On Wed, 11/09 14:06, Eric Blake wrote: > 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.
Alternatively, maybe add a blkdebug option to emulate a small max_transfer at the protocol layer? Fam
