mm: kmem: scoped objcg protection

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.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Roman Gushchin (Cruise) <[email protected]>
Tested-by: Naresh Kamboju <[email protected]>
Acked-by: Shakeel Butt <[email protected]>
Reviewed-by: Vlastimil Babka <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Dennis Zhou <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Muchun Song <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 4 insertions, 0 deletions

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 8d89c8c4fac1..9a19f1b42f64 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -403,6 +403,10 @@ DECLARE_PER_CPU(struct mem_cgroup *, int_active_memcg);
  * __GFP_ACCOUNT allocations till the end of the scope will be charged to the
  * given memcg.
  *
+ * Please, make sure that caller has a reference to the passed memcg structure,
+ * so its lifetime is guaranteed to exceed the scope between two
+ * set_active_memcg() calls.
+ *
  * NOTE: This function can nest. Users must save the return value and
  * reset the previous value after their own charging scope is over.
  */