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Commit 23be7468 authored by Mel Gorman's avatar Mel Gorman Committed by Linus Torvalds
Browse files

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: default avatarMel Gorman <mel@csn.ul.ie>
Acked-by: default avatarPeter Zijlstra <peterz@infradead.org>
Acked-by: default avatarDarren Hart <darren@dvhart.com>
Cc: <stable@kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 9a6a1ecd

include/linux/poison.h

+9 −0
Original line number Diff line number Diff line
@@ -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

mm/hugetlb.c

+4 −1
Original line number Diff line number Diff line
@@ -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 @@ static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma, 
			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;

		}

	}



	/*