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

[David Hildenbrand]

On 05.08.25 03:22, Juan Yescas wrote:
> On Mon, Aug 4, 2025 at 11:50 AM David Hildenbrand <david at redhat.com> 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).
>>
>> 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.
>>
> 
> Thanks David for the quick overview.
> 
>> I still have no clue what this patch here tried to solve: it doesn't
>> make any sense.
>>
> 
> The story started with this out of tree patch that is part of Android.
> 
> https://lore.kernel.org/lkml/cover.1604282969.git.cgoldswo@codeaurora.org/T/#u
> 
> This patch introduced the __GFP_CMA flag that allocates pages from
> MIGRATE_MOVABLE
> or MIGRATE_CMA. What it happens then, it is that the MIGRATE_MOVABLE
> pages in the
> PCP lists were consumed pretty fast. To solve this issue, the PCP
> MIGRATE_CMA list was added.
> This list is initialized by rmqueue_bulk() when it is empty. That's
> how we end up with the PCP MIGRATE_CMA list
> in Android. In addition to this, the THP list for MIGRATE_MOVABLE was
> allowed to contain
> MIGRATE_CMA pages. This is causing THP MIGRATE_CMA pages to be used
> for THP MIGRATE_MOVABLE
> making later allocations from THP MIGRATE_CMA to fail.

Okay, so this patch here really is not suitable for the upstream kernel 
as is. It's purely targeted at the OOT Android patch.

> 
> These workarounds are mainly because we need to solve this issue upstream:
> 
> - When devices reserve big blocks of MIGRATE_CMA pages, the
> underutilized MIGRATE_CMA
> can fall back to MIGRATE_MOVABLE and these pages can be pinned, so if
> we require MIGRATE_CMA
> pages, the allocations might fail.
> 
> I remember that you presented the problem in LPC. Were you able to
> make some progress on that?

There is the problem of CMA pages getting allocated by someone for a 
MOVABLE allocation, to then short-term pin it for DMA. Long-term 
pinnings are disallowed (we just recently fixed a bug where we 
accidentally allowed it).

One concern is that a steady stream of short-term pinnings can turn such 
pages unmovable. We discussed ideas on how to handle that, but there is 
no solution upstream yet.

-- 
Cheers,

David / dhildenb