• Aaditya Kumar's avatar
    mm: fix lost kswapd wakeup in kswapd_stop() · 1c7e7f6c
    Aaditya Kumar authored
    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: default avatarAaditya Kumar <aaditya.kumar@ap.sony.com>
    Acked-by: default avatarKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Reviewed-by: default avatarMinchan Kim <minchan@kernel.org>
    Acked-by: default avatarMel Gorman <mel@csn.ul.ie>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    1c7e7f6c
vmscan.c 93.4 KB