linux-kernel - Re: [PATCH 2/3] mm: page_isolation: remove unnecessary freepage

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Date:	Tue, 09 Dec 2014 10:24:27 +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 2/3] mm: page_isolation: remove unnecessary freepage_migratetype
 check for unused page

On 12/09/2014 08:51 AM, Weijie Yang wrote:
> when we test the pages in a range is free or not, there is a little
> chance we encounter some page which is not in buddy but page_count is 0.
> That means that page could be in the page-freeing path but not in the
> buddy freelist, such as in pcplist

This shouldn't happen anymore IMHO. The pageblock is marked as 
MIGRATE_ISOLATE and then a lru+pcplist drain is performed. Nothing 
should be left on pcplist - anything newly freed goes directly to free 
lists. Hm, maybe it could be on lru cache, but that holds a page 
reference IIRC, so this test won't pass.

> or wait for the zone->lock which the
> tester is holding.

That could maybe happen, but is it worth testing? If yes, please add it 
in a comment to the code.

> Back to the freepage_migratetype, we use it for a cached value for decide
> which free-list the page go when freeing page. If the pageblock is isolated
> the page will go to free-list[MIGRATE_ISOLATE] even if the cached type is
> not MIGRATE_ISOLATE, the commit ad53f92e(fix incorrect isolation behavior
> by rechecking migratetype) patch series have ensure this.
>
> So the freepage_migratetype check for page_count==0 page in
> __test_page_isolated_in_pageblock() is meaningless.
> This patch removes the unnecessary freepage_migratetype check.
>
> Signed-off-by: Weijie Yang <weijie.yang@...sung.com>
> ---
>   mm/page_isolation.c |    3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index 6e5174d..f7c9183 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -223,8 +223,7 @@ __test_page_isolated_in_pageblock(unsigned long pfn, unsigned long end_pfn,
>   		page = pfn_to_page(pfn);
>   		if (PageBuddy(page))
>   			pfn += 1 << page_order(page);
> -		else if (page_count(page) == 0 &&
> -			get_freepage_migratetype(page) == MIGRATE_ISOLATE)
> +		else if (page_count(page) == 0)
>   			pfn += 1;
>   		else if (skip_hwpoisoned_pages && PageHWPoison(page)) {
>   			/*
>

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