[PATCH 3/6] block: share writes on backing child of fleecing node

[Vladimir Sementsov-Ogievskiy]

By default, we share writes on backing child only if our parents share
write permission on us.

Still, with fleecing scheme we want to be able to unshare writes on
fleecing node, which is a kind of immutable snapshot
(copy-before-write operations are write-unchanged). So, let's detect
fleecing node and share writes on its backing child. (we should share
them, otherwise copy-before-write filter can't write to its file
child).

With fleecing scheme we are sure, that writes to backing child goes
through copy-before-write filter, so we are safe to share them.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
---
 block/copy-before-write.h |  1 +
 block.c                   |  3 ++-
 block/copy-before-write.c | 37 +++++++++++++++++++++++++++++++++++++
 3 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/block/copy-before-write.h b/block/copy-before-write.h
index b386fd8f01..ca47af732a 100644
--- a/block/copy-before-write.h
+++ b/block/copy-before-write.h
@@ -36,5 +36,6 @@ BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
                                   BlockCopyState **bcs,
                                   Error **errp);
 void bdrv_cbw_drop(BlockDriverState *bs);
+bool bdrv_is_fleecing_node(BlockDriverState *bs);
 
 #endif /* COPY_BEFORE_WRITE_H */
diff --git a/block.c b/block.c
index 39a5d4be90..c03810e8fa 100644
--- a/block.c
+++ b/block.c
@@ -50,6 +50,7 @@
 #include "qemu/cutils.h"
 #include "qemu/id.h"
 #include "block/coroutines.h"
+#include "block/copy-before-write.h"
 
 #ifdef CONFIG_BSD
 #include <sys/ioctl.h>
@@ -2502,7 +2503,7 @@ static void bdrv_default_perms_for_cow(BlockDriverState 
*bs, BdrvChild *c,
      * writable and resizable backing file.
      * TODO Require !(perm & BLK_PERM_CONSISTENT_READ), too?
      */
-    if (shared & BLK_PERM_WRITE) {
+    if (shared & BLK_PERM_WRITE || bdrv_is_fleecing_node(bs)) {
         shared = BLK_PERM_WRITE | BLK_PERM_RESIZE;
     } else {
         shared = 0;
diff --git a/block/copy-before-write.c b/block/copy-before-write.c
index 1b104545bd..88c9bb0a91 100644
--- a/block/copy-before-write.c
+++ b/block/copy-before-write.c
@@ -269,6 +269,43 @@ void bdrv_cbw_drop(BlockDriverState *bs)
     bdrv_unref(bs);
 }
 
+/*
+ * Detect is bs a fleecing node in some fleecing sceheme like:
+ *
+ * copy-before-write -- target --> fleecing-node
+ *   |                               |
+ *   | file                          | backing
+ * active-node  <---------------------
+ *
+ * In this case, fleecing-node can (and should) safely share writes on its
+ * backing child.
+ */
+bool bdrv_is_fleecing_node(BlockDriverState *bs)
+{
+    BdrvChild *parent;
+    BlockDriverState *parent_bs;
+    BDRVCopyBeforeWriteState *s;
+
+    QLIST_FOREACH(parent, &bs->parents, next_parent) {
+        if (parent->klass != &child_of_bds) {
+            continue;
+        }
+
+        parent_bs = parent->opaque;
+        if (parent_bs->drv != &bdrv_cbw_filter) {
+            continue;
+        }
+
+        s = parent_bs->opaque;
+
+        if (s->target->bs == bs && cbw_is_fleecing(parent_bs)) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
 static void cbw_init(void)
 {
     bdrv_register(&bdrv_cbw_filter);
-- 
2.29.2