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

[Max Reitz <mreitz@redhat.com>]

On 2014-11-15 at 15:50, Eric Blake wrote:
> 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 <mreitz@redhat.com>
>> ---
>>   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.

That's what you intended, but that's harder to test, so I settled for 
this (and the comments are appropriate (note that "target refblock" 
refers to the refblocks after amendment); note that this does indeed 
fail with v1 of this series.

> 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.

That's a good idea! I thought about adding some info, but totally forgot 
about trace points.

I'll see whether I can add a test for increasing the size of the 
original reftable during amendment, too.

Max