Re: [Qemu-devel] [PATCH v2 21/21] iotests: Add test for different refcount widths

[Eric Blake]

On 11/14/2014 06:06 AM, Max Reitz wrote:
> Add a test for conversion between different refcount widths and errors
> specific to certain widths (i.e. snapshots with refcount_width=1).
> 
> Signed-off-by: Max Reitz <address@hidden>
> ---
>  tests/qemu-iotests/112     | 252 
> +++++++++++++++++++++++++++++++++++++++++++++
>  tests/qemu-iotests/112.out | 131 +++++++++++++++++++++++
>  tests/qemu-iotests/group   |   1 +
>  3 files changed, 384 insertions(+)
>  create mode 100755 tests/qemu-iotests/112
>  create mode 100644 tests/qemu-iotests/112.out
> 
> +echo
> +echo '=== Testing too many references for check ==='
> +echo
> +
> +IMGOPTS="$IMGOPTS,refcount_width=1" _make_test_img 64M
> +print_refcount_width
> +
> +# This cluster should be created at 0x50000
> +$QEMU_IO -c 'write 0 64k' "$TEST_IMG" | _filter_qemu_io
> +# Now make the second L2 entry (the L2 table should be at 0x40000) point to 
> that
> +# cluster, so we have two references
> +poke_file "$TEST_IMG" $((0x40008)) "\x80\x00\x00\x00\x00\x05\x00\x00"
> +
> +# This should say "please use amend"
> +_check_test_img -r all
> +
> +# So we do that
> +$QEMU_IMG amend -o refcount_width=2 "$TEST_IMG"
> +print_refcount_width
> +
> +# And try again
> +_check_test_img -r all

I think this section also deserves a test that fuzzes an image with
width=64 to intentionally set the most significant bit of one of the
refcounts, and make sure that we gracefully diagnose it as invalid.

> +
> +echo
> +echo '=== Multiple walks necessary during amend ==='
> +echo
> +
> +IMGOPTS="$IMGOPTS,refcount_width=1,cluster_size=512" _make_test_img 64k
> +
> +# Cluster 0 is the image header, clusters 1 to 4 are used by the L1 table, a
> +# single L2 table, the reftable and a single refblock. This creates 58 data
> +# clusters (actually, the L2 table is created here, too), so in total there 
> are
> +# then 63 used clusters in the image. With a refcount width of 64, one 
> refblock
> +# describes 64 clusters (512 bytes / 64 bits/entry = 64 entries), so this 
> will
> +# make the first target refblock have exactly one free entry.
> +$QEMU_IO -c "write 0 $((58 * 512))" "$TEST_IMG" | _filter_qemu_io
> +
> +# Now change the refcount width; since the first target refblock has exactly 
> one
> +# free entry, that entry will be used to store its own reference. No other
> +# refblocks are needed, so then the new reftable will be allocated; since the
> +# first target refblock is completely filled up, this will require a new
> +# refblock which is why the refcount width changing function will need to run
> +# through everything one more time until the allocations are stable.
> +$QEMU_IMG amend -o refcount_width=64 "$TEST_IMG"
> +print_refcount_width

Umm, that sounds backwards from what you document.  It's a good test of
the _new_ reftable needing a second round of allocations.  So keep it
with corrected comments.  But I think you _intended_ to write a test
that starts with a refcount_width=64 image and resize to a
refcount_width=1, where the _old_ reftable then suffers a reallocation
as part of allocating refblocks for the new table.  It may even help if
you add a tracepoint for every iteration through the walk function
callback, to prove we are indeed executing it 3 times instead of the
usual 2, for these test cases.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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