On Mon, Jul 22, 2024 at 03:08:10PM GMT, 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) > and leave a comment to indicate it is intentional. > Do we expect non-memcg code to access objcg_lock? To me this is some internal implementation detail of memcg and should not be accessible outside memcg code. So, I would recommend to not mention objcg_lock at all. > 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> > --- > include/linux/memcontrol.h | 22 +++++++++++++++++++--- > mm/memcontrol.c | 6 +++--- > 2 files changed, 22 insertions(+), 6 deletions(-) > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > index fc94879db4dff..d616c50025098 100644 > --- a/include/linux/memcontrol.h > +++ b/include/linux/memcontrol.h > @@ -360,11 +360,13 @@ 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(&objcg_lock) && */ > + !lockdep_is_held(&cgroup_mutex)); > return READ_ONCE(objcg->memcg); > } > > @@ -438,6 +440,19 @@ static inline struct mem_cgroup *folio_memcg(struct folio *folio) > return __folio_memcg(folio); > } > > +/* > + * folio_memcg_charged - If a folio is charged to a memory cgroup. > + * @folio: Pointer to the folio. > + * > + * Returns true if folio is charged to a memory cgroup, otherwise returns false. > + */ > +static inline bool folio_memcg_charged(struct folio *folio) > +{ > + if (folio_memcg_kmem(folio)) > + return __folio_objcg(folio) != NULL; > + return __folio_memcg(folio) != NULL; > +} > + > /** > * folio_memcg_rcu - Locklessly get the memory cgroup associated with a folio. > * @folio: Pointer to the folio. > @@ -454,7 +469,6 @@ static inline struct mem_cgroup *folio_memcg_rcu(struct folio *folio) > unsigned long memcg_data = READ_ONCE(folio->memcg_data); > > VM_BUG_ON_FOLIO(folio_test_slab(folio), folio); > - WARN_ON_ONCE(!rcu_read_lock_held()); > > if (memcg_data & MEMCG_DATA_KMEM) { > struct obj_cgroup *objcg; > @@ -463,6 +477,8 @@ static inline struct mem_cgroup *folio_memcg_rcu(struct folio *folio) > return obj_cgroup_memcg(objcg); > } > > + WARN_ON_ONCE(!rcu_read_lock_held()); > + > return (struct mem_cgroup *)(memcg_data & ~OBJEXTS_FLAGS_MASK); > } > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 622d4544edd24..3da0284573857 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -2366,7 +2366,7 @@ void mem_cgroup_cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages) > > static void commit_charge(struct folio *folio, struct mem_cgroup *memcg) > { > - VM_BUG_ON_FOLIO(folio_memcg(folio), folio); > + VM_BUG_ON_FOLIO(folio_memcg_charged(folio), folio); > /* > * Any of the following ensures page's memcg stability: > * > @@ -4617,7 +4617,7 @@ void __mem_cgroup_uncharge(struct folio *folio) > struct uncharge_gather ug; > > /* Don't touch folio->lru of any random page, pre-check: */ > - if (!folio_memcg(folio)) > + if (!folio_memcg_charged(folio)) > return; > > uncharge_gather_clear(&ug); > @@ -4662,7 +4662,7 @@ void mem_cgroup_replace_folio(struct folio *old, struct folio *new) > return; > > /* Page cache replacement: new folio already charged? */ > - if (folio_memcg(new)) > + if (folio_memcg_charged(new)) > return; > > memcg = folio_memcg(old); > -- > 2.20.1 >
