Re: [PATCH 6/9] mm: vmscan: use per memcg nr_deferred of shrinker

[Yang Shi <shy828301@gmail.com>]

On Thu, Dec 3, 2020 at 3:40 AM Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>
> On 02.12.2020 21:27, Yang Shi wrote:
> > Use per memcg's nr_deferred for memcg aware shrinkers.  The shrinker's nr_deferred
> > will be used in the following cases:
> >     1. Non memcg aware shrinkers
> >     2. !CONFIG_MEMCG
> >     3. memcg is disabled by boot parameter
> >
> > Signed-off-by: Yang Shi <shy828301@gmail.com>
> > ---
> >  mm/vmscan.c | 88 +++++++++++++++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 82 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index cba0bc8d4661..d569fdcaba79 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -203,6 +203,12 @@ static DECLARE_RWSEM(shrinker_rwsem);
> >  static DEFINE_IDR(shrinker_idr);
> >  static int shrinker_nr_max;
> >
> > +static inline bool is_deferred_memcg_aware(struct shrinker *shrinker)
> > +{
> > +     return (shrinker->flags & SHRINKER_MEMCG_AWARE) &&
> > +             !mem_cgroup_disabled();
> > +}
> > +
> >  static int prealloc_memcg_shrinker(struct shrinker *shrinker)
> >  {
> >       int id, ret = -ENOMEM;
> > @@ -271,7 +277,58 @@ static bool writeback_throttling_sane(struct scan_control *sc)
> >  #endif
> >       return false;
> >  }
> > +
> > +static inline long count_nr_deferred(struct shrinker *shrinker,
> > +                                  struct shrink_control *sc)
> > +{
> > +     bool per_memcg_deferred = is_deferred_memcg_aware(shrinker) && sc->memcg;
> > +     struct memcg_shrinker_deferred *deferred;
> > +     struct mem_cgroup *memcg = sc->memcg;
> > +     int nid = sc->nid;
> > +     int id = shrinker->id;
> > +     long nr;
> > +
> > +     if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
> > +             nid = 0;
> > +
> > +     if (per_memcg_deferred) {
> > +             deferred = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_deferred,
> > +                                                  true);
>
> My comment is about both 5/9 and 6/9 patches.

Sorry for the late reply, I don't know why Gmail filtered this out to spam.

>
> shrink_slab_memcg() races with mem_cgroup_css_online(). A visibility of CSS_ONLINE flag
> in shrink_slab_memcg()->mem_cgroup_online() does not guarantee that you will see
> memcg->nodeinfo[nid]->shrinker_deferred != NULL in count_nr_deferred(). This may occur
> because of processor reordering on !x86 (there is no a common lock or memory barriers).
>
> Regarding to shrinker_map this is not a problem due to map check in shrink_slab_memcg().
> The map can't be NULL there.
>
> Regarding to shrinker_deferred you should prove either this is not a problem too,
> or to add proper synchronization (maybe, based on barriers) or to add some similar check
> (maybe, in shrink_slab_memcg() too).

It seems shrink_slab_memcg() might see shrinker_deferred as NULL
either due to the same reason. I don't think there is a guarantee it
won't happen.

We just need guarantee CSS_ONLINE is seen after shrinker_maps and
shrinker_deferred are allocated, so I'm supposed barriers before
"css->flags |= CSS_ONLINE" should work.

So the below patch may be ok:

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index df128cab900f..9f7fb0450d69 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5539,6 +5539,12 @@ static int mem_cgroup_css_online(struct
cgroup_subsys_state *css)
                return -ENOMEM;
        }


+       /*
+        * Barrier for CSS_ONLINE, so that shrink_slab_memcg() sees
shirnker_maps
+        * and shrinker_deferred before CSS_ONLINE.
+        */
+       smp_mb();
+
        /* Online state pins memcg ID, memcg ID pins CSS */
        refcount_set(&memcg->id.ref, 1);
        css_get(css);


Or add one more check for shrinker_deferred sounds acceptable as well.

> Kirill
>