From 4175c12664c7950e5649803d3f0d7096bb84cd09 Mon Sep 17 00:00:00 2001 From: Rafael Aquini Date: Thu, 29 Nov 2012 14:17:24 +1100 Subject: [PATCH] mm: fix balloon_page_movable() page->flags check Fix the following crash by fixing and enhancing the way page->flags are tested to identify a ballooned page. BUG: unable to handle kernel NULL pointer dereference at 0000000000000194 IP: [] isolate_migratepages_range+0x344/0x7b0 The NULL pointer deref was taking place because balloon_page_movable() page->flags tests were incomplete and we ended up inadvertently poking at private pages. Signed-off-by: Rafael Aquini Reported-by: Sasha Levin Cc: Mel Gorman Cc: Rusty Russell Cc: "Michael S. Tsirkin" Cc: Rik van Riel Cc: Andi Kleen Cc: Konrad Rzeszutek Wilk Cc: Minchan Kim Signed-off-by: Andrew Morton --- include/linux/balloon_compaction.h | 21 +++++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-) diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h index 2e63d949098b..6d380e3a9f4e 100644 --- a/include/linux/balloon_compaction.h +++ b/include/linux/balloon_compaction.h @@ -106,6 +106,23 @@ static inline void balloon_mapping_free(struct address_space *balloon_mapping) kfree(balloon_mapping); } +/* + * __balloon_page_flags - helper to perform balloon @page ->flags tests. + * + * As balloon pages are got from Buddy, and we do not play with page->flags + * at driver level (exception made when we get the page lock for compaction), + * therefore we can safely identify a ballooned page by checking if the + * NR_PAGEFLAGS rightmost bits from the page->flags are all cleared. + * This approach also helps on skipping ballooned pages that are locked for + * compaction or release, thus mitigating their racy check at + * balloon_page_movable() + */ +#define BALLOON_PAGE_FLAGS_MASK ((1UL << NR_PAGEFLAGS) - 1) +static inline bool __balloon_page_flags(struct page *page) +{ + return page->flags & BALLOON_PAGE_FLAGS_MASK ? false : true; +} + /* * __is_movable_balloon_page - helper to perform @page mapping->flags tests */ @@ -135,8 +152,8 @@ static inline bool balloon_page_movable(struct page *page) * Before dereferencing and testing mapping->flags, lets make sure * this is not a page that uses ->mapping in a different way */ - if (!PageSlab(page) && !PageSwapCache(page) && !PageAnon(page) && - !page_mapped(page)) + if (__balloon_page_flags(page) && !page_mapped(page) && + page_count(page) == 1) return __is_movable_balloon_page(page); return false; -- 2.39.5