Re: [PATCH v3 3/9] block: Make it easier to learn which BDS support bitmaps

[Max Reitz]

On 11.05.20 20:16, Eric Blake wrote:
> On 5/11/20 4:21 AM, Max Reitz wrote:
>> On 08.05.20 20:03, Eric Blake wrote:
>>> Upcoming patches will enhance bitmap support in qemu-img, but in doing
>>> so, it turns out to be nice to suppress output when bitmaps make no
>>> sense (such as on a qcow2 v2 image).  Add a hook to make this easier
>>> to query.
>>>
>>> In the future, when we improve the ability to look up bitmaps through
>>> a filter, we will probably also want to teach the block layer to
>>> automatically let filters pass this request on through.
>>>
>>> Signed-off-by: Eric Blake <address@hidden>
>>> ---
>>>   block/qcow2.h                | 1 +
>>>   include/block/block_int.h    | 1 +
>>>   include/block/dirty-bitmap.h | 1 +
>>>   block/dirty-bitmap.c         | 9 +++++++++
>>>   block/qcow2-bitmap.c         | 7 +++++++
>>>   block/qcow2.c                | 1 +
>>>   6 files changed, 20 insertions(+)
>>>
>>> diff --git a/block/qcow2.h b/block/qcow2.h
>>> index f4de0a27d5c3..fb2b2b5a7b4d 100644
>>> --- a/block/qcow2.h
>>> +++ b/block/qcow2.h
>>> @@ -764,6 +764,7 @@ bool
>>> qcow2_co_can_store_new_dirty_bitmap(BlockDriverState *bs,
>>>   int qcow2_co_remove_persistent_dirty_bitmap(BlockDriverState *bs,
>>>                                               const char *name,
>>>                                               Error **errp);
>>> +bool qcow2_dirty_bitmap_supported(BlockDriverState *bs);
>>>
>>>   ssize_t coroutine_fn
>>>   qcow2_co_compress(BlockDriverState *bs, void *dest, size_t dest_size,
>>> diff --git a/include/block/block_int.h b/include/block/block_int.h
>>> index df6d0273d679..cb1082da4c43 100644
>>> --- a/include/block/block_int.h
>>> +++ b/include/block/block_int.h
>>> @@ -560,6 +560,7 @@ struct BlockDriver {
>>>                                uint64_t parent_perm, uint64_t
>>> parent_shared,
>>>                                uint64_t *nperm, uint64_t *nshared);
>>>
>>> +    bool (*bdrv_dirty_bitmap_supported)(BlockDriverState *bs);
>>
>> All BDSs support bitmaps, but only some support persistent dirty
>> bitmaps, so I think the name should reflect that.
> 
> How about .bdrv_dirty_bitmap_supports_persistent?

Sure.  Or .bdrv_supports_persistent_dirty_bitmaps.  Or
.bdrv_persistent_dirty_bitmaps_supported.  You decide what sounds best. :)

>> Conceptually, this looks reasonable.  This information might indeed be
>> nice to have, and I’m not sure whether we should extend any existing
>> interface to return it.
>>
>> (The interfaces that come to my mind are:
>> (1) bdrv_can_store_new_dirty_bitmap() below, which we could make accept
>> a NULL @name to return basically the same information.  But it’s still a
>> bit different, because I’d expect that function to return whether any
>> bitmap can be stored then, not whether the node supports bitmaps at all.
>>   So e.g. if there are already too many bitmaps, it should return false,
>> even though the node itself does support bitmaps.
> 
> [which reminds me - a while ago, we had patches for qcow2 handling with
> 64k bitmaps, or whatever insane number it took to overflow data
> structures, and I don't know if those ever got applied...]

I think that was a1db8733d28.  As far as I remember I just didn’t merge
the test, because it took like five minutes to run.

>> (2) bdrv_get_info()/BlockDriverInfo: This information would fit in very
>> nicely here, but do we have to put it here just because it does?  I
>> don’t think so.  This patch adds 20 lines of code, that shows that it’s
>> very simple to add a dedicated method, and it’s certainly a bit easier
>> to use than to invoke bdrv_get_info() and throw away all the other
>> information.  Perhaps this patch only shows that BlockDriverInfo doesn’t
>> make much sense in the first place, and most of its fields should have
>> been scalar return values from dedicated functions.)
> 
> Indeed, you (re-)discovered some of the very reasons why I chose to make
> a new interface.  I could tweak the commit message to mention
> alternatives, if that would help.

I suppose it wouldn’t help me, but it can’t harm either.  So if it isn’t
too difficult, why not.

Max

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