Re: [Qemu-devel] [PATCH v2 0.15.0] qcow2: Fix L1 table size after bdrv_snapshot_goto

[Frediano Ziglio <freddy77@gmail.com>]

2011/8/5 Kevin Wolf <kwolf@redhat.com>:
> Am 05.08.2011 08:35, schrieb Frediano Ziglio:
>> 2011/8/4 Kevin Wolf <kwolf@redhat.com>:
>>> When loading an internal snapshot whose L1 table is smaller than the current L1
>>> table, the size of the current L1 would be shrunk to the snapshot's L1 size in
>>> memory, but not on disk. This lead to incorrect refcount updates and eventuelly
>>> to image corruption.
>>>
>>> Instead of writing the new L1 size to disk, this simply retains the bigger L1
>>> size that is currently in use and makes sure that the unused part is zeroed.
>>>
>>> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
>>> ---
>>>
>>> And the moment you send it out, you notice that it's wrong... *sigh*
>>>
>>> v2:
>>> - Check for s->l1_size > sn->l1_size in order to avoid disasters...
>>>
>>> Philipp, I think this should fix your corruption. Please give it a try.
>>>
>>> Anthony, this must go into 0.15. Given the short time until -rc2, do you prefer
>>> to pick it up directly or should I send a pull request tomorrow? The patch
>>> looks obvious, is tested with the given testcase and survives a basic
>>> qemu-iotests run (though qemu-iotests doesn't exercise snapshots a lot)
>>>
>>> Stefan, please review :-)
>>>
>>>  block/qcow2-snapshot.c |    5 ++++-
>>>  1 files changed, 4 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
>>> index 74823a5..6972e66 100644
>>> --- a/block/qcow2-snapshot.c
>>> +++ b/block/qcow2-snapshot.c
>>> @@ -330,8 +330,11 @@ int qcow2_snapshot_goto(BlockDriverState *bs, const char *snapshot_id)
>>>     if (qcow2_grow_l1_table(bs, sn->l1_size, true) < 0)
>>>         goto fail;
>>>
>>> -    s->l1_size = sn->l1_size;
>>> +    if (s->l1_size > sn->l1_size) {
>>> +        memset(s->l1_table + sn->l1_size, 0, s->l1_size - sn->l1_size);
>>> +    }
>>>     l1_size2 = s->l1_size * sizeof(uint64_t);
>>> +
>>>     /* copy the snapshot l1 table to the current l1 table */
>>>     if (bdrv_pread(bs->file, sn->l1_table_offset,
>>>                    s->l1_table, l1_size2) != l1_size2)
>>> --
>>> 1.7.6
>>>
>>
>> This patch looked odd at first sight. First a qcow2_grow_l1_table is
>> called to shrink L1 so perhaps should be qcow2_resize_l1_table.
>
> No, it doesn't shrink the table:
>
>    if (min_size <= s->l1_size)
>        return 0;
>

Yes, but perhaps returning success and not clipping anything is not
that correct, perhaps qcow2_snapshot_goto should not call
qcow2_grow_l1_table with a shorter value.

>> Perhaps also it would be better to clean entries in
>> qcow2_grow_l1_table instead of  qcow2_snapshot_goto to avoid same
>> problem in different calls to qcow2_grow_l1_table. The other oddity
>> (still to understand) is: why does some code use l1_table above
>> l1_size ??
>
> Which code do you mean specifically?
>
> Kevin
>

I think this is the issue

# 1204 -> 128 cluster per L2 entry -> 128k per L2 entry
# 128 cluster per L1 entry -> 16M per L1 entry
qemu-img create -f qcow2 /tmp/sn.qcow2 16m -o cluster_size=1024
qemu-img snapshot -c foo /tmp/sn.qcow2
# extend L1
qemu-io -c 'write -b 0 4M' /tmp/sn.qcow2
# shrink
qemu-img snapshot -a foo /tmp/sn.qcow2
qemu-img check /tmp/sn.qcow2

Well... I was trying to get a leak and got a core instead. I removed
your patch and got leaks.

also, should not be

    memset(s->l1_table + sn->l1_size, 0, (s->l1_size - sn->l1_size) *
sizeof(uint64_t));

instead of

    memset(s->l1_table + sn->l1_size, 0, s->l1_size - sn->l1_size);

Frediano