Re: [PATCH v3 3/7] mm/swap: always account swapped in page into current memcg

[Kairui Song]

On Tue, Jan 30, 2024 at 2:14 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Kairui Song <ryncsn@xxxxxxxxx> writes:
>
> > From: Kairui Song <kasong@xxxxxxxxxxx>
> >
> > Currently, mem_cgroup_swapin_charge_folio is always called with
> > mm == NULL, except in swapin_direct.
> >
> > swapin_direct is only used when swapin should skip readahead
> > and swapcache (SWP_SYNCHRONOUS_IO). All other callers of
> > mem_cgroup_swapin_charge_folio are for swapin that should
> > not skip readahead and cache.
> >
> > This could cause swapin charging to behave differently depending
> > on swap device, which is unexpected.
> >
> > This is currently not happening because the only caller of
> > swapin_direct is the direct anon page fault path, where mm always
> > equals to current->mm, but will no longer be true if swapin_direct
> > is shared and have other callers (eg, swapoff) to share the
> > readahead skipping logic.
> >
> > So make swapin_direct also pass NULL for mm, so swpain charge
> > will behave consistently and not effected by type of swapin device
> > or readahead policy.
> >
> > After this, the second param of mem_cgroup_swapin_charge_folio is
> > never used now, so it can be safely dropped.
> >
> > Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> > ---
> >  include/linux/memcontrol.h | 4 ++--
> >  mm/memcontrol.c            | 5 ++---
> >  mm/swap_state.c            | 7 +++----
> >  3 files changed, 7 insertions(+), 9 deletions(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 20ff87f8e001..540590d80958 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -693,7 +693,7 @@ static inline int mem_cgroup_charge(struct folio *folio, struct mm_struct *mm,
> >  int mem_cgroup_hugetlb_try_charge(struct mem_cgroup *memcg, gfp_t gfp,
> >               long nr_pages);
> >
> > -int mem_cgroup_swapin_charge_folio(struct folio *folio, struct mm_struct *mm,
> > +int mem_cgroup_swapin_charge_folio(struct folio *folio,
> >                                 gfp_t gfp, swp_entry_t entry);
> >  void mem_cgroup_swapin_uncharge_swap(swp_entry_t entry);
> >
> > @@ -1281,7 +1281,7 @@ static inline int mem_cgroup_hugetlb_try_charge(struct mem_cgroup *memcg,
> >  }
> >
> >  static inline int mem_cgroup_swapin_charge_folio(struct folio *folio,
> > -                     struct mm_struct *mm, gfp_t gfp, swp_entry_t entry)
> > +             gfp_t gfp, swp_entry_t entry)
> >  {
> >       return 0;
> >  }
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index e4c8735e7c85..5852742df958 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -7306,8 +7306,7 @@ int mem_cgroup_hugetlb_try_charge(struct mem_cgroup *memcg, gfp_t gfp,
> >   *
> >   * Returns 0 on success. Otherwise, an error code is returned.
> >   */
> > -int mem_cgroup_swapin_charge_folio(struct folio *folio, struct mm_struct *mm,
> > -                               gfp_t gfp, swp_entry_t entry)
> > +int mem_cgroup_swapin_charge_folio(struct folio *folio, gfp_t gfp, swp_entry_t entry)
> >  {
> >       struct mem_cgroup *memcg;
> >       unsigned short id;
> > @@ -7320,7 +7319,7 @@ int mem_cgroup_swapin_charge_folio(struct folio *folio, struct mm_struct *mm,
> >       rcu_read_lock();
> >       memcg = mem_cgroup_from_id(id);
> >       if (!memcg || !css_tryget_online(&memcg->css))
> > -             memcg = get_mem_cgroup_from_mm(mm);
> > +             memcg = get_mem_cgroup_from_current();
>
> The behavior of get_mem_cgroup_from_mm(NULL) and
> get_mem_cgroup_from_current() isn't same exactly.  Are you sure that
> this is OK?

Hi Ying, thank you very much for the careful review.

IIUC, usually get_mem_cgroup_from_mm(NULL) is for allocations without
mm context (after set_active_memcg), so remote charging cgroup is used
first.

But for swap cases it's a bit special, all swapin are issued from
userspace, so remote charging isn't useful. Not sure if it even may
potentially lead to charging into the wrong cgroup.

And for this callsite, it's called only when `if (!memcg ||
!css_tryget_online(&memcg->css))` is true, only case I know is swapoff
(and the memcg is dead) case, or there are some leaks. The behaviour
of swapoff case has been discussed previously, so currently we just
charge it into the current task's memcg.

This is indeed a potential behaviour change though, I can change it
back to get_mem_cgroup_from_mm(NULL), and post another patch later for
this and discuss in more details.

>
> --
> Best Regards,
> Huang, Ying