[Xen-devel] [PATCH v2 1/1] xen/blkfront: always allocate grants first from per-queue persistent grants

[Dongli Zhang <dongli.zhang@xxxxxxxxxx>]

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.

Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>

---
Changed since v1:
  * use "max_grefs - rinfo->persistent_gnts_c" as callback argument

---
 drivers/block/xen-blkfront.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 3945963..4544a1c 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -713,6 +713,7 @@ 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;
 
@@ -724,19 +725,21 @@ 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);
@@ -837,7 +840,7 @@ 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;
-- 
2.7.4


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