14.06.2018 13:46, Kevin Wolf wrote:
Am 12.06.2018 um 20:57 hat Vladimir Sementsov-Ogievskiy geschrieben:
Hi all!
I've faced the following problem:
1. create image with dirty bitmap, a.qcow2 (start qemu and run qmp
command block-dirty-bitmap-add)
2. run the following commands:
qemu-img create -f qcow2 -b a.qcow2 b.qcow2 10M
qemu-io -c 'write 0 512' b.qcow2
qemu-img commit b.qcow2
3. last command fails with the following output:
Formatting 'b.qcow2', fmt=qcow2 size=68719476736 backing_file=a.qcow2
cluster_size=65536 lazy_refcounts=off refcount_bits=16
wrote 512/512 bytes at offset 0
512 bytes, 1 ops; 0.0953 sec (5.243 KiB/sec and 10.4867 ops/sec)
qemu-img: #block397: Failed to make dirty bitmaps writable: Can't
update
bitmap directory: Operation not permitted
qemu-img: Block job failed: Operation not permitted
And problem is that children are reopened _after_ parent. But qcow2
reopen
needs write access to its file, to write IN_USE flag to dirty-bitmaps
extension.
I was aware of a different instance of this problem: Assume a qcow2
image with an unknown autoclear flag (so it will be cleared on r/w
open), which is first opened r/o and then reopened r/w. This will fail
because .bdrv_reopen_prepare doesn't have the permissions yet.
Hm.. If I understand correctly qcow2_reopen_prepare doesn't deal with
autoclear flags, as it doesn't call qcow2_do_open.
Simply changing the order won't fix this because in the r/w -> r/o, the
driver will legitimately flush its caches in .bdrv_reopen_prepare, and
for this it still needs to be able to write.
We may need to have a way for nodes to access both the old and the new
state of their children. I'm not completely sure how to achieve this
best, though.
When I thought only of permissions, the obvious and simple thing to do
was to just get combined permissions for the old and new state, i.e.
'old_perm | new_perm' and 'old_shared & new_shared'. But I don't think
this is actually enough when the child node switches between a r/w and
a r/o file descriptor because even though QEMU's permission system would
allow the write, you still can't successfully write to a r/o file
descriptor.
Kevin
Maybe we want two .bdrv_reopen_prepare:
.bdrv_reopen_prepare_before_children and
.bdrv_reopen_prepare_after_children. But to write something in
reopen_prepare, we need to move bdrv_set_perm from reopen_commit to
reopen_prepare.. Is it possible?
Now, I've found the following workaround, what do you think about
something like this as a temporary fix:
diff --git a/include/block/block.h b/include/block/block.h
index e677080c4e..c21392491d 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -266,7 +266,8 @@ BlockDriverState *bdrv_new_open_driver(BlockDriver
*drv, const char *node_name,
BlockReopenQueue *bdrv_reopen_queue(BlockReopenQueue *bs_queue,
BlockDriverState *bs,
QDict *options, int flags);
-int bdrv_reopen_multiple(AioContext *ctx, BlockReopenQueue *bs_queue,
Error **errp);
+int bdrv_reopen_multiple(AioContext *ctx, BlockReopenQueue *bs_queue,
+ bool cheat_reopen_rw, Error **errp);
int bdrv_reopen(BlockDriverState *bs, int bdrv_flags, Error **errp);
int bdrv_reopen_prepare(BDRVReopenState *reopen_state,
BlockReopenQueue *queue, Error **errp);
diff --git a/block.c b/block.c
index 50887087f3..9b50828cd2 100644
--- a/block.c
+++ b/block.c
@@ -2988,7 +2988,8 @@ BlockReopenQueue
*bdrv_reopen_queue(BlockReopenQueue *bs_queue,
* All affected nodes must be drained between bdrv_reopen_queue() and
* bdrv_reopen_multiple().
*/
-int bdrv_reopen_multiple(AioContext *ctx, BlockReopenQueue *bs_queue,
Error **errp)
+int bdrv_reopen_multiple(AioContext *ctx, BlockReopenQueue *bs_queue,
+ bool cheat_reopen_rw, Error **errp)
{
int ret = -1;
BlockReopenQueueEntry *bs_entry, *next;
@@ -3005,6 +3006,14 @@ int bdrv_reopen_multiple(AioContext *ctx,
BlockReopenQueue *bs_queue, Error **er
bs_entry->prepared = true;
}
+ if (cheat_reopen_rw) {
+ /* reverse queue, to reopen children first */
+ QSIMPLEQ_FOREACH_SAFE(bs_entry, bs_queue, entry, next) {
+ QSIMPLEQ_REMOVE(bs_queue, bs_entry,
BlockReopenQueueEntry, entry);
+ QSIMPLEQ_INSERT_HEAD(bs_queue, bs_entry, entry);
+ }
+ }
+
/* If we reach this point, we have success and just need to apply
the
* changes
*/
@@ -3036,11 +3045,13 @@ int bdrv_reopen(BlockDriverState *bs, int
bdrv_flags, Error **errp)
int ret = -1;
Error *local_err = NULL;
BlockReopenQueue *queue;
+ bool cheat_reopen_rw = bdrv_is_read_only(bs) && (bdrv_flags &
BDRV_O_RDWR);
bdrv_subtree_drained_begin(bs);
queue = bdrv_reopen_queue(NULL, bs, NULL, bdrv_flags);
- ret = bdrv_reopen_multiple(bdrv_get_aio_context(bs), queue,
&local_err);
+ ret = bdrv_reopen_multiple(bdrv_get_aio_context(bs), queue,
cheat_reopen_rw,
+ &local_err);
if (local_err != NULL) {
error_propagate(errp, local_err);
}
diff --git a/block/replication.c b/block/replication.c
index 826db7b304..e528969e2b 100644
--- a/block/replication.c
+++ b/block/replication.c
@@ -411,7 +411,7 @@ static void reopen_backing_file(BlockDriverState
*bs, bool writable,
if (reopen_queue) {
bdrv_reopen_multiple(bdrv_get_aio_context(bs),
- reopen_queue, &local_err);
+ reopen_queue, false, &local_err);
error_propagate(errp, local_err);
}
diff --git a/qemu-io-cmds.c b/qemu-io-cmds.c
index 5bf5f28178..e0c4de323c 100644
--- a/qemu-io-cmds.c
+++ b/qemu-io-cmds.c
@@ -2051,7 +2051,7 @@ static int reopen_f(BlockBackend *blk, int argc,
char **argv)
bdrv_subtree_drained_begin(bs);
brq = bdrv_reopen_queue(NULL, bs, opts, flags);
- bdrv_reopen_multiple(bdrv_get_aio_context(bs), brq, &local_err);
+ bdrv_reopen_multiple(bdrv_get_aio_context(bs), brq, false,
&local_err);
bdrv_subtree_drained_end(bs);
if (local_err) {
