xen/balloon: fix cancelled balloon action

In case a ballooning action is cancelled the new kernel thread handling
the ballooning might end up in a busy loop.

Fix that by handling the cancelled action gracefully.

While at it introduce a short wait for the BP_WAIT case.

Cc: stable@vger.kernel.org
Fixes: 8480ed9c ("xen/balloon: use a kernel thread instead a workqueue")
Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Tested-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20211005133433.32008-1-jgross@suse.comSigned-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/balloon.c

@@ -491,12 +491,12 @@ static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
 }
 
 /*
- * Stop waiting if either state is not BP_EAGAIN and ballooning action is
- * needed, or if the credit has changed while state is BP_EAGAIN.
+ * Stop waiting if either state is BP_DONE and ballooning action is
+ * needed, or if the credit has changed while state is not BP_DONE.
  */
 static bool balloon_thread_cond(enum bp_state state, long credit)
 {
-	if (state != BP_EAGAIN)
+	if (state == BP_DONE)
 		credit = 0;
 
 	return current_credit() != credit || kthread_should_stop();
@@ -516,10 +516,19 @@ static int balloon_thread(void *unused)
 
 	set_freezable();
 	for (;;) {
-		if (state == BP_EAGAIN)
-			timeout = balloon_stats.schedule_delay * HZ;
-		else
+		switch (state) {
+		case BP_DONE:
+		case BP_ECANCELED:
 			timeout = 3600 * HZ;
+			break;
+		case BP_EAGAIN:
+			timeout = balloon_stats.schedule_delay * HZ;
+			break;
+		case BP_WAIT:
+			timeout = HZ;
+			break;
+		}
+
 		credit = current_credit();
 
 		wait_event_freezable_timeout(balloon_thread_wq,