mm: memcg/slab: stop setting page->mem_cgroup pointer for slab pages

Every slab page charged to a non-root memory cgroup has a pointer to the
memory cgroup and holds a reference to it, which protects a non-empty
memory cgroup from being released.  At the same time the page has a
pointer to the corresponding kmem_cache, and also hold a reference to the
kmem_cache.  And kmem_cache by itself holds a reference to the cgroup.

So there is clearly some redundancy, which allows to stop setting the
page->mem_cgroup pointer and rely on getting memcg pointer indirectly via
kmem_cache.  Further it will allow to change this pointer easier, without
a need to go over all charged pages.

So let's stop setting page->mem_cgroup pointer for slab pages, and stop
using the css refcounter directly for protecting the memory cgroup from
going away.  Instead rely on kmem_cache as an intermediate object.

Make sure that vmstats and shrinker lists are working as previously, as
well as /proc/kpagecgroup interface.

Link: http://lkml.kernel.org/r/20190611231813.3148843-10-guro@fb.com
Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Vladimir Davydov <vdavydov.dev@gmail.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Waiman Long <longman@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Andrei Vagin <avagin@gmail.com>
Cc: Qian Cai <cai@lca.pw>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 70 insertions, 19 deletions

diff --git a/mm/list_lru.c b/mm/list_lru.c
index 927d85be32f6..0f1f6b06b7f3 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -12,6 +12,7 @@
 #include <linux/slab.h>
 #include <linux/mutex.h>
 #include <linux/memcontrol.h>
+#include "slab.h"
 
 #ifdef CONFIG_MEMCG_KMEM
 static LIST_HEAD(list_lrus);
@@ -63,7 +64,7 @@ static __always_inline struct mem_cgroup *mem_cgroup_from_kmem(void *ptr)
         if (!memcg_kmem_enabled())
                 return NULL;
         page = virt_to_head_page(ptr);
-        return page->mem_cgroup;
+        return memcg_from_slab_page(page);
 }
 
 static inline struct list_lru_one *
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ce4ce5e7937b..fa39e51b3d94 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -486,7 +486,10 @@ ino_t page_cgroup_ino(struct page *page)
         unsigned long ino = 0;
 
         rcu_read_lock();
-        memcg = READ_ONCE(page->mem_cgroup);
+        if (PageHead(page) && PageSlab(page))
+                memcg = memcg_from_slab_page(page);
+        else
+                memcg = READ_ONCE(page->mem_cgroup);
         while (memcg && !(memcg->css.flags & CSS_ONLINE))
                 memcg = parent_mem_cgroup(memcg);
         if (memcg)
@@ -2802,9 +2805,6 @@ int __memcg_kmem_charge_memcg(struct page *page, gfp_t gfp, int order,
                 cancel_charge(memcg, nr_pages);
                 return -ENOMEM;
         }
-
-        page->mem_cgroup = memcg;
-
         return 0;
 }
 
@@ -2827,8 +2827,10 @@ int __memcg_kmem_charge(struct page *page, gfp_t gfp, int order)
         memcg = get_mem_cgroup_from_current();
         if (!mem_cgroup_is_root(memcg)) {
                 ret = __memcg_kmem_charge_memcg(page, gfp, order, memcg);
-                if (!ret)
+                if (!ret) {
+                        page->mem_cgroup = memcg;
                         __SetPageKmemcg(page);
+                }
         }
         css_put(&memcg->css);
         return ret;
diff --git a/mm/slab.h b/mm/slab.h
index 5d2b8511e6fb..7ead47cb9338 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -255,30 +255,67 @@ static inline struct kmem_cache *memcg_root_cache(struct kmem_cache *s)
         return s->memcg_params.root_cache;
 }
 
