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@xxxxxxxxxxx>
---
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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