backends/hostmem: Fix support of memory-backend-memfd in qemu_maxrampagesize()

It is currently not possible yet to use "memory-backend-memfd" on s390x
with hugepages enabled. This problem is caused by qemu_maxrampagesize()
not taking memory-backend-memfd objects into account yet, so the code
in s390_memory_init() fails to enable the huge page support there via
s390_set_max_pagesize(). Fix it by generalizing the code, so that it
looks at qemu_ram_pagesize(memdev->mr.ram_block) instead of re-trying
to get the information from the filesystem.

Suggested-by: David Hildenbrand <david@redhat.com>
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2116496
Message-Id: <20220810125720.3849835-2-thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

backends/hostmem.c

@@ -306,22 +306,12 @@ bool host_memory_backend_is_mapped(HostMemoryBackend *backend)
     return backend->is_mapped;
 }
 
-#ifdef __linux__
 size_t host_memory_backend_pagesize(HostMemoryBackend *memdev)
 {
-    Object *obj = OBJECT(memdev);
-    char *path = object_property_get_str(obj, "mem-path", NULL);
-    size_t pagesize = qemu_mempath_getpagesize(path);
-
-    g_free(path);
+    size_t pagesize = qemu_ram_pagesize(memdev->mr.ram_block);
+    g_assert(pagesize >= qemu_real_host_page_size());
     return pagesize;
 }
-#else
-size_t host_memory_backend_pagesize(HostMemoryBackend *memdev)
-{
-    return qemu_real_host_page_size();
-}
-#endif
 
 static void
 host_memory_backend_memory_complete(UserCreatable *uc, Error **errp)