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. 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
