From d179e84ba5da1d0024087d1759a2938817a00f3f Mon Sep 17 00:00:00 2001 From: Andrea Arcangeli Date: Wed, 15 Jun 2011 15:08:51 -0700 Subject: [PATCH] mm: vmscan: do not use page_count without a page pin It is unsafe to run page_count during the physical pfn scan because compound_head could trip on a dangling pointer when reading page->first_page if the compound page is being freed by another CPU. [mgorman@suse.de: split out patch] Signed-off-by: Andrea Arcangeli Signed-off-by: Mel Gorman Reviewed-by: Michal Hocko Reviewed-by: Minchan Kim Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/vmscan.c | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index dbe6ea321df4..8ff834e19c24 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -1124,8 +1124,20 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan, nr_lumpy_dirty++; scan++; } else { - /* the page is freed already. */ - if (!page_count(cursor_page)) + /* + * Check if the page is freed already. + * + * We can't use page_count() as that + * requires compound_head and we don't + * have a pin on the page here. If a + * page is tail, we may or may not + * have isolated the head, so assume + * it's not free, it'd be tricky to + * track the head status without a + * page pin. + */ + if (!PageTail(cursor_page) && + !atomic_read(&cursor_page->_count)) continue; break; }