Drivers: hv: Notify the host of permanent hot-add failures

If memory hot-add fails with the error -EEXIST, then this is a permanent
failure. Notify the host of this information, so the host will not attempt
hot-add again. If the failure were a transient failure, host will attempt
a hot-add after some delay.

In this version of the patch, I have added some additional comments
to clarify how the host treats different failure conditions.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hv/hv_balloon.c

@@ -590,6 +590,16 @@ static void hv_mem_hot_add(unsigned long start, unsigned long size,
 
 		if (ret) {
 			pr_info("hot_add memory failed error is %d\n", ret);
+			if (ret == -EEXIST) {
+				/*
+				 * This error indicates that the error
+				 * is not a transient failure. This is the
+				 * case where the guest's physical address map
+				 * precludes hot adding memory. Stop all further
+				 * memory hot-add.
+				 */
+				do_hot_add = false;
+			}
 			has->ha_end_pfn -= HA_CHUNK;
 			has->covered_end_pfn -=  processed_pfn;
 			break;
@@ -849,11 +859,30 @@ static void hot_add_req(struct work_struct *dummy)
 		rg_sz = region_size;
 	}
 
-	resp.page_count = process_hot_add(pg_start, pfn_cnt,
-					rg_start, rg_sz);
+	if (do_hot_add)
+		resp.page_count = process_hot_add(pg_start, pfn_cnt,
+						rg_start, rg_sz);
 #endif
+	/*
+	 * The result field of the response structure has the
+	 * following semantics:
+	 *
+	 * 1. If all or some pages hot-added: Guest should return success.
+	 *
+	 * 2. If no pages could be hot-added:
+	 *
+	 * If the guest returns success, then the host
+	 * will not attempt any further hot-add operations. This
+	 * signifies a permanent failure.
+	 *
+	 * If the guest returns failure, then this failure will be
+	 * treated as a transient failure and the host may retry the
+	 * hot-add operation after some delay.
+	 */
 	if (resp.page_count > 0)
 		resp.result = 1;
+	else if (!do_hot_add)
+		resp.result = 1;
 	else
 		resp.result = 0;