Re: [LSF/MM TOPIC] CMA and larger page sizes

Vlastimil Babka
2018-04-12T06:28:04Z

On 04/12/2018 07:51 AM, Michal Hocko wrote:
> On Wed 11-04-18 18:06:59, Laura Abbott wrote:
>>
>> I took a look at this a little bit more and while it's true we don't
>> have the unmovable restriction anymore, CMA is still tied to the pageblock
>> size (512MB) because we still have MIGRATE_CMA. I guess making the
>> pageblock smaller seems like the most plausible approach?
>
> Maybe I am wrong but my take on what Joonsoo said is that we really do
> not have to care about page blocks and MIGRATE_CMA because GFP_MOVABLE
> can be allocated from that migrate type as it is by definition movable.
> The size of the page block shouldn't matter.

Agree, CMA itself doesn't need mark pageblocks with MIGRATE_CMA anymore.
The only user is now hardened usercopy via check_page_span() ->
is_migrate_cma_page(). If we could give up the CMA check there (or
recognize CMA differently?), MIGRATE_CMA could be removed completely,
together with the pageblock alignment code.