Re: [PATCH] mm,page_alloc,cma: configurable CMA utilization

[Roman Gushchin <roman.gushchin@linux.dev>]

On Fri, Jan 05, 2024 at 03:46:55PM -0800, Sukadev Bhattiprolu wrote:
> 
> On 2/1/2023 3:47 PM, Minchan Kim wrote:
> >
> > I like this patch for different reason but for the specific problem you
> > mentioned, How about making reclaimer/compaction aware of the problem:
> >
> > IOW, when the GFP_KERNEL/DMA allocation happens but not enough memory
> > in the zones, let's migrates movable pages in those zones into CMA
> > area/movable zone if they are plenty of free memory.
> 
> Hi Minchan,
> 
> Coming back to this thread after a while.
> 
> If the CMA region is usually free, allocating pages first in the non-CMA
> region and then moving them into the CMA region would be extra work since
> it would happen most of the time. In such cases, wouldn't it be better to
> allocate from the CMA region itself?

I'm not sure there is a "one size fits all" solution here. 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
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.

Thanks!