[Xen-devel] [PATCH 2/3] xen-blkfront: improve aproximation of required grants per request

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

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.

nr_phys_segments 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.

Signed-off-by: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
---
 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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