xen/grant-table: log the lack of grants

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: NWengang Wang <wen.gang.wang@oracle.com>
Reviewed-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: NJunxiao Bi <junxiao.bi@oracle.com>
Reviewed-by: NJuergen Gross <jgross@suse.com>
Signed-off-by: NJuergen Gross <jgross@suse.com>

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)