Re: [PATCH] memcontrol: only transfer the memcg data for migration

[Nhat Pham <nphamcs@gmail.com>]

On Tue, Oct 3, 2023 at 4:22 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Tue, Oct 3, 2023 at 4:14 PM Nhat Pham <nphamcs@gmail.com> wrote:
> >
> > For most migration use cases, only transfer the memcg data from the old
> > folio to the new folio, and clear the old folio's memcg data. No
> > charging and uncharging will be done. These use cases include the new
> > hugetlb memcg accounting behavior (which was not previously handled).
> >
> > This shaves off some work on the migration path, and avoids the
> > temporary double charging of a folio during its migration.
> >
> > The only exception is replace_page_cache_folio(), which will use the old
> > mem_cgroup_migrate() (now renamed to mem_cgroup_replace_folio). In that
> > context, the isolation of the old page isn't quite as thorough as with
> > migration, so we cannot use our new implementation directly.
> >
> > This patch is the result of the following discussion on the new hugetlb
> > memcg accounting behavior:
> >
> > https://lore.kernel.org/lkml/20231003171329.GB314430@monkey/
> >
> > Reported-by: Mike Kravetz <mike.kravetz@oracle.com>
> > Closes: https://lore.kernel.org/lkml/20231003171329.GB314430@monkey/
> > Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> > Signed-off-by: Nhat Pham <nphamcs@gmail.com>
>
> Does this patch fit before or after your series? In both cases I think
> there might be a problem for bisectability.

Hmm my intention for this patch is as a fixlet.
(i.e it should be eventually squashed to the second patch of that series).
I just include the extra context on the fixlet for review purposes.

My apologies - should have been much clearer.
(Perhaps I should just send out v4 at this point?)

>
> > ---
> >  include/linux/memcontrol.h |  7 ++++++
> >  mm/filemap.c               |  2 +-
> >  mm/memcontrol.c            | 45 +++++++++++++++++++++++++++++++++++---
> >  mm/migrate.c               |  3 +--
> >  4 files changed, 51 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index a827e2129790..e3eaa123256b 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -711,6 +711,8 @@ static inline void mem_cgroup_uncharge_list(struct list_head *page_list)
> >
> >  void mem_cgroup_cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages);
> >
> > +void mem_cgroup_replace_folio(struct folio *old, struct folio *new);
> > +
> >  void mem_cgroup_migrate(struct folio *old, struct folio *new);
> >
> >  /**
> > @@ -1294,6 +1296,11 @@ static inline void mem_cgroup_cancel_charge(struct mem_cgroup *memcg,
> >  {
> >  }
> >
> > +static inline void mem_cgroup_replace_folio(struct folio *old,
> > +               struct folio *new)
> > +{
> > +}
> > +
> >  static inline void mem_cgroup_migrate(struct folio *old, struct folio *new)
> >  {
> >  }
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 9481ffaf24e6..673745219c82 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -819,7 +819,7 @@ void replace_page_cache_folio(struct folio *old, struct folio *new)
> >         new->mapping = mapping;
> >         new->index = offset;
> >
> > -       mem_cgroup_migrate(old, new);
> > +       mem_cgroup_replace_folio(old, new);
> >
> >         xas_lock_irq(&xas);
> >         xas_store(&xas, new);
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 6660684f6f97..cbaa26605b3d 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -7316,16 +7316,17 @@ void __mem_cgroup_uncharge_list(struct list_head *page_list)
> >  }
> >
> >  /**
> > - * mem_cgroup_migrate - Charge a folio's replacement.
> > + * mem_cgroup_replace_folio - Charge a folio's replacement.
> >   * @old: Currently circulating folio.
> >   * @new: Replacement folio.
> >   *
> >   * Charge @new as a replacement folio for @old. @old will
> > - * be uncharged upon free.
> > + * be uncharged upon free. This is only used by the page cache
> > + * (in replace_page_cache_folio()).
> >   *
> >   * Both folios must be locked, @new->mapping must be set up.
> >   */
> > -void mem_cgroup_migrate(struct folio *old, struct folio *new)
> > +void mem_cgroup_replace_folio(struct folio *old, struct folio *new)
> >  {
> >         struct mem_cgroup *memcg;
> >         long nr_pages = folio_nr_pages(new);
> > @@ -7364,6 +7365,44 @@ void mem_cgroup_migrate(struct folio *old, struct folio *new)
> >         local_irq_restore(flags);
> >  }
> >
> > +/**
> > + * mem_cgroup_migrate - Transfer the memcg data from the old to the new folio.
> > + * @old: Currently circulating folio.
> > + * @new: Replacement folio.
> > + *
> > + * Transfer the memcg data from the old folio to the new folio for migration.
> > + * The old folio's data info will be cleared. Note that the memory counters
> > + * will remain unchanged throughout the process.
> > + *
> > + * Both folios must be locked, @new->mapping must be set up.
> > + */
> > +void mem_cgroup_migrate(struct folio *old, struct folio *new)
> > +{
> > +       struct mem_cgroup *memcg;
> > +
> > +       VM_BUG_ON_FOLIO(!folio_test_locked(old), old);
> > +       VM_BUG_ON_FOLIO(!folio_test_locked(new), new);
> > +       VM_BUG_ON_FOLIO(folio_test_anon(old) != folio_test_anon(new), new);
> > +       VM_BUG_ON_FOLIO(folio_nr_pages(old) != folio_nr_pages(new), new);
> > +
> > +       if (mem_cgroup_disabled())
> > +               return;
> > +
> > +       memcg = folio_memcg(old);
> > +       /*
> > +        * Note that it is normal to see !memcg for a hugetlb folio.
> > +        * It could have been allocated when memory_hugetlb_accounting was not
> > +        * selected, for e.g.
> > +        */
> > +       VM_WARN_ON_ONCE_FOLIO(!memcg, old);
> > +       if (!memcg)
> > +               return;
> > +
> > +       /* Transfer the charge and the css ref */
> > +       commit_charge(new, memcg);
> > +       old->memcg_data = 0;
> > +}
> > +
> >  DEFINE_STATIC_KEY_FALSE(memcg_sockets_enabled_key);
> >  EXPORT_SYMBOL(memcg_sockets_enabled_key);
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 7d1804c4a5d9..6034c7ed1d65 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -633,8 +633,7 @@ void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
> >
> >         folio_copy_owner(newfolio, folio);
> >
> > -       if (!folio_test_hugetlb(folio))
> > -               mem_cgroup_migrate(folio, newfolio);
> > +       mem_cgroup_migrate(folio, newfolio);
> >  }
> >  EXPORT_SYMBOL(folio_migrate_flags);
> >
> > --
> > 2.34.1
> >