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block: Generalize should_update_child() rule

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Currently, bdrv_replace_node() refuses to create loops from one BDS to
itself if the BDS to be replaced is the backing node of the BDS to
replace it: Say there is a node A and a node B.  Replacing B by A means
making all references to B point to A.  If B is a child of A (i.e. A has
a reference to B), that would mean we would have to make this reference
point to A itself -- so we'd create a loop.

bdrv_replace_node() (through should_update_child()) refuses to do so if
B is the backing node of A.  There is no reason why we should create
loops if B is not the backing node of A, though.  The BDS graph should
never contain loops, so we should always refuse to create them.

If B is a child of A and B is to be replaced by A, we should simply
leave B in place there because it is the most sensible choice.

A more specific argument would be: Putting filter drivers into the BDS
graph is basically the same as appending an overlay to a backing chain.
But the main child BDS of a filter driver is not "backing" but "file",
so restricting the no-loop rule to backing nodes would fail here.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20180613181823.13618-7-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Max Reitz 2018-06-13 20:18:15 +02:00
parent 138f9fffb8
commit ec9f10fe06
1 changed files with 34 additions and 10 deletions
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44
block.c
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@ -3427,16 +3427,39 @@ static bool should_update_child(BdrvChild *c, BlockDriverState *to)
return false;
}
if (c->role == &child_backing) {
/* If @from is a backing file of @to, ignore the child to avoid
* creating a loop. We only want to change the pointer of other
* parents. */
QLIST_FOREACH(to_c, &to->children, next) {
if (to_c == c) {
break;
}
}
if (to_c) {
/* If the child @c belongs to the BDS @to, replacing the current
* c->bs by @to would mean to create a loop.
*
* Such a case occurs when appending a BDS to a backing chain.
* For instance, imagine the following chain:
*
* guest device -> node A -> further backing chain...
*
* Now we create a new BDS B which we want to put on top of this
* chain, so we first attach A as its backing node:
*
* node B
* |
* v
* guest device -> node A -> further backing chain...
*
* Finally we want to replace A by B. When doing that, we want to
* replace all pointers to A by pointers to B -- except for the
* pointer from B because (1) that would create a loop, and (2)
* that pointer should simply stay intact:
*
* guest device -> node B
* |
* v
* node A -> further backing chain...
*
* In general, when replacing a node A (c->bs) by a node B (@to),
* if A is a child of B, that means we cannot replace A by B there
* because that would create a loop. Silently detaching A from B
* is also not really an option. So overall just leaving A in
* place there is the most sensible choice. */
QLIST_FOREACH(to_c, &to->children, next) {
if (to_c == c) {
return false;
}
}
@ -3462,6 +3485,7 @@ void bdrv_replace_node(BlockDriverState *from, BlockDriverState *to,
/* Put all parents into @list and calculate their cumulative permissions */
QLIST_FOREACH_SAFE(c, &from->parents, next_parent, next) {
assert(c->bs == from);
if (!should_update_child(c, to)) {
continue;
}