If user sets .merge or .dump attributes qemu_madvise() is called
with corresponding advice. But it is never checked for failure
which may mislead users into thinking the attribute is set
correctly.I believe at this point it's too late to report errors
in that case but let's report a warning at least.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
---
 backends/hostmem.c | 27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/backends/hostmem.c b/backends/hostmem.c
index eb9682b4a8..1a6fd1c714 100644
--- a/backends/hostmem.c
+++ b/backends/hostmem.c
@@ -14,6 +14,7 @@
 #include "sysemu/hostmem.h"
 #include "hw/boards.h"
 #include "qapi/error.h"
+#include "qemu/error-report.h"
 #include "qapi/qapi-builtin-visit.h"
 #include "qapi/visitor.h"
 #include "qemu/config-file.h"
@@ -178,8 +179,11 @@ static void host_memory_backend_set_merge(Object *obj, bool value, Error **errp)
         void *ptr = memory_region_get_ram_ptr(&backend->mr);
         uint64_t sz = memory_region_size(&backend->mr);
 
-        qemu_madvise(ptr, sz,
-                     value ? QEMU_MADV_MERGEABLE : QEMU_MADV_UNMERGEABLE);
+        if (qemu_madvise(ptr, sz,
+                         value ? QEMU_MADV_MERGEABLE : QEMU_MADV_UNMERGEABLE)) {
+            warn_report("Couldn't change property 'merge' on '%s': %s",
+                        object_get_typename(obj), strerror(errno));
+        }
         backend->merge = value;
     }
 }
@@ -204,8 +208,11 @@ static void host_memory_backend_set_dump(Object *obj, bool value, Error **errp)
         void *ptr = memory_region_get_ram_ptr(&backend->mr);
         uint64_t sz = memory_region_size(&backend->mr);
 
-        qemu_madvise(ptr, sz,
-                     value ? QEMU_MADV_DODUMP : QEMU_MADV_DONTDUMP);
+        if (qemu_madvise(ptr, sz,
+                         value ? QEMU_MADV_DODUMP : QEMU_MADV_DONTDUMP)) {
+            warn_report("Couldn't change property 'dump' on '%s': %s",
+                        object_get_typename(obj), strerror(errno));
+        }
         backend->dump = value;
     }
 }
@@ -337,11 +344,15 @@ host_memory_backend_memory_complete(UserCreatable *uc, Error **errp)
     ptr = memory_region_get_ram_ptr(&backend->mr);
     sz = memory_region_size(&backend->mr);
 
-    if (backend->merge) {
-        qemu_madvise(ptr, sz, QEMU_MADV_MERGEABLE);
+    if (backend->merge &&
+        qemu_madvise(ptr, sz, QEMU_MADV_MERGEABLE)) {
+        warn_report("Couldn't set property 'merge' on '%s': %s",
+                    object_get_typename(OBJECT(uc)), strerror(errno));
     }
-    if (!backend->dump) {
-        qemu_madvise(ptr, sz, QEMU_MADV_DONTDUMP);
+    if (!backend->dump &&
+        qemu_madvise(ptr, sz, QEMU_MADV_DONTDUMP)) {
+        warn_report("Couldn't set property 'dump' on '%s': %s",
+                    object_get_typename(OBJECT(uc)), strerror(errno));
     }
 #ifdef CONFIG_NUMA
     unsigned long lastbit = find_last_bit(backend->host_nodes, MAX_NODES);
-- 
2.44.1