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Date: Thu, 10 Nov 2016 10:11:45 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [Qemu-block] [PATCH for-2.8] block: Let write
 zeroes fallback work even with small max_transfer
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To: Eric Blake <eblake@redhat.com>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org, Kevin Wolf <kwolf@redhat.com>, qemu-block@nongnu.org, qemu-stable@nongnu.org, Max Reitz <mreitz@redhat.com>, Ed Swierk <eswierk@skyportsystems.com>, Stefan Hajnoczi <stefanha@redhat.com>, "Denis V . Lunev" <den@openvz.org>

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