mm/vmscan: add drop_caches_loop_limit to break loop in drop_slab_node

hulk inclusion
category: bugfix
bugzilla: 175105
CVE: NA

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

Command 'echo 2 > /proc/sys/vm/drop_caches' could lead to a loop in
drop_slab_node for a long while. Add /proc/sys/vm/drop_caches_loop_limit
to break loop in drop_slab_node.
Signed-off-by: NLiu Shixin <liushixin2@huawei.com>
Reviewed-by: Ntong tiangen <tongtiangen@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

fs/drop_caches.c

@@ -13,6 +13,7 @@
 
 /* A global variable is a bit ugly, but it keeps the code simple */
 int sysctl_drop_caches;
+unsigned int sysctl_drop_caches_loop_limit __read_mostly;
 
 static void drop_pagecache_sb(struct super_block *sb, void *unused)
 {

include/linux/mm.h

@@ -2766,6 +2766,7 @@ extern bool process_shares_mm(struct task_struct *p, struct mm_struct *mm);
 
 #ifdef CONFIG_SYSCTL
 extern int sysctl_drop_caches;
+extern unsigned int sysctl_drop_caches_loop_limit;
 int drop_caches_sysctl_handler(struct ctl_table *, int,
 					void __user *, size_t *, loff_t *);
 #endif

kernel/sysctl.c

@@ -1477,6 +1477,14 @@ static struct ctl_table vm_table[] = {
 		.extra1		= &one,
 		.extra2		= &four,
 	},
+	{
+		.procname	= "drop_caches_loop_limit",
+		.data		= &sysctl_drop_caches_loop_limit,
+		.maxlen		= sizeof(unsigned int),
+		.mode		= 0644,
+		.proc_handler	= proc_douintvec,
+	},
+
 #ifdef CONFIG_COMPACTION
 	{
 		.procname	= "compact_memory",

mm/vmscan.c

@@ -735,6 +735,9 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 void drop_slab_node(int nid)
 {
 	unsigned long freed;
+#ifdef CONFIG_SYSCTL
+	unsigned int counts = 0;
+#endif
 
 	do {
 		struct mem_cgroup *memcg = NULL;
@@ -747,6 +750,18 @@ void drop_slab_node(int nid)
 		do {
 			freed += shrink_slab(GFP_KERNEL, nid, memcg, 0);
 		} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
+
+#ifdef CONFIG_SYSCTL
+		if (unlikely(sysctl_drop_caches_loop_limit)) {
+			counts++;
+			if (counts >= sysctl_drop_caches_loop_limit) {
+				pr_info("%s (%d): drop_caches early break: %u loops\n",
+					current->comm, task_pid_nr(current),
+					counts);
+				return;
+			}
+		}
+#endif
 	} while (freed > 10);
 }