block: Make backing files always writeback

First of all, we're generally not writing to backing files, but when we
do, it's in the context of block jobs which know very well when to flush
the image.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

block.c

@@ -738,8 +738,9 @@ static void bdrv_backing_options(int *child_flags, QDict *child_options,
 {
     int flags = parent_flags;
 
-    /* The cache mode is inherited unmodified for backing files */
-    qdict_copy_default(child_options, parent_options, BDRV_OPT_CACHE_WB);
+    /* The cache mode is inherited unmodified for backing files; except WCE,
+     * which is only applied on the top level (BlockBackend) */
+    qdict_set_default_str(child_options, BDRV_OPT_CACHE_WB, "on");
     qdict_copy_default(child_options, parent_options, BDRV_OPT_CACHE_DIRECT);
     qdict_copy_default(child_options, parent_options, BDRV_OPT_CACHE_NO_FLUSH);
 

tests/qemu-iotests/142.out

@@ -62,7 +62,7 @@ cache.direct=on on backing-file
 cache.writeback=off on none0
     Cache mode:       writethrough
     Cache mode:       writeback
-    Cache mode:       writethrough
+    Cache mode:       writeback
     Cache mode:       writeback
 
 cache.writeback=off on file
@@ -143,7 +143,7 @@ cache.writeback=off on none0
     Cache mode:       writethrough
     Cache mode:       writethrough
     Cache mode:       writeback
-    Cache mode:       writethrough
+    Cache mode:       writeback
     Cache mode:       writeback
 
 cache.writeback=off on file
@@ -302,7 +302,7 @@ cache.direct=on on backing-file
 cache.writeback=off on none0
     Cache mode:       writethrough, direct
     Cache mode:       writeback, direct
-    Cache mode:       writethrough, direct
+    Cache mode:       writeback, direct
     Cache mode:       writeback, direct
 
 cache.writeback=off on file
@@ -383,7 +383,7 @@ cache.writeback=off on none0
     Cache mode:       writeback, direct
     Cache mode:       writethrough
     Cache mode:       writeback
-    Cache mode:       writethrough
+    Cache mode:       writeback
     Cache mode:       writeback
 
 cache.writeback=off on file
@@ -718,7 +718,7 @@ cache.direct=on on backing-file
 cache.writeback=off on none0
     Cache mode:       writethrough
     Cache mode:       writeback
-    Cache mode:       writethrough, direct
+    Cache mode:       writeback, direct
     Cache mode:       writeback, direct
 
 cache.writeback=off on file