[PATCH v1] xen/grant-table: log the lack of grants

[Wengang Wang <wen.gang.wang@oracle.com>]

log a message when we enter this situation:
1) we already allocated the max number of available grants from hypervisor
and
2) we still need more (but the request fails because of 1)).

Sometimes the lack of grants causes IO hangs in xen_blkfront devices.
Adding this log would help debuging.

Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Junxiao Bi <junxiao.bi@oracle.com>
---
 drivers/xen/grant-table.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
index d6786b8..2c6a911 100644
--- a/drivers/xen/grant-table.c
+++ b/drivers/xen/grant-table.c
@@ -42,6 +42,7 @@
 #include <linux/delay.h>
 #include <linux/hardirq.h>
 #include <linux/workqueue.h>
+#include <linux/ratelimit.h>
 
 #include <xen/xen.h>
 #include <xen/interface/xen.h>
@@ -1072,8 +1073,14 @@ static int gnttab_expand(unsigned int req_entries)
 	cur = nr_grant_frames;
 	extra = ((req_entries + (grefs_per_grant_frame-1)) /
 		 grefs_per_grant_frame);
-	if (cur + extra > gnttab_max_grant_frames())
+	if (cur + extra > gnttab_max_grant_frames()) {
+		pr_warn_ratelimited("xen/grant-table: max_grant_frames reached"
+				    " cur=%u extra=%u limit=%u"
+				    " gnttab_free_count=%u req_entries=%u\n",
+				    cur, extra, gnttab_max_grant_frames(),
+				    gnttab_free_count, req_entries);
 		return -ENOSPC;
+	}
 
 	rc = gnttab_map(cur, cur + extra - 1);
 	if (rc == 0)
-- 
2.9.4