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

[Max Reitz <mreitz@redhat.com>]

On 2014-11-17 at 09:34, Max Reitz wrote:
> 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.

...On second thought, trace doesn't work so well with qemu-img. My best 
bet would be blkdebug, but that seems kind of ugly to me...

Max

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