Xen devel - Jul 2013 - [PATCH RFC 2/4] xen-blkfront: improve aproximation of required grants per request

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.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 drivers/block/xen-blkfront.c |   11 +++++++----
 1 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 2e1ee34..187a437 100644
--- 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) {
-- 
1.7.7.5 (Apple Git-26)


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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) {

On 11/07/13 15:20, David Vrabel wrote:
> 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?
This contains the number of scatter-gather DMA addr+len pairs, which is
basically what we put at every granted page. for_each_sg iterates over
the DMA addr+len pairs and uses a grant page for each of them.