Re: [PATCH v2 03/13] libxenguest: deal with log-dirty op stats overflow

[Jan Beulich <jbeulich@suse.com>]

On 05.07.2021 19:26, Olaf Hering wrote:
> Am Mon, 5 Jul 2021 17:13:28 +0200
> schrieb Jan Beulich <jbeulich@suse.com>:
> 
>>      ctx->save.stats = (struct precopy_stats){
>> -        .dirty_count = ctx->save.p2m_size,
>> +        .dirty_count = -1,
>>      };
> 
> As said earlier, a consumer of these data may now be unable to initialize itself properly. Without the patch it would be able to size its private data structures properly to p2m_size. With the patch it can not know in advance what the upper limit might be.

And as said before, besides the intention of this being usable for
sizing purposes not being spelled out anywhere (instead it is
explicitly documented that -1 means "unknown", and the value really
_is_ unknown at this point; p2m_size is only a wrongly assumed
upper bound), this is useless information as the size may change in
the course of migration. It is a present limitation that this isn't
handled properly, which I think would better not be baked into
further places.

> There is no in-tree consumer that is affected, and I do not have an out-of-tree consumer that might be broken by this change.
> 
> Just saying..

Sure, but (as is the case for Andrew's replies) if you want me to
change the patch, you need to not just "say" something, but provide
convincing arguments. So far I've provided counter arguments which
weren't proven wrong or at least inapplicable.

Jan