[PATCH] mm: fork: fix kernel_stack memcg stats for various stack implementations

[Roman Gushchin <guro@fb.com>]

Depending on CONFIG_VMAP_STACK and the THREAD_SIZE / PAGE_SIZE ratio
the space for task stacks can be allocated using __vmalloc_node_range(),
alloc_pages_node() and kmem_cache_alloc_node(). In the first and the
second cases page->mem_cgroup pointer is set, but in the third it's
not: memcg membership of a slab page should be determined using the
memcg_from_slab_page() function, which looks at
page->slab_cache->memcg_params.memcg . In this case, using
mod_memcg_page_state() (as in account_kernel_stack()) is incorrect:
page->mem_cgroup pointer is NULL even for pages charged to a non-root
memory cgroup.

It can lead to kernel_stack per-memcg counters permanently showing 0
on some architectures (depending on the configuration).

In order to fix it, let's introduce a mod_memcg_obj_state() helper,
which takes a pointer to a kernel object as a first argument, uses
mem_cgroup_from_obj() to get a RCU-protected memcg pointer and
calls mod_memcg_state(). It allows to handle all possible
configurations (CONFIG_VMAP_STACK and various THREAD_SIZE/PAGE_SIZE
values) without spilling any memcg/kmem specifics into fork.c .

Note: this patch has been first posted as a part of the new slab
controller patchset. This is a slightly updated version: the fixes
tag has been added and the commit log was extended by the advice
of Johannes Weiner. Because it's a fix that makes sense by itself,
I'm re-posting it as a standalone patch.

Fixes: 4d96ba353075 ("mm: memcg/slab: stop setting page->mem_cgroup pointer for slab pages")
Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: stable@vger.kernel.org
---
 include/linux/memcontrol.h |  5 +++++
 kernel/fork.c              |  4 ++--
 mm/memcontrol.c            | 11 +++++++++++
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 3253d5de8243..817ea1d93e0e 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -695,6 +695,7 @@ static inline unsigned long lruvec_page_state_local(struct lruvec *lruvec,
 void __mod_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 			int val);
 void __mod_lruvec_slab_state(void *p, enum node_stat_item idx, int val);
+void mod_memcg_obj_state(void *p, int idx, int val);
 
 static inline void mod_lruvec_state(struct lruvec *lruvec,
 				    enum node_stat_item idx, int val)
@@ -1129,6 +1130,10 @@ static inline void __mod_lruvec_slab_state(void *p, enum node_stat_item idx,
 	__mod_node_page_state(page_pgdat(page), idx, val);
 }
 
+static inline void mod_memcg_obj_state(void *p, int idx, int val)
+{
+}
+
 static inline
 unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
 					    gfp_t gfp_mask,
diff --git a/kernel/fork.c b/kernel/fork.c
index a1f2f5205a61..bdc5004effa4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -404,8 +404,8 @@ static void account_kernel_stack(struct task_struct *tsk, int account)
 		mod_zone_page_state(page_zone(first_page), NR_KERNEL_STACK_KB,
 				    THREAD_SIZE / 1024 * account);
 
-		mod_memcg_page_state(first_page, MEMCG_KERNEL_STACK_KB,
-				     account * (THREAD_SIZE / 1024));
+		mod_memcg_obj_state(stack, MEMCG_KERNEL_STACK_KB,
+				    account * (THREAD_SIZE / 1024));
 	}
 }
 
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d1ae46838af1..6514df549433 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -776,6 +776,17 @@ void __mod_lruvec_slab_state(void *p, enum node_stat_item idx, int val)
 	rcu_read_unlock();
 }
 
+void mod_memcg_obj_state(void *p, int idx, int val)
+{
+	struct mem_cgroup *memcg;
+
+	rcu_read_lock();
+	memcg = mem_cgroup_from_obj(p);
+	if (memcg)
+		mod_memcg_state(memcg, idx, val);
+	rcu_read_unlock();
+}
+
 /**
  * __count_memcg_events - account VM events in a cgroup
  * @memcg: the memory cgroup
-- 
2.24.1