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

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Message-ID: <3675d394-371c-cff6-ca3e-2e11d0e80642@redhat.com>
Date:   Mon, 2 Nov 2020 18:35:08 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Chris Goldsworthy <cgoldswo@...eaurora.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>,
        Nitin Gupta <ngupta@...are.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] Increasing CMA Utilization with a GFP Flag

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@...hat.com>

-- 
Thanks,

David / dhildenb