Am 15.06.2018 um 20:42 hat Vladimir Sementsov-Ogievskiy geschrieben: > 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.
Hm, right, not sure what I really meant back then when I added it to my to-do list... Maybe I confused reopen and invalidate_cache. > > 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 > .. Is it possible? Getting the permission problems out of the way can be solved by changing permissions twice, like I said above: First to the combined permissions of old and new, and finally to only the new permissions. The problem I see with .bdrv_reopen_prepare_after_children is that I don't see how it actually buys you anything: Even if the children already prepared the reopen, any access of the child node still refers to the old file descriptor because the new one only becomes valid with .bdrv_reopen_commit. > Now, I've found the following workaround, what do you think about something > like this as a temporary fix: I honestly don't understand why this workaround makes any difference. Shouldn't all .bdrv_reopen_prepare() callbacks still work on the old version of the child node? Even if I understood the reason, it looks a bit too hacky probably. Maybe I'll change may opinion once I understand it. Kevin
