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SubjectRe: [PATCH 0/2] Increasing CMA Utilization with a GFP Flag
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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@redhat.com>

--
Thanks,

David / dhildenb

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