Re: [PATCH] mm: cma: allocate pages from CMA if NR_FREE_PAGES approaches low water mark

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 12 Nov 2012 09:59:42 +0100
Marek Szyprowski <m.szyprowski@samsung.com> wrote:

> It has been observed that system tends to keep a lot of CMA free pages
> even in very high memory pressure use cases. The CMA fallback for movable
> pages is used very rarely, only when system is completely pruned from
> MOVABLE pages, what usually means that the out-of-memory even will be
> triggered very soon. To avoid such situation and make better use of CMA
> pages, a heuristics is introduced which turns on CMA fallback for movable
> pages when the real number of free pages (excluding CMA free pages)
> approaches low water mark.
> 
> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Reviewed-by: Kyungmin Park <kyungmin.park@samsung.com>
> CC: Michal Nazarewicz <mina86@mina86.com>
> ---
>  mm/page_alloc.c |    9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index fcb9719..90b51f3 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1076,6 +1076,15 @@ static struct page *__rmqueue(struct zone *zone, unsigned int order,
>  {
>  	struct page *page;
>  
> +#ifdef CONFIG_CMA
> +	unsigned long nr_free = zone_page_state(zone, NR_FREE_PAGES);
> +	unsigned long nr_cma_free = zone_page_state(zone, NR_FREE_CMA_PAGES);
> +
> +	if (migratetype == MIGRATE_MOVABLE && nr_cma_free &&
> +	    nr_free - nr_cma_free < 2 * low_wmark_pages(zone))
> +		migratetype = MIGRATE_CMA;
> +#endif /* CONFIG_CMA */
> +
>  retry_reserve:
>  	page = __rmqueue_smallest(zone, order, migratetype);

erk, this is right on the page allocator hotpath.  Bad.

At the very least, we could code it so it is not quite so dreadfully
inefficient:

	if (migratetype == MIGRATE_MOVABLE) {
		unsigned long nr_cma_free;

		nr_cma_free = zone_page_state(zone, NR_FREE_CMA_PAGES);
		if (nr_cma_free) {
			unsigned long nr_free;

			nr_free = zone_page_state(zone, NR_FREE_PAGES);

			if (nr_free - nr_cma_free < 2 * low_wmark_pages(zone))
				migratetype = MIGRATE_CMA;
		}
	}

but it still looks pretty bad.

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