mm: mark the OOM reaper thread as freezable

The OOM reaper alters user address space which might theoretically alter
the snapshot if reaping is allowed to happen after the freezer quiescent
state.  To this end, the reaper kthread uses wait_event_freezable()
while waiting for any work so that it cannot run while the system
freezes.

However, the current implementation doesn't respect the freezer because
all kernel threads are created with the PF_NOFREEZE flag, so they are
automatically excluded from freezing operations.  This means that the
OOM reaper can race with system snapshotting if it has work to do while
the system is being frozen.

Fix this by adding a set_freezable() call which will clear the
PF_NOFREEZE flag and thus make the OOM reaper visible to the freezer.

Please note that the OOM reaper altering the snapshot this way is mostly
a theoretical concern and has not been observed in practice.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: aac4536 ("mm, oom: introduce oom reaper")
Signed-off-by: Sultan Alsawaf <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Mel Gorman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/oom_kill.c

@@ -641,6 +641,8 @@ static void oom_reap_task(struct task_struct *tsk)
 
 static int oom_reaper(void *unused)
 {
+	set_freezable();
+
 	while (true) {
 		struct task_struct *tsk = NULL;