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Re: [Qemu-devel] [PATCH v3 3/9] block: Let write zeroes fallback work ev

From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH v3 3/9] block: Let write zeroes fallback work even with small max_transfer
Date: Thu, 17 Nov 2016 22:40:00 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

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 <address@hidden>
> CC: address@hidden
> CC: Denis V. Lunev <address@hidden>
> Signed-off-by: Eric Blake <address@hidden>
> ---
>  block/io.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)

Reviewed-by: Max Reitz <address@hidden>

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