On 11/09/2016 07:49 AM, Stefan Hajnoczi wrote: > On Tue, Nov 08, 2016 at 04:52:15PM -0600, Eric Blake wrote: >> Commit 443668ca rewrote the write_zeroes logic to guarantee that >> an unaligned request never crosses a cluster boundary. But >> in the rewrite, the new code assumed that at most one iteration >> would be needed to get to an alignment boundary. >> >> However, it is easy to trigger an assertion failure: the Linux >> kernel limits loopback devices to advertise a max_transfer of >> only 64k. Any operation that requires falling back to writes >> rather than more efficient zeroing must obey max_transfer during >> that fallback, which means an unaligned head may require multiple >> iterations of the write fallbacks before reaching the aligned >> boundaries, when layering a format with clusters larger than 64k >> atop the protocol of file access to a loopback device. >> >> Test case: >> >> $ qemu-img create -f qcow2 -o cluster_size=1M file 10M >> $ losetup /dev/loop2 /path/to/file >> $ qemu-io -f qcow2 /dev/loop2 >> qemu-io> w 7m 1k >> qemu-io> w -z 8003584 2093056 > > Please include a qemu-iotests test case to protect against regressions.
None of the existing qemu-iotests use losetup; I guess the closest thing to do is crib from a test that uses passwordless sudo? It will certainly be a separate commit, but I'll give it my best shot to post something soon. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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