Re: [Qemu-devel] [PATCH v6 20/42] block/snapshot: Fix fallback

[Max Reitz]

On 10.09.19 13:56, Kevin Wolf wrote:
> Am 09.08.2019 um 18:13 hat Max Reitz geschrieben:
>> If the top node's driver does not provide snapshot functionality and we
>> want to fall back to a node down the chain, we need to snapshot all
>> non-COW children.  For simplicity's sake, just do not fall back if there
>> is more than one such child.
>>
>> bdrv_snapshot_goto() becomes a bit weird because we may have to redirect
>> the actual child pointer, so it only works if the fallback child is
>> bs->file or bs->backing (and then we have to find out which it is).
>>
>> Suggested-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>> Signed-off-by: Max Reitz <address@hidden>
>> ---
>>  block/snapshot.c | 100 +++++++++++++++++++++++++++++++++++++----------
>>  1 file changed, 79 insertions(+), 21 deletions(-)
>>
>> diff --git a/block/snapshot.c b/block/snapshot.c
>> index f2f48f926a..35403c167f 100644
>> --- a/block/snapshot.c
>> +++ b/block/snapshot.c
>> @@ -146,6 +146,32 @@ bool bdrv_snapshot_find_by_id_and_name(BlockDriverState 
>> *bs,
>>      return ret;
>>  }
>>  
>> +/**
>> + * Return the child BDS to which we can fall back if the given BDS
>> + * does not support snapshots.
>> + * Return NULL if there is no BDS to (safely) fall back to.
>> + */
>> +static BlockDriverState *bdrv_snapshot_fallback(BlockDriverState *bs)
>> +{
>> +    BlockDriverState *child_bs = NULL;
>> +    BdrvChild *child;
>> +
>> +    QLIST_FOREACH(child, &bs->children, next) {
>> +        if (child == bdrv_filtered_cow_child(bs)) {
>> +            /* Ignore: COW children need not be included in snapshots */
>> +            continue;
>> +        }
>> +
>> +        if (child_bs) {
>> +            /* Cannot fall back to a single child if there are multiple */
>> +            return NULL;
>> +        }
>> +        child_bs = child->bs;
>> +    }
>> +
>> +    return child_bs;
>> +}
> 
> Why do we return child->bs here when bdrv_snapshot_goto() then needs to
> reconstruct what the associated BdrvChild was? Wouldn't it make more
> sense to return BdrvChild** from here and maybe have a small wrapper for
> the other functions that only need a BDS?

What would you return instead?  &child doesn’t work.

We could limit ourselves to bs->file and bs->backing.  It just seemed
like a bit of an artificial limit to me, because we only really have it
for bdrv_snapshot_goto().

Max

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