[PATCH v2] mm: percpu: fix incorrect size in pcpu_obj_full_size()

[PATCH v2] mm: percpu: fix incorrect size in pcpu_obj_full_size()

[Yafang Shao]

The extra space which is used to store the obj_cgroup membership is only
valid when kmemcg is enabled. The kmemcg can be disabled via the kernel
parameter "cgroup.memory=nokmem" at boot time.
This helper is also used in non-memcg code, for example the tracepoint,
so we should fix it.

It was found by code review when I was implementing bpf memory usage[1].
No real issue happens in production environment.

[1]. https://lwn.net/Articles/921991/

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: Dennis Zhou <dennis@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Vasily Averin <vvs@openvz.org>
---
 mm/percpu-internal.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

---
v1->v2: improve the commit log and comments (Andrew)

diff --git a/mm/percpu-internal.h b/mm/percpu-internal.h
index 70b1ea2..f9847c1 100644
--- a/mm/percpu-internal.h
+++ b/mm/percpu-internal.h
@@ -4,6 +4,7 @@
 
 #include <linux/types.h>
 #include <linux/percpu.h>
+#include <linux/memcontrol.h>
 
 /*
  * pcpu_block_md is the metadata block struct.
@@ -118,14 +119,15 @@ static inline int pcpu_chunk_map_bits(struct pcpu_chunk *chunk)
  * @size: size of area to allocate in bytes
  *
  * For each accounted object there is an extra space which is used to store
- * obj_cgroup membership. Charge it too.
+ * obj_cgroup membership if kmemcg is not disabled. Charge it too.
  */
 static inline size_t pcpu_obj_full_size(size_t size)
 {
 	size_t extra_size = 0;
 
 #ifdef CONFIG_MEMCG_KMEM
-	extra_size += size / PCPU_MIN_ALLOC_SIZE * sizeof(struct obj_cgroup *);
+	if (!mem_cgroup_kmem_disabled())
+		extra_size += size / PCPU_MIN_ALLOC_SIZE * sizeof(struct obj_cgroup *);
 #endif
 
 	return size * num_possible_cpus() + extra_size;
-- 
1.8.3.1