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

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Message-ID: <CAJDx_ri28-_SqPoYgm2DHEwkA8Y8PU9+YjbRZqMdGpy_k0DKuA@mail.gmail.com>
Date: Mon, 4 Aug 2025 18:24:51 -0700
From: Juan Yescas <jyescas@...gle.com>
To: Zi Yan <ziy@...dia.com>
Cc: David Hildenbrand <david@...hat.com>, akash.tyagi@...iatek.com, 
	Andrew Morton <akpm@...ux-foundation.org>, angelogioacchino.delregno@...labora.com, 
	hannes@...xchg.org, Brendan Jackman <jackmanb@...gle.com>, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linux-mediatek@...ts.infradead.org, 
	Linux Memory Management List <linux-mm@...ck.org>, matthias.bgg@...il.com, Michal Hocko <mhocko@...e.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, wsd_upstream@...iatek.com, 
	Kalesh Singh <kaleshsingh@...gle.com>, "T.J. Mercier" <tjmercier@...gle.com>, 
	Isaac Manjarres <isaacmanjarres@...gle.com>
Subject: Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

On Mon, Aug 4, 2025 at 12:00 PM Zi Yan <ziy@...dia.com> 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.
>
> >
> > 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.
>

Thanks Zi for confirming!

> >
> > I still have no clue what this patch here tried to solve: it doesn't make any sense.
>
>
> Best Regards,
> Yan, Zi