Commit d648bcc7 authored Aug 06, 2020 by Roman Gushchin Committed by Linus Torvalds Aug 07, 2020

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 <naresh.kamboju@linaro.org>
Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Link: http://lkml.kernel.org/r/20200702180926.1330769-1-guro@fb.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent ea3271f7

mm/memcontrol.c

+2 −6

Original line number	Diff line number	Diff line
		@@ -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)

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