Commit aa016d14 authored Sep 08, 2015 by Vlastimil Babka Committed by Linus Torvalds Sep 08, 2015

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
51bb1a40 ("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nowadays, those are freed directly to free lists, so the check is
obsolete.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@suse.cz>
Acked-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Minchan Kim <minchan@kernel.org>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Cc: Laura Abbott <lauraa@codeaurora.org>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Seungho Park <seungho1.park@lge.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 156408c0

mm/page_alloc.c

+4 −0

Original line number	Diff line number	Diff line
		@@ -788,7 +788,11 @@ static void free_pcppages_bulk(struct zone *zone, int count,
		page = list_entry(list->prev, struct page, lru);
		/* must delete as __free_one_page list manipulates */
		list_del(&page->lru);

		mt = get_freepage_migratetype(page);
		/* MIGRATE_ISOLATE page should not go to pcplists */
		VM_BUG_ON_PAGE(is_migrate_isolate(mt), page);
		/* Pageblock could have been isolated meanwhile */
		if (unlikely(has_isolate_pageblock(zone)))
		mt = get_pageblock_migratetype(page);

mm/page_isolation.c

+6 −24

Original line number	Diff line number	Diff line
		@@ -223,34 +223,16 @@ __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;
		}

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