From aa016d145d4c3b8a7273429528f19d5b423ddbc7 Mon Sep 17 00:00:00 2001 From: Vlastimil Babka Date: Tue, 8 Sep 2015 15:01:22 -0700 Subject: [PATCH] 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pcplists don't contain MIGRATE_ISOLATE freepages nowadays, those are freed directly to free lists, so the check is obsolete.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 Acked-by: Joonsoo Kim Cc: Minchan Kim Acked-by: Michal Nazarewicz Cc: Laura Abbott Reviewed-by: Naoya Horiguchi Cc: Seungho Park Cc: Johannes Weiner Cc: "Kirill A. Shutemov" Acked-by: Mel Gorman Cc: David Rientjes Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/page_alloc.c | 4 ++++ mm/page_isolation.c | 30 ++++++------------------------ 2 files changed, 10 insertions(+), 24 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 5f9394df19bf..a329cfaf634d 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -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); diff --git a/mm/page_isolation.c b/mm/page_isolation.c index 303c908790ef..32fdc1df05e5 100644 --- a/mm/page_isolation.c +++ b/mm/page_isolation.c @@ -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; } -- 2.20.1