Re: [PATCH] mm: cma: free cma page to buddy instead of being cpu hotpage

[Mel Gorman]

On Mon, Oct 28, 2013 at 07:42:49PM +0800, zhang.mingjun@xxxxxxxxxx wrote:
> From: Mingjun Zhang <troy.zhangmingjun@xxxxxxxxxx>
> 
> free_contig_range frees cma pages one by one and MIGRATE_CMA pages will be
> used as MIGRATE_MOVEABLE pages in the pcp list, it causes unnecessary
> migration action when these pages reused by CMA.
> 
> Signed-off-by: Mingjun Zhang <troy.zhangmingjun@xxxxxxxxxx>
> ---
>  mm/page_alloc.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0ee638f..84b9d84 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1362,7 +1362,8 @@ void free_hot_cold_page(struct page *page, int cold)
>  	 * excessively into the page allocator
>  	 */
>  	if (migratetype >= MIGRATE_PCPTYPES) {
> -		if (unlikely(is_migrate_isolate(migratetype))) {
> +		if (unlikely(is_migrate_isolate(migratetype))
> +			|| is_migrate_cma(migratetype))
>  			free_one_page(zone, page, 0, migratetype);
>  			goto out;

This slightly impacts the page allocator free path for a marginal gain
on CMA which are relatively rare allocations. There is no obvious
benefit to this patch as I expect CMA allocations to flush the PCP lists
when a range of pages have been isolated and migrated. Is there any
measurable benefit to this patch?

-- 
Mel Gorman
SUSE Labs
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