xen/blkfront: always allocate grants first from per-queue persistent grants

This patch partially reverts 3df0e50 ("xen/blkfront: pseudo support for
multi hardware queues/rings"). The xen-blkfront queue/ring might hang due
to grants allocation failure in the situation when gnttab_free_head is
almost empty while many persistent grants are reserved for this queue/ring.

As persistent grants management was per-queue since 73716df ("xen/blkfront:
make persistent grants pool per-queue"), we should always allocate from
persistent grants first.

Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

1 files changed, 11 insertions, 8 deletions

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 04eeb540490f..98e34e4c62b8 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -708,6 +708,7 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
          * existing persistent grants, or if we have to get new grants,
          * as there are not sufficiently many free.
          */
+        bool new_persistent_gnts = false;
         struct scatterlist *sg;
         int num_sg, max_grefs, num_grant;
 
@@ -719,19 +720,21 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
                  */
                 max_grefs += INDIRECT_GREFS(max_grefs);
 
-        /*
-         * We have to reserve 'max_grefs' grants because persistent
-         * grants are shared by all rings.
-         */
-        if (max_grefs > 0)
-                if (gnttab_alloc_grant_references(max_grefs, &setup.gref_head) < 0) {
+        /* Check if we have enough persistent grants to allocate a requests */
+        if (rinfo->persistent_gnts_c < max_grefs) {
+                new_persistent_gnts = true;
+
+                if (gnttab_alloc_grant_references(
+                    max_grefs - rinfo->persistent_gnts_c,
+                    &setup.gref_head) < 0) {
                         gnttab_request_free_callback(
                                 &rinfo->callback,
                                 blkif_restart_queue_callback,
                                 rinfo,
-                                max_grefs);
+                                max_grefs - rinfo->persistent_gnts_c);
                         return 1;
                 }
+        }
 
         /* Fill out a communications ring structure. */
         id = blkif_ring_get_request(rinfo, req, &ring_req);
@@ -832,7 +835,7 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
         if (unlikely(require_extra_req))
                 rinfo->shadow[extra_id].req = *extra_ring_req;
 
-        if (max_grefs > 0)
+        if (new_persistent_gnts)
                 gnttab_free_grant_references(setup.gref_head);
 
         return 0;