[RFC PATCH v5 2/3] memcg: Add memory.swap.max.effective like hierarchical_memsw_limit

["Michal Koutný" <mkoutny@suse.com>]

cgroup v1 used memory.stat:hierarchical_memsw_limit to report the value
of effecitve memsw limit. cgroup v2 has no combined charing but swap.max
limit, add a new memcg attribute file that contains the effective value
of memory limit for given cgroup (following cpuset.cpus.effective
pattern) for cases when whole hierarchy cannot be traversed up due to
cgroupns visibility.

Signed-off-by: Jan Kratochvil (Azul) <jkratochvil@azul.com>
[ mkoutny: rewrite commit message, only memory.swap.max change]
Signed-off-by: Michal Koutný <mkoutny@suse.com>
---
 mm/memcontrol.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 86bcec84fe7b..a889385f6033 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -8279,6 +8279,19 @@ static ssize_t swap_max_write(struct kernfs_open_file *of,
 	return nbytes;
 }
 
+static int swap_max_effective_show(struct seq_file *m, void *v)
+{
+	unsigned long swap;
+	struct mem_cgroup *mi;
+
+	/* Hierarchical information */
+	swap = PAGE_COUNTER_MAX;
+	for (mi = mem_cgroup_from_seq(m); mi; mi = parent_mem_cgroup(mi))
+		swap = min(swap, READ_ONCE(mi->swap.max));
+
+	return seq_puts_memcg_tunable(m, swap);
+}
+
 static int swap_events_show(struct seq_file *m, void *v)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
@@ -8311,6 +8324,11 @@ static struct cftype swap_files[] = {
 		.seq_show = swap_max_show,
 		.write = swap_max_write,
 	},
+	{
+		.name = "swap.max.effective",
+		.flags = CFTYPE_NOT_ON_ROOT,
+		.seq_show = swap_max_effective_show,
+	},
 	{
 		.name = "swap.peak",
 		.flags = CFTYPE_NOT_ON_ROOT,
-- 
2.45.1