On Wed, Jul 24, 2024 at 05:20:09PM GMT, Vlastimil Babka (SUSE) wrote: > On 7/24/24 11:53 AM, Muchun Song wrote: > > The obj_cgroup_memcg() is supposed to safe to prevent the returned > > memory cgroup from being freed only when the caller is holding the > > rcu read lock or objcg_lock or cgroup_mutex. It is very easy to > > ignore thoes conditions when users call some upper APIs which call > > obj_cgroup_memcg() internally like mem_cgroup_from_slab_obj() (See > > the link below). So it is better to add lockdep assertion to > > obj_cgroup_memcg() to find those issues ASAP. > > > > Because there is no user of obj_cgroup_memcg() holding objcg_lock > > to make the returned memory cgroup safe, do not add objcg_lock > > assertion (We should export objcg_lock if we really want to do). > > Additionally, this is some internal implementation detail of memcg > > and should not be accessible outside memcg code. > > > > Some users like __mem_cgroup_uncharge() do not care the lifetime > > of the returned memory cgroup, which just want to know if the > > folio is charged to a memory cgroup, therefore, they do not need > > to hold the needed locks. In which case, introduce a new helper > > folio_memcg_charged() to do this. Compare it to folio_memcg(), it > > could eliminate a memory access of objcg->memcg for kmem, actually, > > a really small gain. > > > > Link: https://lore.kernel.org/all/20240718083607.42068-1-songmuchun@xxxxxxxxxxxxx/ > > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx> > > --- > > v2: > > - Remove mention of objcg_lock in obj_cgroup_memcg()(Shakeel Butt). > > > > include/linux/memcontrol.h | 20 +++++++++++++++++--- > > mm/memcontrol.c | 6 +++--- > > 2 files changed, 20 insertions(+), 6 deletions(-) > > > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > > index fc94879db4dff..742351945f683 100644 > > --- a/include/linux/memcontrol.h > > +++ b/include/linux/memcontrol.h > > @@ -360,11 +360,11 @@ static inline bool folio_memcg_kmem(struct folio *folio); > > * After the initialization objcg->memcg is always pointing at > > * a valid memcg, but can be atomically swapped to the parent memcg. > > * > > - * The caller must ensure that the returned memcg won't be released: > > - * e.g. acquire the rcu_read_lock or css_set_lock. > > + * The caller must ensure that the returned memcg won't be released. > > */ > > static inline struct mem_cgroup *obj_cgroup_memcg(struct obj_cgroup *objcg) > > { > > + WARN_ON_ONCE(!rcu_read_lock_held() && !lockdep_is_held(&cgroup_mutex)); > > Maybe lockdep_assert_once() would be a better fit? > So something like: lockdep_assert_once(rcu_read_lock_held() || lockdep_is_held(&cgroup_mutex));
