linux-kernel - Re: [PATCH RFC] mm: compaction: avoid migrating non-cma pages to a cma area

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Date:   Mon, 13 Apr 2020 09:36:08 -0700
From:   Roman Gushchin <guro@...com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     Michal Hocko <mhocko@...nel.org>, <linux-mm@...ck.org>,
        <kernel-team@...com>, <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...riel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>, Qian Cai <cai@....pw>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Joonsoo Kim <js1304@...il.com>
Subject: Re: [PATCH RFC] mm: compaction: avoid migrating non-cma pages to a
 cma area

On Wed, Apr 08, 2020 at 12:41:19PM -0700, Roman Gushchin wrote:
> Compaction does treat cma pageblocks on pair with any movable
> pageblocks. It means it can easily move non-cma pages into a cma zone.
> 
> It can create problems for the cma allocator.
> 
> The particular problem I'm looking at is related to btrfs metadata
> pages, which are allocated without __GFP_MOVABLE, but beside that
> are generic pagecache pages. In fact, they are sometimes movable
> and sometimes not, depending on whether they are dirty and also
> on the extent buffer reference counter.
> 
> Compaction moves them to the hugetlb_cma area, and then sometimes
> the cma allocator fails to move them back from the cma area. It
> results in failures of gigantic hugepages allocations.
> 
> Also in general cma areas are reserved close to the end of a zone,
> and it's where compaction tries to migrate pages. It means
> compaction will aggressively fill cma areas, which makes not much
> sense.
> 
> So to avoid it, let's preserve non-cma pages from being moved into
> a cma area. Because cma areas are usually quite large and the number
> of areas is small, it should not significantly affect the memory
> fragmentation.
> 
> Signed-off-by: Roman Gushchin <guro@...com>

Friendly ping... Any thoughts, comments, ideas?

Thanks!

Roman