From: Martin Schwidefsky <schwidefsky@de.ibm.com>

The patch avoids the instantiation of pagetables for not-present pages in
get_user_pages().  Without this, the coredump code can cause total memory
exhaustion in pagetables.  Consider a store to current stack - 1TB.  The
stack vma is extended to include this address because of VM_GROWSDOWN.  If
such a process dies (which is likely for a defunc process) then the elf core
dumper will cause the system to hang because of too many page tables.

We especially recognise this situation and simply return a ref to the zero
page.