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Subject[013/117] hugetlb: fix infinite loop in get_futex_key() when backed by huge pages
2.6.33-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Mel Gorman <mel@csn.ul.ie>

commit 23be7468e8802a2ac1de6ee3eecb3ec7f14dc703 upstream.

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
include/linux/poison.h | 9 +++++++++
mm/hugetlb.c | 5 ++++-
2 files changed, 13 insertions(+), 1 deletion(-)
--- 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

--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -546,6 +546,7 @@ static void free_huge_page(struct 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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