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Subject: Re: [PATCH v7 0/7] Introduce ZONE_CMA
To: Michal Hocko <mhocko@kernel.org>
Cc: Joonsoo Kim <js1304@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <203a7f52-d444-dbcc-0cc0-0c77dffbcadd@suse.cz>
Date: Thu, 11 May 2017 10:51:17 +0200
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On 05/04/2017 02:46 PM, Michal Hocko wrote:
> On Thu 04-05-17 14:33:24, Vlastimil Babka wrote:
>>>
>>> I am pretty sure s390 and ppc support NUMA and aim at supporting really
>>> large systems. 
>>
>> I don't see ppc there,
> 
> config KVM_BOOK3S_64_HV
>         tristate "KVM for POWER7 and later using hypervisor mode in host"
>         depends on KVM_BOOK3S_64 && PPC_POWERNV
>         select KVM_BOOK3S_HV_POSSIBLE
>         select MMU_NOTIFIER
>         select CMA
> 
> fa61a4e376d21 tries to explain some more

Uh, that's unfortunate then.

> [...]
>>> Are we really ready to add another thing like that? How are distribution
>>> kernels going to handle that?
>>
>> I still hope that generic enterprise/desktop distributions can disable
>> it, and it's only used for small devices with custom kernels.
>>
>> The config burden is already there in any case, it just translates to
>> extra migratetype and fastpath hooks, not extra zone and potentially
>> less nodes.
> 
> AFAIU the extra migrate type costs nothing when there are no cma
> reservations. And those hooks can be made noop behind static branch
> as well. So distribution kernels do not really have to be afraid of
> enabling CMA currently.

The tradeoff is unfortunate :/