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

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Message-ID: <20201102144449.GM27442@casper.infradead.org>
Date:   Mon, 2 Nov 2020 14:44:49 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     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 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.