Re: [Qemu-devel] [PATCH] qcow2-refcount: Snapshot update for zero clusters

[Max Reitz <mreitz@redhat.com>]

Am 29.08.2013 14:53, schrieb Kevin Wolf:
> Am 29.08.2013 um 14:38 hat Max Reitz geschrieben:
>> Am 29.08.2013 14:33, schrieb Kevin Wolf:
>>> Am 29.08.2013 um 12:16 hat Max Reitz geschrieben:
>>>> Do not try to update the refcount for zero clusters in
>>>> qcow2_update_snapshot_refcount.
>>>>
>>>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>>>> ---
>>>>   block/qcow2-refcount.c | 16 +++++++++++++---
>>>>   1 file changed, 13 insertions(+), 3 deletions(-)
>>> Please don't forget to add a test case for v2.
>> Okay.
>>
>>>> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
>>>> index 1244693..7555242 100644
>>>> --- a/block/qcow2-refcount.c
>>>> +++ b/block/qcow2-refcount.c
>>>> @@ -863,9 +863,13 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
>>>>               for(j = 0; j < s->l2_size; j++) {
>>>>                   offset = be64_to_cpu(l2_table[j]);
>>>>                   if (offset != 0) {
>>>> +                    uint64_t cluster_index;
>>>> +
>>>>                       old_offset = offset;
>>>>                       offset &= ~QCOW_OFLAG_COPIED;
>>>> -                    if (offset & QCOW_OFLAG_COMPRESSED) {
>>>> +
>>>> +                    switch (qcow2_get_cluster_type(offset)) {
>>>> +                    case QCOW2_CLUSTER_COMPRESSED:
>>>>                           nb_csectors = ((offset >> s->csize_shift) &
>>>>                                          s->csize_mask) + 1;
>>>>                           if (addend != 0) {
>>>> @@ -880,8 +884,10 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
>>>>                           }
>>>>                           /* compressed clusters are never modified */
>>>>                           refcount = 2;
>>>> -                    } else {
>>>> -                        uint64_t cluster_index = (offset & L2E_OFFSET_MASK) >> s->cluster_bits;
>>>> +                        break;
>>>> +
>>>> +                    case QCOW2_CLUSTER_NORMAL:
>>>> +                        cluster_index = (offset & L2E_OFFSET_MASK) >> s->cluster_bits;
>>>>                           if (addend != 0) {
>>>>                               refcount = update_cluster_refcount(bs, cluster_index, addend,
>>>>                                                                  QCOW2_DISCARD_SNAPSHOT);
>>>> @@ -893,6 +899,10 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
>>>>                               ret = refcount;
>>>>                               goto fail;
>>>>                           }
>>>> +                        break;
>>> I think this part isn't quite right. Even zero clusters can have an
>>> allocated offset, so I think what's really needed is that the same code
>>> runs from QCOW2_CLUSTER_ZERO. However, add an if (cluster_index == 0)
>>> case for the unallocated case.
>> So then it would be enough to change:
>>
>> - if (offset != 0) {
>> + if ((offset & L2E_OFFSET_MASK) != 0) {
>>
>> and just execute the same code for QCOW2_CLUSTER_NORMAL and
>> QCOW2_CLUSTER_ZERO?
> Doing only this change in the original code might happen to work, but
> it's not really clean as the L2 entry format is different for compressed
> clusters (L2E_OFFSET_MASK isn't valid for it). So having the switch for
> the cluster type is a good thing, but the implementation for normal and
> zero clusters should be shared indeed.
Yes, of course I meant to include the switch as well.

> The offset != 0 check ends up redundant when you introduce the switch,
> so in theory it could as well be removed.
>
Well, I don't know; for zero clusters, the if statement is required 
anyway (since qcow2_get_cluster_type does not distinguish between 
allocated and non-allocated zero clusters), so I though we might as well 
just leave it where it is (just slightly adjusted in the way I proposed).


Max