[PATCH] fix OOM killing of swapoff

These days, if you swapoff when there isn't enough memory, OOM killer gives
"BUG: scheduling while atomic" and the machine hangs: badness() needs to do
its PF_SWAPOFF return after the task_unlock (tasklist_lock is also held
here, so p isn't going to be freed: PF_SWAPOFF might get turned off at any
moment, but that doesn't really matter).

Signed-off-by: Hugh Dickins <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/oom_kill.c

···
 	}
 
 	/*
-	 * swapoff can easily use up all memory, so kill those first.
-	 */
-	if (p->flags & PF_SWAPOFF)
-		return ULONG_MAX;
-
-	/*
 	 * The memory size of the process is the basis for the badness.
 	 */
 	points = mm->total_vm;
···
 	 * After this unlock we can no longer dereference local variable `mm'
 	 */
 	task_unlock(p);
+
+	/*
+	 * swapoff can easily use up all memory, so kill those first.
+	 */
+	if (p->flags & PF_SWAPOFF)
+		return ULONG_MAX;
 
 	/*
 	 * Processes which fork a lot of child processes are likely