On 17.11.2016 21:13, 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 > > In fairness to Denis (as the original listed author of the culprit > commit), the faulty logic for at most one iteration is probably all > my fault in reworking his idea. But the solution is to restore what > was in place prior to that commit: when dealing with an unaligned > head or tail, iterate as many times as necessary while fragmenting > the operation at max_transfer boundaries. > > Reported-by: Ed Swierk <eswi...@skyportsystems.com> > CC: qemu-sta...@nongnu.org > CC: Denis V. Lunev <d...@openvz.org> > Signed-off-by: Eric Blake <ebl...@redhat.com> > --- > block/io.c | 13 ++++++++----- > 1 file changed, 8 insertions(+), 5 deletions(-)
Reviewed-by: Max Reitz <mre...@redhat.com>
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