Re: [PATCH] mm: verify page type before getting memcg from it

[Roman Gushchin <guro@fb.com>]

On Thu, Jan 16, 2020 at 04:50:56PM +0100, Michal Hocko wrote:
> [Cc Roman]

Thanks!

> 
> On Thu 16-01-20 09:10:11, Yafang Shao wrote:
> > Per disccusion with Dave[1], we always assume we only ever put objects from
> > memcg associated slab pages in the list_lru. In list_lru_from_kmem() it
> > calls memcg_from_slab_page() which makes no attempt to verify the page is
> > actually a slab page. But currently the binder coder (in
> > drivers/android/binder_alloc.c) stores normal pages in the list_lru, rather
> > than slab objects. The only reason the binder doesn't catch issue is that
> > the list_lru is not configured to be memcg aware.
> > In order to make it more stable, we should verify the page type before
> > getting memcg from it. In this patch, a new helper is introduced and the
> > old helper is modified. Now we have two helpers as bellow,
> > 
> > struct mem_cgroup *__memcg_from_slab_page(struct page *page);
> > struct mem_cgroup *memcg_from_slab_page(struct page *page);
> > 
> > The first helper is used when we are sure the page is a slab page and also
> > a head page, while the second helper is used when we are not sure the page
> > type.
> > 
> > [1].
> > https://lore.kernel.org/linux-mm/20200106213103.GJ23195@dread.disaster.area/
> > 
> > Suggested-by: Dave Chinner <david@fromorbit.com>
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>

Hello Yafang!

I actually have something similar in my patch queue, but I'm adding
a helper which takes a kernel pointer rather than a page:
  struct mem_cgroup *mem_cgroup_from_obj(void *p);

Will it work for you? If so, I can send it separately.

(I'm working on switching to per-object accounting of slab object,
so that slab pages will be shared between multiple cgroups. So it will
require a change like this).

Thanks!

--

From fc2b1ec53285edcb0017275019d60bd577bf64a9 Mon Sep 17 00:00:00 2001
From: Roman Gushchin <guro@fb.com>
Date: Thu, 2 Jan 2020 15:22:19 -0800
Subject: [PATCH] mm: memcg/slab: introduce mem_cgroup_from_obj()

Sometimes we need to get a memcg pointer from a charged kernel object.
The right way to do it depends on whether it's a proper slab object
or it's backed by raw pages (e.g. it's a vmalloc alloction). In the
first case the kmem_cache->memcg_params.memcg indirection should be
used, however in the the second case it's just page->mem_cgroup.

To simplify this task and hide these implementation details let's
introduce the mem_cgroup_from_obj() helper, which takes a pointer
to any kernel object and returns a valid memcg pointer or NULL.

The caller is still responsible to ensure that the returned memcg
isn't going away underneath: take the rcu read lock, cgroup mutex etc.

mem_cgroup_from_kmem() defined in mm/list_lru.c is now obsolete
and can be removed.

Signed-off-by: Roman Gushchin <guro@fb.com>
---
 include/linux/memcontrol.h |  7 +++++++
 mm/list_lru.c              | 12 +-----------
 mm/memcontrol.c            | 32 +++++++++++++++++++++++++++++---
 3 files changed, 37 insertions(+), 14 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index c372bed6be80..0f6f8e18029e 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -420,6 +420,8 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *, struct pglist_data *);
 
 struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p);
 
+struct mem_cgroup *mem_cgroup_from_obj(void *p);
+
 struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm);
 
 struct mem_cgroup *get_mem_cgroup_from_page(struct page *page);
@@ -912,6 +914,11 @@ static inline bool mm_match_cgroup(struct mm_struct *mm,
 	return true;
 }
 
+static inline struct mem_cgroup *mem_cgroup_from_obj(void *p)
+{
+	return NULL;
+}
+
 static inline struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
 {
 	return NULL;
diff --git a/mm/list_lru.c b/mm/list_lru.c
index 0f1f6b06b7f3..8de5e3784ee4 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -57,16 +57,6 @@ list_lru_from_memcg_idx(struct list_lru_node *nlru, int idx)
 	return &nlru->lru;
 }
 
-static __always_inline struct mem_cgroup *mem_cgroup_from_kmem(void *ptr)
-{
-	struct page *page;
-
-	if (!memcg_kmem_enabled())
-		return NULL;
-	page = virt_to_head_page(ptr);
-	return memcg_from_slab_page(page);
-}
-
 static inline struct list_lru_one *
 list_lru_from_kmem(struct list_lru_node *nlru, void *ptr,
 		   struct mem_cgroup **memcg_ptr)
@@ -77,7 +67,7 @@ list_lru_from_kmem(struct list_lru_node *nlru, void *ptr,
 	if (!nlru->memcg_lrus)
 		goto out;
 
-	memcg = mem_cgroup_from_kmem(ptr);
+	memcg = mem_cgroup_from_obj(ptr);
 	if (!memcg)
 		goto out;
 
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6e1ee8577ecf..99d6fe9d7026 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -757,13 +757,12 @@ void __mod_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 
 void __mod_lruvec_slab_state(void *p, enum node_stat_item idx, int val)
 {
-	struct page *page = virt_to_head_page(p);
-	pg_data_t *pgdat = page_pgdat(page);
+	pg_data_t *pgdat = page_pgdat(virt_to_page(p));
 	struct mem_cgroup *memcg;
 	struct lruvec *lruvec;
 
 	rcu_read_lock();
-	memcg = memcg_from_slab_page(page);
+	memcg = mem_cgroup_from_obj(p);
 
 	/* Untracked pages have no memcg, no lruvec. Update only the node */
 	if (!memcg || memcg == root_mem_cgroup) {
@@ -2636,6 +2635,33 @@ static void commit_charge(struct page *page, struct mem_cgroup *memcg,
 		unlock_page_lru(page, isolated);
 }
 
+/*
+ * Returns a pointer to the memory cgroup to which the kernel object is charged.
+ *
+ * The caller must ensure the memcg lifetime, e.g. by owning a charged object,
+ * taking rcu_read_lock() or cgroup_mutex.
+ */
+struct mem_cgroup *mem_cgroup_from_obj(void *p)
+{
+	struct page *page;
+
+	if (mem_cgroup_disabled())
+		return NULL;
+
+	page = virt_to_head_page(p);
+
+	/*
+	 * Slab pages don't have page->mem_cgroup set because corresponding
+	 * kmem caches can be reparented during the lifetime. That's why
+	 * cache->memcg_params.memcg pointer should be used instead.
+	 */
+	if (PageSlab(page))
+		return memcg_from_slab_page(page);
+
+	/* All other pages use page->mem_cgroup */
+	return page->mem_cgroup;
+}
+
 #ifdef CONFIG_MEMCG_KMEM
 static int memcg_alloc_cache_id(void)
 {
-- 
2.21.1