Re: [PATCH v3 4/7] per-cgroup tcp buffers control

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We can't change net.ipv4.tcp_mem if a cgroup with memory controller isn't mounted.

[root@dhcp-10-30-20-19 ~]# sysctl -w net.ipv4.tcp_mem="3 2 3"
error: "Invalid argument" setting key "net.ipv4.tcp_mem"

It's because tcp_max_memory is initialized in mem_cgroup_populate:

mem_cgroup_populate->register_kmem_files->sockets_populate->tcp_init_cgroup

+int sockets_populate(struct cgroup *cgrp, struct cgroup_subsys *ss)
+{
+	struct proto *proto;
+	int ret = 0;
+
+	read_lock(&proto_list_lock);
+	list_for_each_entry(proto,&proto_list, node) {
+		if (proto->init_cgroup)
+			ret |= proto->init_cgroup(proto, cgrp, ss);
+	}
+	if (!ret)
+		goto out;
+
+	list_for_each_entry_continue_reverse(proto,&proto_list, node)
+		if (proto->destroy_cgroup)
+			proto->destroy_cgroup(proto, cgrp, ss);
+
+out:
+	read_unlock(&proto_list_lock);
+	return ret;
+}

@@ -198,6 +203,21 @@ static int ipv4_tcp_mem(ctl_table *ctl, int write,
  	if (ret)
  		return ret;

+#ifdef CONFIG_CGROUP_MEM_RES_CTLR_KMEM
+	rcu_read_lock();
+	cg = mem_cgroup_from_task(current);
+	for (i = 0; i<  3; i++)
+		if (vec[i]>  tcp_max_memory(cg)) {
+			rcu_read_unlock();
+			return -EINVAL;
+		}

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