Re: [PATCH 0/5] mm: hugetlb: add CMA support for PMD-sized page

[David Hildenbrand (Arm)]

On 10/19/26 11:17, Jose A. Perez de Azpillaga wrote:
> On 10/19/26 10:43 AM, Muchun Song wrote:
>>
>>
>>>
>>>
>>> wouldn't a shared pool still be sized and aligned for 1G? then the 2M
>>> headroom stays a multiple of 1G and the 1G-aligned reservation can still
>>> fail, which is what the series is trying to avoid.
>>
>> If we decide to use a shared pool, I think the current alignment strategy
>> should be adapted as well, it should be changed to align with the minimal
>> acquirement, like 2M.
>>
>> For example, if a user reserves 1G+2M through hugetlb_cma, then the memory
>> reserved by the pool is also 1G+2M, except that the starting address needs
>> to be aligned to the largest gigantic page (1G). Therefore, this pool is
>> suitable for both 1GB and 2MB allocation. The only problem I see with a
>> shared pool is that it may not support mixed-use scenarios very well.
>>
>> What do you think?
> 
> makes sense to me. existing hugetlb_cma= users with WM pages would get
> mixed use without asking for it though.

Yeah, my intuition tells me that having 1 allocation size per CMA area might be
cleaner.

Assume you allocate 512 2 MiB hugetlb folios. You'd think that you would then be
able to allocate 1 GiB hugetlb folio less. But that's not guaranteed, as CMA
might just fallback to other areas fairly easily.

If someone wants to convert from 1 GiB to 512 x 2MiB, I think the demotion
interface allows for that.

-- 
Cheers,

David