Commit 23be7468 authored Apr 23, 2010 by Mel Gorman Committed by Linus Torvalds Apr 24, 2010

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 <mel@csn.ul.ie>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Darren Hart <darren@dvhart.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus 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 @@ 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;
		}
		}

		/*

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