Re: [Xen-devel] [PATCH RFC 2/4] xen-blkfront: improve aproximation of required grants per request

[David Vrabel <david.vrabel@citrix.com>]

On 08/07/13 14:03, Roger Pau Monne wrote:
> Improve the calculation of required grants to process a request by
> using nr_phys_segments instead of always assuming a request is going
> to use all posible segments.

This isn't obviously correct to me.  Why is this correct value for the
number of pages that need to be granted?

David

> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -400,10 +400,13 @@ static int blkif_queue_request(struct request *req)
>  	if (unlikely(info->connected != BLKIF_STATE_CONNECTED))
>  		return 1;
>  
> -	max_grefs = info->max_indirect_segments ?
> -		    info->max_indirect_segments +
> -		    INDIRECT_GREFS(info->max_indirect_segments) :
> -		    BLKIF_MAX_SEGMENTS_PER_REQUEST;
> +	max_grefs = req->nr_phys_segments;
> +	if (max_grefs > BLKIF_MAX_SEGMENTS_PER_REQUEST)
> +		/*
> +		 * If we are using indirect segments we need to account
> +		 * for the indirect grefs used in the request.
> +		 */
> +		max_grefs += INDIRECT_GREFS(req->nr_phys_segments);
>  
>  	/* Check if we have enough grants to allocate a requests */
>  	if (info->persistent_gnts_c < max_grefs) {