 13 deletions(-)
>
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 6318134aab76..c336d9bb9105 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -30,20 +30,8 @@
>   #define VIRTIO_BLK_INLINE_SG_CNT	2
>   #endif
>   
> -static int virtblk_queue_count_set(const char *val,
> -		const struct kernel_param *kp)
> -{
> -	return param_set_uint_minmax(val, kp, 1, nr_cpu_ids);
> -}
> -
> -static const struct kernel_param_ops queue_count_ops = {
> -	.set = virtblk_queue_count_set,
> -	.get = param_get_uint,
> -};
> -
>   static unsigned int num_request_queues;
> -module_param_cb(num_request_queues, &queue_count_ops, &num_request_queues,
> -		0644);
> +module_param(num_request_queues, uint, 0644);

Not the best solution.

You can set the param to 1024 but in practice nr_cpu_ids can be 32 for 
example.


>   MODULE_PARM_DESC(num_request_queues,
>   		 "Limit the number of request queues to use for blk device. "
>   		 "0 for no limit. "