etmem: fix the div 0 problem in swapcache reclaim process

euleros inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7JI6K
CVE: NA

------------------------------------------------------------

In the swapcache recycling process, the number of pages
to be reclaimed on each node is obtained as follows:

nr_to_reclaim[nid_num] = (swapcache_to_reclaim /
			  (swapcache_total_reclaimable / nr[nid_num]));

However, nr[nid_num] is obtained by traversing the number
of swapcache pages on each node.
If there are multiple nodes in the environment and
no swap process occurs on a node, no swapcache page exists.
The value of nr[nid_num] may be 0.

Therefore, division by zero errors may occur.
Signed-off-by: Nliubo <liubo254@huawei.com>

mm/vmscan.c

@@ -4819,9 +4819,12 @@ int do_swapcache_reclaim(unsigned long *swapcache_watermark,
 		for_each_node_state(nid, N_MEMORY) {
 			cond_resched();
 
-			nr_to_reclaim[nid_num] = (swapcache_to_reclaim / (swapcache_total_reclaimable / nr[nid_num]));
+			nr_to_reclaim[nid_num] = (swapcache_total_reclaimable == 0) ? 0 :
+						 ((swapcache_to_reclaim * nr[nid_num]) /
+						   swapcache_total_reclaimable);
 			reclaim_page_count += reclaim_swapcache_pages_from_list(nid,
-						&swapcache_list[nid_num], nr_to_reclaim[nid_num], false);
+						&swapcache_list[nid_num],
+						nr_to_reclaim[nid_num], false);
 			nid_num++;
 		}