[RFC] workqueue: allow system workqueue be used in memory reclaim

[Junxiao Bi <junxiao.bi@oracle.com>]

The following deadlock was triggered on Intel IMSM raid1 volumes.

The sequence of the event is this:

1. memory reclaim was waiting xfs journal flushing and get stucked by
md flush work.

2. md flush work was queued into "md" workqueue, but never get executed,
kworker thread can not be created and also the rescuer thread was executing
md flush work for another md disk and get stuck because
"MD_SB_CHANGE_PENDING" flag was set.

3. That flag should be set by some md write process which was asking to
update md superblock to change in_sync status to 0, and then it used
kernfs_notify to ask "mdmon" process to update superblock, after that,
write process waited that flag to be cleared.

4. But "mdmon" was never wake up, because kernfs_notify() depended on
system wide workqueue "system_wq" to do the notify, but since that
workqueue doesn't have a rescuer thread, notify will not happen.

Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>
---
 kernel/workqueue.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 6e578f576a6f..e3338e3be700 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -6597,7 +6597,7 @@ void __init workqueue_init_early(void)
 		ordered_wq_attrs[i] = attrs;
 	}
 
-	system_wq = alloc_workqueue("events", 0, 0);
+	system_wq = alloc_workqueue("events", WQ_MEM_RECLAIM, 0);
 	system_highpri_wq = alloc_workqueue("events_highpri", WQ_HIGHPRI, 0);
 	system_long_wq = alloc_workqueue("events_long", 0, 0);
 	system_unbound_wq = alloc_workqueue("events_unbound", WQ_UNBOUND,
-- 
2.39.3 (Apple Git-145)