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[PATCH 20/21] block: introduce bdrv_drop_filter()


From: Vladimir Sementsov-Ogievskiy
Subject: [PATCH 20/21] block: introduce bdrv_drop_filter()
Date: Mon, 23 Nov 2020 23:12:32 +0300

Using bdrv_replace_node() for removing filter is not good enough: it
keeps child reference of the filter, which may conflict with original
top node during permission update.

Instead let's create new interface, which will do all graph
modifications first and then update permissions.

Let's modify bdrv_replace_node_common(), allowing it additionally drop
backing chain child link pointing to new node. This is quite
appropriate for bdrv_drop_intermediate() and makes possible to add
new bdrv_drop_filter() as a simple wrapper.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 include/block/block.h |  1 +
 block.c               | 35 ++++++++++++++++++++++++++++++++---
 2 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/include/block/block.h b/include/block/block.h
index 6c1efce0c3..981a07e29d 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -348,6 +348,7 @@ int bdrv_append(BlockDriverState *bs_new, BlockDriverState 
*bs_top,
                 Error **errp);
 int bdrv_replace_node(BlockDriverState *from, BlockDriverState *to,
                       Error **errp);
+int bdrv_drop_filter(BlockDriverState *bs, Error **errp);
 
 int bdrv_parse_aio(const char *mode, int *flags);
 int bdrv_parse_cache_mode(const char *mode, int *flags, bool *writethrough);
diff --git a/block.c b/block.c
index 6ea926e2c1..ee0143aff2 100644
--- a/block.c
+++ b/block.c
@@ -4923,15 +4923,30 @@ static int bdrv_replace_node_noperm(BlockDriverState 
*from,
  *
  * With auto_skip=false the error is returned if from has a parent which should
  * not be updated.
+ *
+ * With detach_subchain to must be in a backing chain of from. In this case
+ * backing link of the cow-parent of @to is removed.
  */
 static int bdrv_replace_node_common(BlockDriverState *from,
                                     BlockDriverState *to,
-                                    bool auto_skip, Error **errp)
+                                    bool auto_skip, bool detach_subchain,
+                                    Error **errp)
 {
     int ret = -EPERM;
     GSList *tran = NULL;
     g_autoptr(GHashTable) found = NULL;
     g_autoptr(GSList) refresh_list = NULL;
+    BlockDriverState *to_cow_parent;
+
+    if (detach_subchain) {
+        assert(bdrv_chain_contains(from, to));
+        for (to_cow_parent = from;
+             bdrv_filter_or_cow_bs(to_cow_parent) != to;
+             to_cow_parent = bdrv_filter_or_cow_bs(to_cow_parent))
+        {
+            ;
+        }
+    }
 
     /* Make sure that @from doesn't go away until we have successfully attached
      * all of its parents to @to. */
@@ -4952,6 +4967,10 @@ static int bdrv_replace_node_common(BlockDriverState 
*from,
         goto out;
     }
 
+    if (detach_subchain) {
+        bdrv_remove_backing(to_cow_parent, &tran);
+    }
+
     found = g_hash_table_new(NULL, NULL);
 
     refresh_list = bdrv_topological_dfs(refresh_list, found, to);
@@ -4971,7 +4990,13 @@ out:
 int bdrv_replace_node(BlockDriverState *from, BlockDriverState *to,
                       Error **errp)
 {
-    return bdrv_replace_node_common(from, to, true, errp);
+    return bdrv_replace_node_common(from, to, true, false, errp);
+}
+
+int bdrv_drop_filter(BlockDriverState *bs, Error **errp)
+{
+    return bdrv_replace_node_common(bs, bdrv_filter_or_cow_bs(bs), true, true,
+                                    errp);
 }
 
 /*
@@ -5303,7 +5328,11 @@ int bdrv_drop_intermediate(BlockDriverState *top, 
BlockDriverState *base,
         updated_children = g_slist_prepend(updated_children, c);
     }
 
-    bdrv_replace_node_common(top, base, false, &local_err);
+    /*
+     * It seems correct to pass detach_subchain=true here, but it triggers one
+     * more yet not fixed bug
+     */
+    bdrv_replace_node_common(top, base, false, false, &local_err);
     if (local_err) {
         error_report_err(local_err);
         goto exit;
-- 
2.21.3




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