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

[Juan Yescas <jyescas@google.com>]

On Tue, Aug 5, 2025 at 2:58 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 05.08.25 03:22, Juan Yescas wrote:
> > On Mon, Aug 4, 2025 at 11:50 AM David Hildenbrand <david@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.
>
Right, it is a temporary solution for the pinned MIGRATE_CMA pages.

> >
> > 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).
>
Nice, it is great the issue got caught and fixed upstream :)

> 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.

Are there any plans to continue the discussion? Is it in the priority
list? Maybe
a topic we can discuss in LPC Japan?

Thanks
Juan

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