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

[Michal Hocko <mhocko@kernel.org>]

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

Laura, are you still interested discussing this or other CMA related
topic?

-- 
Michal Hocko
SUSE Labs