s390/mm: Add cond_resched() to cmm_alloc/free_pages()

Adding/removing large amount of pages at once to/from the CMM balloon
can result in rcu_sched stalls or workqueue lockups, because of busy
looping w/o cond_resched().

Prevent this by adding a cond_resched(). cmm_free_pages() holds a
spin_lock while looping, so it cannot be added directly to the existing
loop. Instead, introduce a wrapper function that operates on maximum 256
pages at once, and add it there.
Signed-off-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/mm/cmm.c

@@ -95,11 +95,12 @@ static long cmm_alloc_pages(long nr, long *counter,
 		(*counter)++;
 		spin_unlock(&cmm_lock);
 		nr--;
+		cond_resched();
 	}
 	return nr;
 }
 
-static long cmm_free_pages(long nr, long *counter, struct cmm_page_array **list)
+static long __cmm_free_pages(long nr, long *counter, struct cmm_page_array **list)
 {
 	struct cmm_page_array *pa;
 	unsigned long addr;
@@ -123,6 +124,21 @@ static long cmm_free_pages(long nr, long *counter, struct cmm_page_array **list)
 	return nr;
 }
 
+static long cmm_free_pages(long nr, long *counter, struct cmm_page_array **list)
+{
+	long inc = 0;
+
+	while (nr) {
+		inc = min(256L, nr);
+		nr -= inc;
+		inc = __cmm_free_pages(inc, counter, list);
+		if (inc)
+			break;
+		cond_resched();
+	}
+	return nr + inc;
+}
+
 static int cmm_oom_notify(struct notifier_block *self,
 			  unsigned long dummy, void *parm)
 {