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[Qemu-block] [PATCH v2 2/3] block/mirror: Fix target backing BDS

From: Max Reitz
Subject: [Qemu-block] [PATCH v2 2/3] block/mirror: Fix target backing BDS
Date: Mon, 6 Jun 2016 16:42:11 +0200

Currently, we are trying to move the backing BDS from the source to the
target in bdrv_replace_in_backing_chain() which is called from
mirror_exit(). However, mirror_complete() already tries to open the
target's backing chain with a call to bdrv_open_backing_file().

First, we should only set the target's backing BDS once. Second, the
mirroring block job has a better idea of what to set it to than the
generic code in bdrv_replace_in_backing_chain() (in fact, the latter's
conditions on when to move the backing BDS from source to target are not
really correct).

Therefore, remove that code from bdrv_replace_in_backing_chain() and
leave it to mirror_complete().

However, mirror_complete() in turn pursues a questionable strategy by
employing bdrv_open_backing_file(): On the one hand, because this may
open the wrong backing file with drive-mirror in "existing" mode, or
because it will not override a possibly wrong backing file in the
blockdev-mirror case.

On the other hand, we want to reuse the existing backing chain of the
source instead of opening everything anew, because the latter results in
having multiple BDSs for a single physical file and thus potentially
concurrent access which we should try to avoid.

Thus, instead of invoking bdrv_open_backing_file(), just set the correct
backing BDS directly via bdrv_set_backing_hd(). Also, do so only when
mirror_complete() is certain to succeed.

In contrast to what bdrv_replace_in_backing_chain() did so far, we do
not need to drop the source's backing file.

Signed-off-by: Max Reitz <address@hidden>
 block.c        |  8 --------
 block/mirror.c | 21 +++++++++++++--------
 2 files changed, 13 insertions(+), 16 deletions(-)

diff --git a/block.c b/block.c
index 16463aa..792f5dd 100644
--- a/block.c
+++ b/block.c
@@ -2288,14 +2288,6 @@ void bdrv_replace_in_backing_chain(BlockDriverState 
*old, BlockDriverState *new)
     change_parent_backing_link(old, new);
-    /* Change backing files if a previously independent node is added to the
-     * chain. For active commit, we replace top by its own (indirect) backing
-     * file and don't do anything here so we don't build a loop. */
-    if (new->backing == NULL && !bdrv_chain_contains(backing_bs(old), new)) {
-        bdrv_set_backing_hd(new, backing_bs(old));
-        bdrv_set_backing_hd(old, NULL);
-    }
diff --git a/block/mirror.c b/block/mirror.c
index 80fd3c7..217475b 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -742,15 +742,11 @@ static void mirror_set_speed(BlockJob *job, int64_t 
speed, Error **errp)
 static void mirror_complete(BlockJob *job, Error **errp)
     MirrorBlockJob *s = container_of(job, MirrorBlockJob, common);
-    Error *local_err = NULL;
-    int ret;
+    BlockDriverState *src, *target;
+    src = blk_bs(job->blk);
+    target = blk_bs(s->target);
-    ret = bdrv_open_backing_file(blk_bs(s->target), NULL, "backing",
-                                 &local_err);
-    if (ret < 0) {
-        error_propagate(errp, local_err);
-        return;
-    }
     if (!s->synced) {
         error_setg(errp, QERR_BLOCK_JOB_NOT_READY, job->id);
@@ -777,6 +773,15 @@ static void mirror_complete(BlockJob *job, Error **errp)
+    /* Now we need to adjust the target's backing BDS. This is not necessary
+     * when having performed a commit operation. */
+    if (!bdrv_chain_contains(backing_bs(src), target)) {
+        BlockDriverState *backing = s->is_none_mode ? src : s->base;
+        if (backing_bs(target) != backing) {
+            bdrv_set_backing_hd(target, backing);
+        }
+    }
     s->should_complete = true;

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