From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37446) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fTPmU-00022A-RK for qemu-devel@nongnu.org; Thu, 14 Jun 2018 06:47:10 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fTPmU-0003ig-1l for qemu-devel@nongnu.org; Thu, 14 Jun 2018 06:47:06 -0400 Date: Thu, 14 Jun 2018 12:46:56 +0200 From: Kevin Wolf Message-ID: <20180614104656.GD8564@localhost.localdomain> References: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] bug in reopen arch List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Vladimir Sementsov-Ogievskiy Cc: qemu block , qemu-devel , Max Reitz , "Denis V. Lunev" Am 12.06.2018 um 20:57 hat Vladimir Sementsov-Ogievskiy geschrieben: > Hi all! >=20 > I've faced the following problem: >=20 > =A0=A0=A0 1. create image with dirty bitmap, a.qcow2 (start qemu and ru= n qmp > =A0=A0=A0 command block-dirty-bitmap-add) >=20 > =A0=A0=A0 2. run the following commands: >=20 > =A0=A0=A0=A0=A0=A0=A0 qemu-img create -f qcow2 -b a.qcow2 b.qcow2 10M > =A0=A0=A0=A0=A0=A0=A0 qemu-io -c 'write 0 512' b.qcow2 > =A0=A0=A0=A0=A0=A0=A0 qemu-img commit b.qcow2 >=20 > =A0=A0=A0 3. last command fails with the following output: >=20 > Formatting 'b.qcow2', fmt=3Dqcow2 size=3D68719476736 backing_file=3Da.q= cow2 > cluster_size=3D65536 lazy_refcounts=3Doff refcount_bits=3D16 > 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 updat= e > bitmap directory: Operation not permitted > qemu-img: Block job failed: Operation not permitted >=20 > And problem is that children are reopened _after_ parent. But qcow2 reo= pen > 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