linux-kernel - Re: [PATCH 1/2] mm, page_isolation: remove bogus tests for isolated pages

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Date:	Tue, 21 Jul 2015 15:43:42 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Vlastimil Babka <vbabka@...e.cz>
cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	"minkyung88.kim" <minkyung88.kim@....com>, kmk3210@...il.com,
	Seungho Park <seungho1.park@....com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Minchan Kim <minchan@...nel.org>,
	Michal Nazarewicz <mina86@...a86.com>,
	Laura Abbott <lauraa@...eaurora.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH 1/2] mm, page_isolation: remove bogus tests for isolated
 pages

On Tue, 21 Jul 2015, Vlastimil Babka wrote:

> The __test_page_isolated_in_pageblock() is used to verify whether all pages
> in pageblock were either successfully isolated, or are hwpoisoned. Two of the
> possible state of pages, that are tested, are however bogus and misleading.
> 
> Both tests rely on get_freepage_migratetype(page), which however has no
> guarantees about pages on freelists. Specifically, it doesn't guarantee that
> the migratetype returned by the function actually matches the migratetype of
> the freelist that the page is on. Such guarantee is not its purpose and would
> have negative impact on allocator performance.
> 
> The first test checks whether the freepage_migratetype equals MIGRATE_ISOLATE,
> supposedly to catch races between page isolation and allocator activity. These
> races should be fixed nowadays with 51bb1a4093 ("mm/page_alloc: add freepage
> on isolate pageblock to correct buddy list") and related patches. As explained
> above, the check wouldn't be able to catch them reliably anyway. For the same
> reason false positives can happen, although they are harmless, as the
> move_freepages() call would just move the page to the same freelist it's
> already on. So removing the test is not a bug fix, just cleanup. After this
> patch, we assume that all PageBuddy pages are on the correct freelist and that
> the races were really fixed. A truly reliable verification in the form of e.g.
> VM_BUG_ON() would be complicated and is arguably not needed.
> 
> The second test (page_count(page) == 0 && get_freepage_migratetype(page)
> == MIGRATE_ISOLATE) is probably supposed (the code comes from a big memory
> isolation patch from 2007) to catch pages on MIGRATE_ISOLATE pcplists.
> However, pcplists don't contain MIGRATE_ISOLATE freepages nowadays, those are
> freed directly to free lists, so the check is obsolete. Remove it as well.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Michal Nazarewicz <mina86@...a86.com>
> Cc: Laura Abbott <lauraa@...eaurora.org>
> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> ---
>  mm/page_isolation.c | 30 ++++++------------------------
>  1 file changed, 6 insertions(+), 24 deletions(-)
> 
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index 0e69d25..9eaa489c 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -226,34 +226,16 @@ __test_page_isolated_in_pageblock(unsigned long pfn, unsigned long end_pfn,
>  			continue;
>  		}
>  		page = pfn_to_page(pfn);
> -		if (PageBuddy(page)) {
> +		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 the page is on a free list, it has to be on
> +			 * the correct MIGRATE_ISOLATE freelist. There is no
> +			 * simple way to verify that as VM_BUG_ON(), though.
>  			 */
> -			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);
> -			}
>  			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)) {
> -			/*
> -			 * The HWPoisoned page may be not in buddy
> -			 * system, and page_count() is not 0.
> -			 */
> +		else if (skip_hwpoisoned_pages && PageHWPoison(page))
> +			/* A HWPoisoned page cannot be also PageBuddy */
>  			pfn++;
> -			continue;
> -		}
>  		else
>  			break;
>  	}

You may want to consider stating your assumptions explicitly in the code, 
perhaps with VM_BUG_ON(), such as in free_pcppages_bulk() to ensure things 
like get_freepage_migratetype(page) != MIGRATE_ISOLATE.
--
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