[PATCH v2 0/2] xen-blkfront: fixes for blkfront

[PATCH v2 0/2] xen-blkfront: fixes for blkfront

[Roger Pau Monne]

This series contains a couple of improvements to blkfront to make it 
work better if there's a shortage on available free grants.

[PATCH v2 1/2] xen-blkfront: improve aproximation of required grants per request

[Roger Pau Monne]

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 Monné <roger.pau@citrix.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.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)

[PATCH v2 2/2] xen-blkfront: revoke foreign access for grants not mapped by the backend

[Roger Pau Monne]

There's no need to keep the foreign access in a grant if it is not
persistently mapped by the backend. This allows us to free grants that
are not mapped by the backend, thus preventing blkfront from hoarding
all grants.

The main effect of this is that blkfront will only persistently map
the same grants as the backend, and it will always try to use grants
that are already mapped by the backend. Also the number of persistent
grants in blkfront is the same as in blkback (and is controlled by the
value in blkback).

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Matt Wilson <msw@amazon.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
---
 drivers/block/xen-blkfront.c |   33 +++++++++++++++++++++++++++++----
 1 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 187a437..bc9fc7d 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -1016,13 +1016,38 @@ static void blkif_completion(struct blk_shadow *s, struct blkfront_info *info,
 	}
 	/* Add the persistent grant into the list of free grants */
 	for (i = 0; i < nseg; i++) {
-		list_add(&s->grants_used[i]->node, &info->persistent_gnts);
-		info->persistent_gnts_c++;
+		if (gnttab_query_foreign_access(s->grants_used[i]->gref)) {
+			/*
+			 * If the grant is still mapped by the backend (the
+			 * backend has chosen to make this grant persistent)
+			 * we add it at the head of the list, so it will be
+			 * reused first.
+			 */
+			list_add(&s->grants_used[i]->node, &info->persistent_gnts);
+			info->persistent_gnts_c++;
+		} else {
+			/*
+			 * If the grant is not mapped by the backend we end the
+			 * foreign access and add it to the tail of the list,
+			 * so it will not be picked again unless we run out of
+			 * persistent grants.
+			 */
+			gnttab_end_foreign_access(s->grants_used[i]->gref, 0, 0UL);
+			s->grants_used[i]->gref = GRANT_INVALID_REF;
+			list_add_tail(&s->grants_used[i]->node, &info->persistent_gnts);
+		}
 	}
 	if (s->req.operation == BLKIF_OP_INDIRECT) {
 		for (i = 0; i < INDIRECT_GREFS(nseg); i++) {
-			list_add(&s->indirect_grants[i]->node, &info->persistent_gnts);
-			info->persistent_gnts_c++;
+			if (gnttab_query_foreign_access(s->indirect_grants[i]->gref)) {
+				list_add(&s->indirect_grants[i]->node, &info->persistent_gnts);
+				info->persistent_gnts_c++;
+			} else {
+				gnttab_end_foreign_access(s->indirect_grants[i]->gref, 0, 0UL);
+				s->indirect_grants[i]->gref = GRANT_INVALID_REF;
+				list_add_tail(&s->indirect_grants[i]->node,
+				              &info->persistent_gnts);
+			}
 		}
 	}
 }
-- 
1.7.7.5 (Apple Git-26)