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>

1 files changed, 31 insertions, 2 deletions

diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
index d1b4a4624d81..4c605c70ebf9 100644
--- a/drivers/hv/hv_balloon.c
+++ b/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;