Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[David Hildenbrand <david@redhat.com>]

On 04.08.25 21:00, Zi Yan wrote:
> On 4 Aug 2025, at 14:49, David Hildenbrand wrote:
> 
>> On 04.08.25 20:20, Juan Yescas wrote:
>>> Hi David/Zi,
>>>
>>> Is there any reason why the MIGRATE_CMA pages are not in the PCP lists?
>>>
>>> There are many devices that need fast allocation of MIGRATE_CMA pages,
>>> and they have to get them from the buddy allocator, which is a bit
>>> slower in comparison to the PCP lists.
>>>
>>> We also have cases where the MIGRATE_CMA memory requirements are big.
>>> For example, GPUs need MIGRATE_CMA memory in the ranges of 30MiB to 500MiBs.
>>> These cases would benefit if we have THPs for CMAs.
>>>
>>> Could we add the support for MIGRATE_CMA pages on the PCP and THP lists?
>>
>> Remember how CMA memory is used:
>>
>> The owner allocates it through cma_alloc() and friends, where the CMA allocator will try allocating *specific physical memory regions* using alloc_contig_range(). It doesn't just go ahead and pick a random CMA page from the buddy (or PCP) lists. Doesn't work (just imagine having different CMA areas etc).
> 
> Yeah, unless some code is relying on gfp_to_alloc_flags_cma() to get ALLOC_CMA
> to try to get CMA pages from buddy.

Right, but that's just for internal purposes IIUC, to grab pages from 
the CMA lists when serving movable allocations.

> 
>>
>> Anybody else is free to use CMA pages for MOVABLE allocations. So we treat them as being MOVABLE on the PCP.
>>
>> Having a separate CMA PCP list doesn't solve or speedup anything, really.
> 
> It can be slower when small CMA pages are on PCP lists and large CMA pages
> cannot be allocated, one needs to drain PCP lists. This assumes the code is
> trying to get CMA pages from buddy, which is not how CMA memory is designed
> to be used like David mentioned above.

Right. And alloc_contig_range_noprof() already does a 
drain_all_pages(cc.zone).

-- 
Cheers,

David / dhildenb