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>

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 04eeb540490f39c32de88cbd19ee6ed43ad27d7e..98e34e4c62b8b228a6ee6d37256e860ee7d3c2b7 100644 (file)
--- 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;