[PATCH v1 4/5] mm: kmem: scoped objcg protection

Roman Gushchin
2023-09-29T18:01:47Z

Switch to a scope-based protection of the objcg pointer on slab/kmem
allocation paths. Instead of using the get_() semantics in the
pre-allocation hook and put the reference afterwards, let's rely
on the fact that objcg is pinned by the scope.

It's possible because:
1) if the objcg is received from the current task struct, the task is
keeping a reference to the objcg.
2) if the objcg is received from an active memcg (remote charging),
the memcg is pinned by the scope and has a reference to the
corresponding objcg.

Signed-off-by: Roman Gushchin (Cruise) <roman.gushchin@linux.dev>
---
include/linux/memcontrol.h | 6 +++++
mm/memcontrol.c | 46 ++++++++++++++++++++++++++++++++++++--
mm/slab.h | 10 +++------
3 files changed, 53 insertions(+), 9 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index e59dea9d8666..5a52327ab09a 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1779,6 +1779,12 @@ bool mem_cgroup_kmem_disabled(void);
int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order);
void __memcg_kmem_uncharge_page(struct page *page, int order);

+/*
+ * The returned objcg pointer is safe to use without additional
+ * protection within a scope, refer to the implementation for the
+ * additional details.
+ */
+struct obj_cgroup *current_obj_cgroup(void);
struct obj_cgroup *get_obj_cgroup_from_current(void);
struct obj_cgroup *get_obj_cgroup_from_folio(struct folio *folio);

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e9890f6e4da7..78ab36b5899f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3074,6 +3074,48 @@ __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
return objcg;
}

+__always_inline struct obj_cgroup *current_obj_cgroup(void)
+{
+ struct mem_cgroup *memcg;
+ struct obj_cgroup *objcg;
+
+ if (in_task()) {
+ memcg = current->active_memcg;
+ if (unlikely(memcg))
+ goto from_memcg;
+
+ objcg = READ_ONCE(current->objcg);
+ if (unlikely(current_objcg_needs_update(objcg)))
+ objcg = current_objcg_update(objcg);
+ /*
+ * Objcg reference is kept by the task, so it's safe
+ * to use the objcg by the current task.
+ */
+ return objcg;
+ } else {
+ memcg = this_cpu_read(int_active_memcg);
+ if (unlikely(memcg))
+ goto from_memcg;
+ }
+ return NULL;
+
+from_memcg:
+ for (; !mem_cgroup_is_root(memcg); memcg = parent_mem_cgroup(memcg)) {
+ /*
+ * Memcg pointer is protected by scope (see set_active_memcg())
+ * and is pinning the corresponding objcg, so objcg can't go
+ * away and can be used within the scope without any additional
+ * protection.
+ */
+ objcg = rcu_dereference_check(memcg->objcg, 1);
+ if (likely(objcg))
+ break;
+ objcg = NULL;
+ }
+
+ return objcg;
+}
+
struct obj_cgroup *get_obj_cgroup_from_folio(struct folio *folio)
{
struct obj_cgroup *objcg;
@@ -3168,15 +3210,15 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
struct obj_cgroup *objcg;
int ret = 0;

- objcg = get_obj_cgroup_from_current();
+ objcg = current_obj_cgroup();
if (objcg) {
ret = obj_cgroup_charge_pages(objcg, gfp, 1 << order);
if (!ret) {
+ obj_cgroup_get(objcg);
page->memcg_data = (unsigned long)objcg |
MEMCG_DATA_KMEM;
return 0;
}
- obj_cgroup_put(objcg);
}
return ret;
}
diff --git a/mm/slab.h b/mm/slab.h
index 799a315695c6..8cd3294fedf5 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -484,7 +484,7 @@ static inline bool memcg_slab_pre_alloc_hook(struct kmem_cache *s,
if (!(flags & __GFP_ACCOUNT) && !(s->flags & SLAB_ACCOUNT))
return true;

- objcg = get_obj_cgroup_from_current();
+ objcg = current_obj_cgroup();
if (!objcg)
return true;

@@ -497,17 +497,14 @@ static inline bool memcg_slab_pre_alloc_hook(struct kmem_cache *s,
css_put(&memcg->css);

if (ret)
- goto out;
+ return false;
}

if (obj_cgroup_charge(objcg, flags, objects * obj_full_size(s)))
- goto out;
+ return false;

*objcgp = objcg;
return true;
-out:
- obj_cgroup_put(objcg);
- return false;
}

static inline void memcg_slab_post_alloc_hook(struct kmem_cache *s,
@@ -542,7 +539,6 @@ static inline void memcg_slab_post_alloc_hook(struct kmem_cache *s,
obj_cgroup_uncharge(objcg, obj_full_size(s));
}
}
- obj_cgroup_put(objcg);
}

static inline void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
--
2.42.0