mm: kmem: make memcg keep a reference to the original objcg

Keep a reference to the original objcg object for the entire life of a
memcg structure.

This allows to simplify the synchronization on the kernel memory
allocation paths: pinning a (live) memcg will also pin the corresponding
objcg.

The memory overhead of this change is minimal because object cgroups
usually outlive their corresponding memory cgroups even without this
change, so it's only an additional pointer per memcg.

Link: https://lkml.kernel.org/r/20231019225346.1822282-4-roman.gushchin@linux.dev
Signed-off-by: Roman Gushchin (Cruise) <roman.gushchin@linux.dev>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Acked-by: Shakeel Butt <shakeelb@google.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: David Rientjes <rientjes@google.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 6674c12725d56c29d986c2dcd74569281258ac3e..cc110cc8fdfc3d284cec61245fc34cb8cc533e74 100644 (file)
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -289,7 +289,13 @@ struct mem_cgroup {
 
 #ifdef CONFIG_MEMCG_KMEM
        int kmemcg_id;
-       struct obj_cgroup __rcu *objcg;
+       /*
+        * memcg->objcg is wiped out as a part of the objcg repaprenting
+        * process. memcg->orig_objcg preserves a pointer (and a reference)
+        * to the original objcg until the end of live of memcg.
+        */
+       struct obj_cgroup __rcu *objcg;
+       struct obj_cgroup       *orig_objcg;
        /* list of inherited objcgs, protected by objcg_lock */
        struct list_head objcg_list;
 #endif

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 96f4c319f02572421683c290f0b95a5fc0d14d74..ff036d5d339d94e465fc8badfcef14e534dbdb09 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3899,6 +3899,8 @@ static int memcg_online_kmem(struct mem_cgroup *memcg)
 
        objcg->memcg = memcg;
        rcu_assign_pointer(memcg->objcg, objcg);
+       obj_cgroup_get(objcg);
+       memcg->orig_objcg = objcg;
 
        static_branch_enable(&memcg_kmem_online_key);
 
@@ -5406,6 +5408,9 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
 {
        int node;
 
+       if (memcg->orig_objcg)
+               obj_cgroup_put(memcg->orig_objcg);
+
        for_each_node(node)
                free_mem_cgroup_per_node_info(memcg, node);
        kfree(memcg->vmstats);