[RFC][PATCH 1/2] add res_counter_usage_safe

[Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

This series is a cleaned up patches discussed in a few days ago, the topic
was how to make compaction works well even if there is a memcg under OOM.
==
memcg: add res_counter_usage_safe()

I think usage > limit means a sign of BUG. But, sometimes,
res_counter_charge_nofail() is very convenient. tcp_memcg uses it.
And I'd like to use it for helping page migration.

This patch adds res_counter_usage_safe() which returns min(usage,limit).
By this we can use res_counter_charge_nofail() without breaking
user experience.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 include/linux/res_counter.h |    2 ++
 kernel/res_counter.c        |   15 +++++++++++++++
 net/ipv4/tcp_memcontrol.c   |    2 +-
 3 files changed, 18 insertions(+), 1 deletions(-)

diff --git a/include/linux/res_counter.h b/include/linux/res_counter.h
index 7d7fbe2..a6f8cc5 100644
--- a/include/linux/res_counter.h
+++ b/include/linux/res_counter.h
@@ -226,4 +226,6 @@ res_counter_set_soft_limit(struct res_counter *cnt,
 	return 0;
 }
 
+u64 res_counter_usage_safe(struct res_counter *cnt);
+
 #endif
diff --git a/kernel/res_counter.c b/kernel/res_counter.c
index ad581aa..e84149b 100644
--- a/kernel/res_counter.c
+++ b/kernel/res_counter.c
@@ -171,6 +171,21 @@ u64 res_counter_read_u64(struct res_counter *counter, int member)
 }
 #endif
 
+/*
+ * Returns usage. If usage > limit, limit is returned.
+ * This is useful not to break user experiance if the excess
+ * is temporal.
+ */
+u64 res_counter_usage_safe(struct res_counter *counter)
+{
+	u64 usage, limit;
+
+	limit = res_counter_read_u64(counter, RES_LIMIT);
+	usage = res_counter_read_u64(counter, RES_USAGE);
+
+	return min(usage, limit);
+}
+
 int res_counter_memparse_write_strategy(const char *buf,
 					unsigned long long *res)
 {
diff --git a/net/ipv4/tcp_memcontrol.c b/net/ipv4/tcp_memcontrol.c
index b6f3583..a73dce6 100644
--- a/net/ipv4/tcp_memcontrol.c
+++ b/net/ipv4/tcp_memcontrol.c
@@ -180,7 +180,7 @@ static u64 tcp_read_usage(struct mem_cgroup *memcg)
 		return atomic_long_read(&tcp_memory_allocated) << PAGE_SHIFT;
 
 	tcp = tcp_from_cgproto(cg_proto);
-	return res_counter_read_u64(&tcp->tcp_memory_allocated, RES_USAGE);
+	return res_counter_usage_safe(&tcp->tcp_memory_allocated);
 }
 
 static u64 tcp_cgroup_read(struct cgroup *cont, struct cftype *cft)
-- 
1.7.4.1


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