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

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Message-Id: <1437483218-18703-1-git-send-email-vbabka@suse.cz>
Date:	Tue, 21 Jul 2015 14:53:37 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Cc:	linux-kernel@...r.kernel.org,
	"minkyung88.kim" <minkyung88.kim@....com>, kmk3210@...il.com,
	Seungho Park <seungho1.park@....com>,
	Vlastimil Babka <vbabka@...e.cz>,
	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: [PATCH 1/2] mm, page_isolation: remove bogus tests for isolated pages

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;
 	}
-- 
2.4.5

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