This avoid userspace mm corruption during COWs with threads (i.e. 
malloc;fork;clone) on x86 PAE with >4G of ram
Signed-Off-By: Andrea Arcangeli <andrea@novell.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>