linux-kernel - Re: [PATCH] mm,page_alloc,cma: conditionally prefer cma pageblocks for movable allocations

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Date:   Wed, 11 Mar 2020 09:51:07 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Rik van Riel <riel@...riel.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, Roman Gushchin <guro@...com>,
        Qian Cai <cai@....pw>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH] mm,page_alloc,cma: conditionally prefer cma pageblocks
 for movable allocations

On 3/6/20 9:01 PM, Rik van Riel wrote:
> Posting this one for Roman so I can deal with any upstream feedback and
> create a v2 if needed, while scratching my head over the next piece of
> this puzzle :)
> 
> ---8<---
> 
> From: Roman Gushchin <guro@...com>
> 
> Currently a cma area is barely used by the page allocator because
> it's used only as a fallback from movable, however kswapd tries
> hard to make sure that the fallback path isn't used.

Few years ago Joonsoo wanted to fix these kinds of weird MIGRATE_CMA corner
cases by using ZONE_MOVABLE instead [1]. Unfortunately it was reverted due to
unresolved bugs. Perhaps the idea could be resurrected now?

[1]
https://lore.kernel.org/linux-mm/1512114786-5085-1-git-send-email-iamjoonsoo.kim@lge.com/

> This results in a system evicting memory and pushing data into swap,
> while lots of CMA memory is still available. This happens despite the
> fact that alloc_contig_range is perfectly capable of moving any movable
> allocations out of the way of an allocation.
> 
> To effectively use the cma area let's alter the rules: if the zone
> has more free cma pages than the half of total free pages in the zone,
> use cma pageblocks first and fallback to movable blocks in the case of
> failure.
> 
> Signed-off-by: Rik van Riel <riel@...riel.com>
> Co-developed-by: Rik van Riel <riel@...riel.com>
> Signed-off-by: Roman Gushchin <guro@...com>
> ---
>  mm/page_alloc.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3c4eb750a199..0fb3c1719625 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2711,6 +2711,18 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype,
>  {
>  	struct page *page;
>  
> +	/*
> +	 * Balance movable allocations between regular and CMA areas by
> +	 * allocating from CMA when over half of the zone's free memory
> +	 * is in the CMA area.
> +	 */
> +	if (migratetype == MIGRATE_MOVABLE &&
> +	    zone_page_state(zone, NR_FREE_CMA_PAGES) >
> +	    zone_page_state(zone, NR_FREE_PAGES) / 2) {
> +		page = __rmqueue_cma_fallback(zone, order);
> +		if (page)
> +			return page;
> +	}
>  retry:
>  	page = __rmqueue_smallest(zone, order, migratetype);
>  	if (unlikely(!page)) {
>