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

Michal Hocko
2018-01-27T03:51:46Z

[Ccing Joonsoo]

On Fri 26-01-18 02:08:14, Laura Abbott wrote:
> CMA as it's currently designed requires alignment to the pageblock size c.f.
>
> /*
> * Sanitise input arguments.
> * Pages both ends in CMA area could be merged into adjacent unmovable
> * migratetype page by page allocator's buddy algorithm. In the case,
> * you couldn't get a contiguous memory, which is not what we want.
> */
> alignment = max(alignment, (phys_addr_t)PAGE_SIZE <<
> max_t(unsigned long, MAX_ORDER - 1, pageblock_order));
>
>
> On arm64 with 64K page size and transparent huge page, this gives an alignment
> of 512MB. This is quite restrictive and can eat up significant portions of
> memory on smaller memory targets. Adjusting the configuration options really
> isn't ideal for distributions that aim to have a single image which runs on
> all targets.
>
> Approaches I've thought about:
> - Making CMA alignment less restrictive (and dealing with the fallout from
> the comment above)
> - Command line option to force a reasonable alignment
>
> There's been some interest in other CMA topics so this might go along well.
>
> Thanks,
> Laura
>
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--
Michal Hocko
SUSE Labs

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