oom: stop allocating user memory if TIF_MEMDIE is set

get_user_pages() can try to allocate a nearly unlimited amount of memory on
behalf of a user process, even if that process has been OOM killed.  The
OOM kill occurs upon return to user space via a SIGKILL, but
get_user_pages() will try allocate all its memory before returning.  Change
get_user_pages() to check for TIF_MEMDIE, and if set then return
immediately.
Signed-off-by: NEthan Solomita <solo@google.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -1055,6 +1055,14 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 		do {
 			struct page *page;
 
+			/*
+			 * If tsk is ooming, cut off its access to large memory
+			 * allocations. It has a pending SIGKILL, but it can't
+			 * be processed until returning to user space.
+			 */
+			if (unlikely(test_tsk_thread_flag(tsk, TIF_MEMDIE)))
+				return -ENOMEM;
+
 			if (write)
 				foll_flags |= FOLL_WRITE;