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Re: [PATCH v7 23/47] block/snapshot: Fix fallback

From: Andrey Shinkevich
Subject: Re: [PATCH v7 23/47] block/snapshot: Fix fallback
Date: Thu, 16 Jul 2020 00:22:37 +0300
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:68.0) Gecko/20100101 Thunderbird/68.9.0

On 25.06.2020 18:21, Max Reitz wrote:
If the top node's driver does not provide snapshot functionality and we
want to fall back to a node down the chain, we need to snapshot all
non-COW children.  For simplicity's sake, just do not fall back if there

I guess it comes to COW children like BDRV_CHILD_DATA | BDRV_CHILD_METADATA  rather than non-COW ones, does it?


is more than one such child.  Furthermore, we really only can fall back
to bs->file and bs->backing, because bdrv_snapshot_goto() has to modify
the child link (notably, set it to NULL).

Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
  block/snapshot.c | 104 +++++++++++++++++++++++++++++++++++++----------
  1 file changed, 83 insertions(+), 21 deletions(-)

diff --git a/block/snapshot.c b/block/snapshot.c
+    /*
+     * Check that there are no other children that would need to be
+     * snapshotted.  If there are, it is not safe to fall back to
+     * *fallback.
+     */
+    QLIST_FOREACH(child, &bs->children, next) {
+        if (child->role & (BDRV_CHILD_DATA | BDRV_CHILD_METADATA |
+                           BDRV_CHILD_FILTERED) &&
+            child != *fallback)
+        {
+            return NULL;
+        }
+    }
+    return fallback;


Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>

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