linux-kernel - Re: [PATCH 1/3] mm: page_isolation: remove redundant moving for isolated buddy pages

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Date:	Tue, 09 Dec 2014 10:14:45 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Weijie Yang <weijie.yang@...sung.com>, iamjoonsoo.kim@....com
CC:	'Andrew Morton' <akpm@...ux-foundation.org>, mgorman@...e.de,
	'Rik van Riel' <riel@...hat.com>,
	'Johannes Weiner' <hannes@...xchg.org>,
	'Minchan Kim' <minchan@...nel.org>,
	'Weijie Yang' <weijie.yang.kh@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/3] mm: page_isolation: remove redundant moving for isolated
 buddy pages

On 12/09/2014 08:50 AM, Weijie Yang wrote:
> The commit ad53f92e(fix incorrect isolation behavior by rechecking migratetype)
> patch series describe the race between page isolation and alloc/free path, and
> fix the race.
>
> Now, after the pageblock has been isolated, free buddy pages are already in
> the free_list[MIGRATE_ISOLATE] and will not be allocated for usage. So the
> current freepage_migratetype check is unnecessary and it will cause redundant
> page move. That is to say, even if the buddy page's migratetype is not
> MIGRATE_ISOLATE, the page is in free_list[MIGRATE_ISOLATE], we just move it
> from free_list[MIGRATE_ISOLATE] to free_list[MIGRATE_ISOLATE].
>
> This patch removes the unnecessary freepage_migratetype check and the
> redundant page moving.
>
> Signed-off-by: Weijie Yang <weijie.yang@...sung.com>

With great hope that Joonsoo won't find another corner case J

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>   mm/page_isolation.c |   17 +----------------
>   1 file changed, 1 insertion(+), 16 deletions(-)
>
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index c8778f7..6e5174d 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -221,23 +221,8 @@ __test_page_isolated_in_pageblock(unsigned long pfn, unsigned long end_pfn,
>   			continue;
>   		}
>   		page = pfn_to_page(pfn);
> -		if (PageBuddy(page)) {
> -			/*
> -			 * If race between isolatation and allocation happens,
> -			 * some free pages could be in MIGRATE_MOVABLE list
> -			 * although pageblock's migratation type of the page
> -			 * is MIGRATE_ISOLATE. Catch it and move the page into
> -			 * MIGRATE_ISOLATE list.
> -			 */
> -			if (get_freepage_migratetype(page) != MIGRATE_ISOLATE) {
> -				struct page *end_page;
> -
> -				end_page = page + (1 << page_order(page)) - 1;
> -				move_freepages(page_zone(page), page, end_page,
> -						MIGRATE_ISOLATE);
> -			}
> +		if (PageBuddy(page))
>   			pfn += 1 << page_order(page);
> -		}
>   		else if (page_count(page) == 0 &&
>   			get_freepage_migratetype(page) == MIGRATE_ISOLATE)
>   			pfn += 1;
>

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