Re: [PATCH] memcg: workingset: remove folio_memcg_rcu usage

[Shakeel Butt <shakeel.butt@linux.dev>]

On Sat, Oct 26, 2024 at 09:37:00AM GMT, Yu Zhao wrote:
> On Sat, Oct 26, 2024 at 1:12 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >
> > The function workingset_activation() is called from
> > folio_mark_accessed() with the guarantee that the given folio can not be
> > freed under us in workingset_activation(). In addition, the association
> > of the folio and its memcg can not be broken here because charge
> > migration is no more. There is no need to use folio_memcg_rcu. Simply
> > use folio_memcg_charged() because that is what this function cares
> > about.
> >
> > Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> > Suggested-by: Yu Zhao <yuzhao@google.com>
> > ---
> >
> > Andrew, please put this patch after the charge migration deprecation
> > series.
> >
> >  include/linux/memcontrol.h | 35 -----------------------------------
> >  mm/workingset.c            | 10 ++--------
> >  2 files changed, 2 insertions(+), 43 deletions(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 932534291ca2..89a1e9f10e1b 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -443,35 +443,6 @@ static inline bool folio_memcg_charged(struct folio *folio)
> >         return __folio_memcg(folio) != NULL;
> >  }
> >
> > -/**
> > - * folio_memcg_rcu - Locklessly get the memory cgroup associated with a folio.
> > - * @folio: Pointer to the folio.
> > - *
> > - * This function assumes that the folio is known to have a
> > - * proper memory cgroup pointer. It's not safe to call this function
> > - * against some type of folios, e.g. slab folios or ex-slab folios.
> > - *
> > - * Return: A pointer to the memory cgroup associated with the folio,
> > - * or NULL.
> > - */
> > -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);
> > -
> > -       if (memcg_data & MEMCG_DATA_KMEM) {
> > -               struct obj_cgroup *objcg;
> > -
> > -               objcg = (void *)(memcg_data & ~OBJEXTS_FLAGS_MASK);
> > -               return obj_cgroup_memcg(objcg);
> > -       }
> > -
> > -       WARN_ON_ONCE(!rcu_read_lock_held());
> > -
> > -       return (struct mem_cgroup *)(memcg_data & ~OBJEXTS_FLAGS_MASK);
> > -}
> > -
> >  /*
> >   * folio_memcg_check - Get the memory cgroup associated with a folio.
> >   * @folio: Pointer to the folio.
> > @@ -1084,12 +1055,6 @@ static inline struct mem_cgroup *folio_memcg(struct folio *folio)
> >         return NULL;
> >  }
> >
> > -static inline struct mem_cgroup *folio_memcg_rcu(struct folio *folio)
> > -{
> > -       WARN_ON_ONCE(!rcu_read_lock_held());
> > -       return NULL;
> > -}
> > -
> >  static inline struct mem_cgroup *folio_memcg_check(struct folio *folio)
> >  {
> >         return NULL;
> > diff --git a/mm/workingset.c b/mm/workingset.c
> > index 4b58ef535a17..c47aa482fad5 100644
> > --- a/mm/workingset.c
> > +++ b/mm/workingset.c
> > @@ -591,9 +591,6 @@ void workingset_refault(struct folio *folio, void *shadow)
> >   */
> >  void workingset_activation(struct folio *folio)
> >  {
> > -       struct mem_cgroup *memcg;
> > -
> > -       rcu_read_lock();
> >         /*
> >          * Filter non-memcg pages here, e.g. unmap can call
> >          * mark_page_accessed() on VDSO pages.
> > @@ -601,12 +598,9 @@ void workingset_activation(struct folio *folio)
> >          * XXX: See workingset_refault() - this should return
> >          * root_mem_cgroup even for !CONFIG_MEMCG.
> 
> The "XXX" part of the comments doesn't apply anymore.
> 
> >          */
> > -       memcg = folio_memcg_rcu(folio);
> > -       if (!mem_cgroup_disabled() && !memcg)
> > -               goto out;
> > +       if (mem_cgroup_disabled() || !folio_memcg_charged(folio))
> > +               return;
> >         workingset_age_nonresident(folio_lruvec(folio), folio_nr_pages(folio));
> 
>  if (mem_cgroup_disabled() || folio_memcg_charged(folio))
>     workingset_age_nonresident()
> 
> We still want to call workingset_age_nonresident() when memcg is
> disabled. (We just want to "Filter non-memcg pages here" when memcg is
> enabled, as the comment above says, which I assume still applies?)
> 

Yes, you are right. Let me send the v2 with the fix.