Re: [PATCH 0/2] Increasing CMA Utilization with a GFP Flag

[David Hildenbrand]

On 02.11.20 15:44, Matthew Wilcox wrote:
> On Mon, Nov 02, 2020 at 06:39:20AM -0800, Chris Goldsworthy wrote:
> > The current approach to increasing CMA utilization introduced in
> > commit 16867664936e ("mm,page_alloc,cma: conditionally prefer cma
> > pageblocks for movable allocations") increases CMA utilization by
> > redirecting MIGRATE_MOVABLE allocations to a CMA region, when
> > greater than half of the free pages in a given zone are CMA pages.
> > The issue in this approach is that allocations with type
> > MIGRATE_MOVABLE can still succumb to pinning. To get around
> > this, one approach is to re-direct allocations to the CMA areas, that
> > are known not to be victims of pinning.
> >
> > To this end, this series brings in __GFP_CMA, which we mark with
> > allocations that we know are safe to be redirected to a CMA area.
>
> This feels backwards to me.  What you're essentially saying is "Some
> allocations marked with GFP_MOVABLE turn out not to be movable, so we're
> going to add another GFP_REALLY_MOVABLE flag" instead of tracking down
> which GFP_MOVABLE allocations aren't really movable.

Right, this just sounds wrong. We have the exact same issues with 
long-term pinnings on ZONE_MOVABLE. We have known issues with short-term 
pinnings and movable allocations (e.g., when a process dies) that should 
be tackled instead.

This is just trying to work around the original issue.

Nacked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Thanks,

David / dhildenb