Re: [Qemu-devel] [PATCH v6 09/11] qcow2: Clean up after refcount rebuild

[Max Reitz <mreitz@redhat.com>]

On 2014-10-21 at 12:16, Max Reitz wrote:
> On 2014-10-21 at 11:59, Kevin Wolf wrote:
>> Am 20.10.2014 um 16:35 hat Max Reitz geschrieben:
>>> Because the old refcount structure will be leaked after having rebuilt
>>> it, we need to recalculate the refcounts and run a leak-fixing 
>>> operation
>>> afterwards (if leaks should be fixed at all).
>>>
>>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>>> Reviewed-by: Benoît Canet <benoit.canet@nodalink.com>
>>> ---
>>>   block/qcow2-refcount.c | 35 +++++++++++++++++++++++++++++++++++
>>>   1 file changed, 35 insertions(+)
>>>
>>> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
>>> index 75e726b..3730be2 100644
>>> --- a/block/qcow2-refcount.c
>>> +++ b/block/qcow2-refcount.c
>>> @@ -1956,12 +1956,47 @@ int qcow2_check_refcounts(BlockDriverState 
>>> *bs, BdrvCheckResult *res,
>>>                         nb_clusters);
>>>         if (rebuild && (fix & BDRV_FIX_ERRORS)) {
>>> +        BdrvCheckResult old_res = *res;
>>> +
>>>           fprintf(stderr, "Rebuilding refcount structure\n");
>>>           ret = rebuild_refcount_structure(bs, res, &refcount_table,
>>>                                            &nb_clusters);
>>>           if (ret < 0) {
>>>               goto fail;
>>>           }
>>> +
>>> +        res->corruptions = 0;
>>> +        res->leaks = 0;
>>> +
>>> +        /* Because the old reftable has been exchanged for a new 
>>> one the
>>> +         * references have to be recalculated */
>>> +        rebuild = false;
>>> +        memset(refcount_table, 0, nb_clusters * sizeof(uint16_t));
>>> +        ret = calculate_refcounts(bs, res, 0, &rebuild, 
>>> &refcount_table,
>>> +                                  &nb_clusters);
>>> +        if (ret < 0) {
>>> +            goto fail;
>>> +        }
>>> +
>>> +        if (fix & BDRV_FIX_LEAKS) {
>>> +            /* The old refcount structures are now leaked, fix it; 
>>> the result
>>> +             * can be ignored */
>>> +            pre_compare_res = *res;
>> I would prefer using another local variable here. At the first sight
>> it's not quite clear which references to pre_compare_res correspond to
>> which state.
>
> Why not.
>
>>> +            compare_refcounts(bs, res, BDRV_FIX_LEAKS, &rebuild,
>>> +                              &highest_cluster, refcount_table, 
>>> nb_clusters);
>>> +            if (rebuild) {
>>> +                fprintf(stderr, "ERROR rebuilt refcount structure 
>>> is still "
>>> +                        "broken\n");
>>> +            }
>>> +            *res = pre_compare_res;
>>> +        }
>>> +
>>> +        if (res->corruptions < old_res.corruptions) {
>>> +            res->corruptions_fixed += old_res.corruptions - 
>>> res->corruptions;
>>> +        }
>>> +        if (res->leaks < old_res.leaks) {
>>> +            res->leaks_fixed += old_res.leaks - res->leaks;
>>> +        }
>> For these numbers to be accurate, don't we need to run
>> compare_refcounts() unconditionally and only make BDRV_FIX_LEAKS
>> conditional?
>
> Actually, there is no difference, because at the point of this patch, 
> you cannot use BDRV_FIX_ERRORS without BDRV_FIX_LEAKS. But it'd be 
> more correct, right.

Wait, it would not be more correct. The result of the 
compare_refcounts() call inside of the "if (fix & BDRV_FIX_LEAKS)" 
conditional block is ignored, its only purpose is to fix leaks resulting 
from rebuild_refcount_structure().

So the question is whether we should discard the result of that 
compare_refcounts() call. I think we should. Its sole purpose is to fix 
leaks due to the rebuilt refcount structures, and qemu-img will double 
check anyway.

Max

> Thanks,
>
> Max
>
>> Now that I've read the rest of the series, comparing res and old_res
>> actually makes sense, so maybe it's not necessary to introduce a
>> Qcow2CheckResult.
>>
>>>       } else if (fix) {
>>>           if (rebuild) {
>>>               fprintf(stderr, "ERROR need to rebuild refcount 
>>> structures\n");
>>> -- 
>>> 2.1.2
>> Kevin
>