From: Igor Mammedov <imammedo@redhat.com>

Fallback might affect guest or worse whole host performance
or functionality if backing file were used to share guest RAM
with another process.

Patch deprecates fallback so that we could remove it in future
and ensure that QEMU will provide expected behavior and fail if
it can't use user provided backing file.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190626074228.11558-1-imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 hw/core/numa.c       | 6 ++++--
 qemu-deprecated.texi | 9 +++++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/hw/core/numa.c b/hw/core/numa.c
index b74262b743..7faec75a07 100644
--- a/hw/core/numa.c
+++ b/hw/core/numa.c
@@ -479,8 +479,10 @@ static void allocate_system_memory_nonnuma(MemoryRegion *mr, Object *owner,
             if (mem_prealloc) {
                 exit(1);
             }
-            error_report("falling back to regular RAM allocation.");
-
+            warn_report("falling back to regular RAM allocation");
+            error_printf("This is deprecated. Make sure that -mem-path "
+                         " specified path has sufficient resources to allocate"
+                         " -m specified RAM amount");
             /* Legacy behavior: if allocation failed, fall back to
              * regular RAM allocation.
              */
diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi
index 2fe9b72121..9cba82d5ec 100644
--- a/qemu-deprecated.texi
+++ b/qemu-deprecated.texi
@@ -112,6 +112,15 @@ QEMU using implicit generic or board specific splitting rule.
 Use @option{memdev} with @var{memory-backend-ram} backend or @option{mem} (if
 it's supported by used machine type) to define mapping explictly instead.
 
+@subsection -mem-path fallback to RAM (since 4.1)
+Currently if guest RAM allocation from file pointed by @option{mem-path}
+fails, QEMU falls back to allocating from RAM, which might result
+in unpredictable behavior since the backing file specified by the user
+is ignored. In the future, users will be responsible for making sure
+the backing storage specified with @option{-mem-path} can actually provide
+the guest RAM configured with @option{-m} and QEMU will fail to start up if
+RAM allocation is unsuccessful.
+
 @section QEMU Machine Protocol (QMP) commands
 
 @subsection block-dirty-bitmap-add "autoload" parameter (since 2.12.0)
-- 
2.18.0.rc1.1.g3f1ff2140