Am 10.11.2016 um 03:11 hat Fam Zheng geschrieben: > 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?
This sounds like the better option indeed. Kevin