[PATCH] mm: offlining memory may block forever

[Aaditya Kumar <aaditya.kumar.30@gmail.com>]

Offlining memory may block forever, waiting for kswapd() to wake up because
kswapd() does not check the event kthread->should_stop before sleeping.

The proper pattern, from Documentation/memory-barriers.txt, is:
   ---  waker  ---
   event_indicated = 1;
   wake_up_process(event_daemon);

   ---  sleeper  ---
   for (;;) {
      set_current_state(TASK_UNINTERRUPTIBLE);
      if (event_indicated)
         break;
      schedule();
   }

   set_current_state() may be wrapped by:
      prepare_to_wait();

In the kswapd() case, event_indicated is kthread->should_stop.
---  offlining memory (waker)  ---
   kswapd_stop()
      kthread_stop()
         kthread->should_stop = 1
         wake_up_process()
         wait_for_completion()


---  kswapd_try_to_sleep (sleeper)  ---
   kswapd_try_to_sleep()
      prepare_to_wait()
           .
           .
      schedule()
           .
           .
      finish_wait()

   The schedule() needs to be protected by a test of kthread->should_stop,
   which is wrapped by kthread_should_stop().

Reproducer:
   Do heavy file I/O in background.
   Do a memory offline/online in a tight loop


Signed-off-by: Aaditya Kumar <aaditya.kumar@ap.sony.com>

---
diff --git a/mm/vmscan.c b/mm/vmscan.c
index eeb3bc9..b60691e 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2688,7 +2688,10 @@ static void kswapd_try_to_sleep(pg_data_t
*pgdat, int order, int classzone_idx)
 		 * them before going back to sleep.
 		 */
 		set_pgdat_percpu_threshold(pgdat, calculate_normal_threshold);
-		schedule();
+
+		if (!kthread_should_stop())
+			schedule();
+
 		set_pgdat_percpu_threshold(pgdat, calculate_pressure_threshold);
 	} else {
 		if (remaining)

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