+/*
+ * Expects a pointer to a slab page. Please note, that PageSlab() check
+ * isn't sufficient, as it returns true also for tail compound slab pages,
+ * which do not have slab_cache pointer set.
+ * So this function assumes that the page can pass PageHead() and PageSlab()
+ * checks.
+ */
+static inline struct mem_cgroup *memcg_from_slab_page(struct page *page)
+{
+        struct kmem_cache *s;
+
+        s = READ_ONCE(page->slab_cache);
+        if (s && !is_root_cache(s))
+                return s->memcg_params.memcg;
+
+        return NULL;
+}
+
+/*
+ * Charge the slab page belonging to the non-root kmem_cache.
+ * Can be called for non-root kmem_caches only.
+ */
 static __always_inline int memcg_charge_slab(struct page *page,
                                              gfp_t gfp, int order,
                                              struct kmem_cache *s)
 {
+        struct mem_cgroup *memcg;
+        struct lruvec *lruvec;
         int ret;
 
-        if (is_root_cache(s))
-                return 0;
-
-        ret = memcg_kmem_charge_memcg(page, gfp, order, s->memcg_params.memcg);
+        memcg = s->memcg_params.memcg;
+        ret = memcg_kmem_charge_memcg(page, gfp, order, memcg);
         if (ret)
                 return ret;
 
+        lruvec = mem_cgroup_lruvec(page_pgdat(page), memcg);
+        mod_lruvec_state(lruvec, cache_vmstat_idx(s), 1 << order);
+
+        /* transer try_charge() page references to kmem_cache */
         percpu_ref_get_many(&s->memcg_params.refcnt, 1 << order);
+        css_put_many(&memcg->css, 1 << order);
 
         return 0;
 }
 
+/*
+ * Uncharge a slab page belonging to a non-root kmem_cache.
+ * Can be called for non-root kmem_caches only.
+ */
 static __always_inline void memcg_uncharge_slab(struct page *page, int order,
                                                 struct kmem_cache *s)
 {
-        if (!is_root_cache(s))
-                percpu_ref_put_many(&s->memcg_params.refcnt, 1 << order);
-        memcg_kmem_uncharge(page, order);
+        struct mem_cgroup *memcg;
+        struct lruvec *lruvec;
+
+        memcg = s->memcg_params.memcg;
+        lruvec = mem_cgroup_lruvec(page_pgdat(page), memcg);
+        mod_lruvec_state(lruvec, cache_vmstat_idx(s), -(1 << order));
+        memcg_kmem_uncharge_memcg(page, order, memcg);
+
+        percpu_ref_put_many(&s->memcg_params.refcnt, 1 << order);
 }
 
 extern void slab_init_memcg_params(struct kmem_cache *);
@@ -314,6 +351,11 @@ static inline struct kmem_cache *memcg_root_cache(struct kmem_cache *s)
         return s;
 }
 
+static inline struct mem_cgroup *memcg_from_slab_page(struct page *page)
+{
+        return NULL;
+}
+
 static inline int memcg_charge_slab(struct page *page, gfp_t gfp, int order,
                                     struct kmem_cache *s)
 {
@@ -351,18 +393,24 @@ static __always_inline int charge_slab_page(struct page *page,
                                             gfp_t gfp, int order,
                                             struct kmem_cache *s)
 {
-        int ret = memcg_charge_slab(page, gfp, order, s);
-
-        if (!ret)
-                mod_lruvec_page_state(page, cache_vmstat_idx(s), 1 << order);
+        if (is_root_cache(s)) {
+                mod_node_page_state(page_pgdat(page), cache_vmstat_idx(s),
+                                    1 << order);
+                return 0;
+        }
 
-        return ret;
+        return memcg_charge_slab(page, gfp, order, s);
 }
 
 static __always_inline void uncharge_slab_page(struct page *page, int order,
                                                struct kmem_cache *s)
 {
-        mod_lruvec_page_state(page, cache_vmstat_idx(s), -(1 << order));
+        if (is_root_cache(s)) {
+                mod_node_page_state(page_pgdat(page), cache_vmstat_idx(s),
+                                    -(1 << order));
+                return;
+        }
+
         memcg_uncharge_slab(page, order, s);
 }