linux-kernel - Re: [PATCH v2 2/8] mm/isolation: remove unstable check for isolated page

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Date:	Thu, 07 Aug 2014 15:49:17 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan@...nel.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Tang Chen <tangchen@...fujitsu.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Wen Congyang <wency@...fujitsu.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Laura Abbott <lauraa@...eaurora.org>,
	Heesub Shin <heesub.shin@...sung.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Ritesh Harjani <ritesh.list@...il.com>,
	t.stanislaws@...sung.com, Gioh Kim <gioh.kim@....com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/8] mm/isolation: remove unstable check for isolated
 page

On 08/06/2014 09:18 AM, Joonsoo Kim wrote:
> The check '!PageBuddy(page) && page_count(page) == 0 &&
> migratetype == MIGRATE_ISOLATE' would mean the page on free processing.

What is "the page on free processing"? I thought this test means the 
page is on some CPU's pcplist?

> Although it could go into buddy allocator within a short time,
> futher operation such as isolate_freepages_range() in CMA, called after
> test_page_isolated_in_pageblock(), could be failed due to this unstability

By "unstability" you mean the page can be allocated again from the 
pcplist instead of being freed to buddy list?

> since it requires that the page is on buddy. I think that removing
> this unstability is good thing.
>
> And, following patch makes isolated freepage has new status matched with
> this condition and this check is the obstacle to that change. So remove
> it.

You could also say that pages from isolated pageblocks can no longer 
appear on pcplists after the later patches.

> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> ---
>   mm/page_isolation.c |    6 +-----
>   1 file changed, 1 insertion(+), 5 deletions(-)
>
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index d1473b2..3100f98 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -198,11 +198,7 @@ __test_page_isolated_in_pageblock(unsigned long pfn, unsigned long end_pfn,
>   						MIGRATE_ISOLATE);
>   			}
>   			pfn += 1 << page_order(page);
> -		}
> -		else if (page_count(page) == 0 &&
> -			get_freepage_migratetype(page) == MIGRATE_ISOLATE)
> -			pfn += 1;
> -		else if (skip_hwpoisoned_pages && PageHWPoison(page)) {
> +		} else if (skip_hwpoisoned_pages && PageHWPoison(page)) {
>   			/*
>   			 * The HWPoisoned page may be not in buddy
>   			 * system, and page_count() is not 0.
>

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