mm: kmem: make memcg_kmem_enabled() irreversible

Historically the kernel memory accounting was an opt-in feature, which
could be enabled for individual cgroups.  But now it's not true, and it's
on by default both on cgroup v1 and cgroup v2.  And as long as a user has
at least one non-root memory cgroup, the kernel memory accounting is on.
So in most setups it's either always on (if memory cgroups are in use and
kmem accounting is not disabled), either always off (otherwise).

memcg_kmem_enabled() is used in many places to guard the kernel memory
accounting code.  If memcg_kmem_enabled() can reverse from returning true
to returning false (as now), we can't rely on it on release paths and have
to check if it was on before.

If we'll make memcg_kmem_enabled() irreversible (always returning true
after returning it for the first time), it'll make the general logic more
simple and robust.  It also will allow to guard some checks which
otherwise would stay unguarded.

Reported-by: Naresh Kamboju <[email protected]>
Signed-off-by: Roman Gushchin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Tested-by: Naresh Kamboju <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8cc617ede7e2ec408970e0a6c9b0a30cd54ab90a..24892a14cc75cc3b701ce4225c5e88a1a80a149d 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3416,7 +3416,8 @@ static int memcg_online_kmem(struct mem_cgroup *memcg)
        if (memcg_id < 0)
                return memcg_id;
 
-       static_branch_inc(&memcg_kmem_enabled_key);
+       static_branch_enable(&memcg_kmem_enabled_key);
+
        /*
         * A memory cgroup is considered kmem-online as soon as it gets
         * kmemcg_id. Setting the id after enabling static branching will
@@ -3486,11 +3487,6 @@ static void memcg_free_kmem(struct mem_cgroup *memcg)
        /* css_alloc() failed, offlining didn't happen */
        if (unlikely(memcg->kmem_state == KMEM_ONLINE))
                memcg_offline_kmem(memcg);
-
-       if (memcg->kmem_state == KMEM_ALLOCATED) {
-               WARN_ON(!list_empty(&memcg->kmem_caches));
-               static_branch_dec(&memcg_kmem_enabled_key);
-       }
 }
 #else
 static int memcg_online_kmem(struct mem_cgroup *memcg)