From 23be7468e8802a2ac1de6ee3eecb3ec7f14dc703 Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Fri, 23 Apr 2010 13:17:56 -0400 Subject: [PATCH] hugetlb: fix infinite loop in get_futex_key() when backed by huge pages If a futex key happens to be located within a huge page mapped MAP_PRIVATE, get_futex_key() can go into an infinite loop waiting for a page->mapping that will never exist. See https://bugzilla.redhat.com/show_bug.cgi?id=552257 for more details about the problem. This patch makes page->mapping a poisoned value that includes PAGE_MAPPING_ANON mapped MAP_PRIVATE. This is enough for futex to continue but because of PAGE_MAPPING_ANON, the poisoned value is not dereferenced or used by futex. No other part of the VM should be dereferencing the page->mapping of a hugetlbfs page as its page cache is not on the LRU. This patch fixes the problem with the test case described in the bugzilla. [akpm@linux-foundation.org: mel cant spel] Signed-off-by: Mel Gorman Acked-by: Peter Zijlstra Acked-by: Darren Hart Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/poison.h | 9 +++++++++ mm/hugetlb.c | 5 ++++- 2 files changed, 13 insertions(+), 1 deletion(-) diff --git a/include/linux/poison.h b/include/linux/poison.h index 2110a81c5e2a..34066ffd893d 100644 --- a/include/linux/poison.h +++ b/include/linux/poison.h @@ -48,6 +48,15 @@ #define POISON_FREE 0x6b /* for use-after-free poisoning */ #define POISON_END 0xa5 /* end-byte of poisoning */ +/********** mm/hugetlb.c **********/ +/* + * Private mappings of hugetlb pages use this poisoned value for + * page->mapping. The core VM should not be doing anything with this mapping + * but futex requires the existence of some page->mapping value even though it + * is unused if PAGE_MAPPING_ANON is set. + */ +#define HUGETLB_POISON ((void *)(0x00300300 + POISON_POINTER_DELTA + PAGE_MAPPING_ANON)) + /********** arch/$ARCH/mm/init.c **********/ #define POISON_FREE_INITMEM 0xcc diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 6034dc9e9796..ffbdfc86aedf 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -546,6 +546,7 @@ static void free_huge_page(struct page *page) mapping = (struct address_space *) page_private(page); set_page_private(page, 0); + page->mapping = NULL; BUG_ON(page_count(page)); INIT_LIST_HEAD(&page->lru); @@ -2447,8 +2448,10 @@ retry: spin_lock(&inode->i_lock); inode->i_blocks += blocks_per_huge_page(h); spin_unlock(&inode->i_lock); - } else + } else { lock_page(page); + page->mapping = HUGETLB_POISON; + } } /*