mm/memcontrol: export memcg->watermark via sysfs for v2 memcg

We run a lot of automated tests when building our software and run into OOM scenarios when the tests run unbounded. v1 memcg exports memcg->watermark as "memory.max_usage_in_bytes" in sysfs. We use this metric to heuristically limit the number of tests that can run in parallel based on per test historical data. This metric is currently not exported for v2 memcg and there is no other easy way of getting this information. getrusage() syscall returns "ru_maxrss" which can be used as an approximation but that's the max RSS of a single child process across all children instead of the aggregated max for all child processes. The only work around is to periodically poll "memory.current" but that's not practical for short-lived one-off cgroups. Hence, expose memcg->watermark as "memory.peak" for v2 memcg. Link: https://lkml.kernel.org/r/20220507050916.GA13577@us192.sjc.aristanetworks.com Signed-off-by: Ganesan Rajagopal <rganesan@arista.com> Acked-by: Shakeel Butt <shakeelb@google.com> Acked-by: Johannes Weiner <hannes@cmpxchg.org> Acked-by: Roman Gushchin <roman.gushchin@linux.dev> Acked-by: Michal Hocko <mhocko@suse.com> Cc: Roman Gushchin <roman.gushchin@linux.dev> Reviewed-by: Michal Koutný <mkoutny@suse.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: rganesan

Documentation/admin-guide/cgroup-v2.rst

@@ -1229,6 +1229,13 @@ PAGE_SIZE multiple when read back.
 the target cgroup. If less bytes are reclaimed than the
 specified amount, -EAGAIN is returned.
 
+ memory.peak
+A read-only single value file which exists on non-root
+cgroups.
+
+The max memory usage recorded for the cgroup and its
+descendants since the creation of the cgroup.
+
  memory.oom.group
 A read-write single value file which exists on non-root
 cgroups. The default value is "0".

mm/memcontrol.c

@@ -6103,6 +6103,14 @@ static u64 memory_current_read(struct cgroup_subsys_state *css,
 return (u64)page_counter_read(&memcg->memory) * PAGE_SIZE;
 }
 
+static u64 memory_peak_read(struct cgroup_subsys_state *css,
+ struct cftype *cft)
+{
+struct mem_cgroup *memcg = mem_cgroup_from_css(css);
+
+return (u64)memcg->memory.watermark * PAGE_SIZE;
+}
+
 static int memory_min_show(struct seq_file *m, void *v)
 {
 return seq_puts_memcg_tunable(m,
@@ -6406,6 +6414,11 @@ static struct cftype memory_files[] = {
 .flags = CFTYPE_NOT_ON_ROOT,
 .read_u64 = memory_current_read,
 },
+{
+.name = "peak",
+.flags = CFTYPE_NOT_ON_ROOT,
+.read_u64 = memory_peak_read,
+},
 {
 .name = "min",
 .flags = CFTYPE_NOT_ON_ROOT,