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>

diff --git a/include/linux/poison.h b/include/linux/poison.h
index 2110a81..34066ff 100644 (file)
--- 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 6034dc9..ffbdfc8 100644 (file)
--- 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;
+               }
        }
 
        /*