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

[Juan Yescas]

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.

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?

Thanks
Juan





> --
> Cheers,
>
> David / dhildenb
>