[PATCH] futexs: fix infinite loop in get_futex_key on huge page

[Sonny Rao <sonnyrao@us.ibm.com>]

get_futex_key() can infinitely loop if it is called on a virtual address
that is within a huge page but not aligned to the beginning of that
page.  The call to get_user_pages_fast will return the struct page for
a sub-page within the huge page and the check for page->mapping will
always fail.

The fix is to call compound_head on the page before checking that it's mapped.

Signed-off-by: Sonny Rao <sonnyrao@us.ibm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: stable@kernel.org

Index: linux-2.6.31-rc2/kernel/futex.c
===================================================================
--- linux-2.6.31-rc2.orig/kernel/futex.c	2009-07-10 17:45:46.181084475 -0500
+++ linux-2.6.31-rc2/kernel/futex.c	2009-07-10 17:46:47.345084062 -0500
@@ -247,6 +247,7 @@
 	if (err < 0)
 		return err;
 
+	page = compound_head(page);
 	lock_page(page);
 	if (!page->mapping) {
 		unlock_page(page);