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Date: Mon, 8 Jan 2024 18:59:27 -0800
From: Roman Gushchin <roman.gushchin@linux.dev>
To: Sukadev Bhattiprolu <quic_sukadev@quicinc.com>
Cc: Minchan Kim <minchan@kernel.org>,
	Chris Goldsworthy <quic_cgoldswo@quicinc.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Rik van Riel <riel@surriel.com>, Roman Gushchin <guro@fb.com>,
	Vlastimil Babka <vbabka@suse.cz>, Joonsoo Kim <js1304@gmail.com>,
	Georgi Djakov <quic_c_gdjako@quicinc.com>, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm,page_alloc,cma: configurable CMA utilization
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On Mon, Jan 08, 2024 at 12:15:05PM -0800, Sukadev Bhattiprolu wrote:
> 
> On 1/5/2024 4:05 PM, Roman Gushchin wrote:
> > I'm not sure there is a "one size fits all" solution here. 
> agree - that's why we are thinking a configurable cma utilization would be
> useful.
> > There are two distinctive cases:
> > 1) A relatively small cma area used for a specific purpose. This is how cma
> >    was used until recently. And it was barely used by the kernel for non-cma
> >    allocations.
> > 2) A relatively large cma area which is used to allocate gigantic hugepages
> >    and as an anti-fragmentation mechanism in general (basically as a movable
> >    zone). In this case it might be preferable to use cma for movable
> >    allocations, because the space for non-movable allocations might be limited.
> >
> > I see two options here:
> > 1) introduce per-cma area flags which will define the usage policy
> Could you please elaborate on this - how would we use the per-cma flags
> when allocating pages?

I mean potentially we can add some per-cma area configuration options which will
define the "priority" of using the memory from this cma area.

> > 2) redesign the page allocator to better take care of fragmentation at 1Gb scale
> >
> > The latter is obviously not a small endeavour.
> > The fundamentally missing piece is a notion of an anti-fragmentation cost.
> > E.g. how much work does it makes sense to put into page migration
> > before "polluting" a new large block of memory with an unmovable folio.
> 
> Stepping back, we are trying to solve for a situation where system:
> ??????? - has lot of movable allocs in zone normal
> ??????? - has lot of idle memory in CMA region
> ??????? - but is low on memory for unmovable allocs, leading to oom-kills
> 
> On devices where cma region is mostly idle, allocating movable pages from
> the cma region would have lesser overhead?

It's not that easy: imagine booting up a small system with a cma area reserved
for some hardware-related operations. This is pretty much what cma was initially
designed. How to not fill the cma area up with the page cache?

Thanks!