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

[Frediano Ziglio]

2011/8/5 Frediano Ziglio <address@hidden>:
> 2011/8/5 Kevin Wolf <address@hidden>:
>> Am 05.08.2011 08:35, schrieb Frediano Ziglio:
>>> 2011/8/4 Kevin Wolf <address@hidden>:
>>>> 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 <address@hidden>
>>>> ---
>>>>
>>>> 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
>

This patch fix Philipp and my problem

diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
index 74823a5..0e5fc13 100644
--- a/block/qcow2-snapshot.c
+++ b/block/qcow2-snapshot.c
@@ -330,14 +330,16 @@ 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;
-    l1_size2 = s->l1_size * sizeof(uint64_t);
+    if (s->l1_size > sn->l1_size) {
+        memset(s->l1_table + sn->l1_size, 0, (s->l1_size -
sn->l1_size) * sizeof(uint64_t));
+    }
+    l1_size2 = sn->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)
         goto fail;
     if (bdrv_pwrite_sync(bs->file, s->l1_table_offset,
-                    s->l1_table, l1_size2) < 0)
+                    s->l1_table, s->l1_size * sizeof(uint64_t)) < 0)
         goto fail;
     for(i = 0;i < s->l1_size; i++) {
         be64_to_cpus(&s->l1_table[i]);


Frediano