linux-kernel - Re: [v8 PATCH 05/13] mm: vmscan: use kvfree_rcu instead of call

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Date:   Mon, 8 Mar 2021 06:54:44 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Yang Shi <shy828301@...il.com>, Roman Gushchin <guro@...com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dave Chinner <david@...morbit.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [v8 PATCH 05/13] mm: vmscan: use kvfree_rcu instead of call_rcu

On Sun, Mar 07, 2021 at 10:13:04PM -0800, Shakeel Butt wrote:
> On Tue, Feb 16, 2021 at 4:13 PM Yang Shi <shy828301@...il.com> wrote:
> >
> > Using kvfree_rcu() to free the old shrinker_maps instead of call_rcu().
> > We don't have to define a dedicated callback for call_rcu() anymore.
> >
> > Signed-off-by: Yang Shi <shy828301@...il.com>
> > ---
> >  mm/vmscan.c | 7 +------
> >  1 file changed, 1 insertion(+), 6 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 2e753c2516fa..c2a309acd86b 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -192,11 +192,6 @@ static inline int shrinker_map_size(int nr_items)
> >         return (DIV_ROUND_UP(nr_items, BITS_PER_LONG) * sizeof(unsigned long));
> >  }
> >
> > -static void free_shrinker_map_rcu(struct rcu_head *head)
> > -{
> > -       kvfree(container_of(head, struct memcg_shrinker_map, rcu));
> > -}
> > -
> >  static int expand_one_shrinker_map(struct mem_cgroup *memcg,
> >                                    int size, int old_size)
> >  {
> > @@ -219,7 +214,7 @@ static int expand_one_shrinker_map(struct mem_cgroup *memcg,
> >                 memset((void *)new->map + old_size, 0, size - old_size);
> >
> >                 rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_map, new);
> > -               call_rcu(&old->rcu, free_shrinker_map_rcu);
> > +               kvfree_rcu(old);
> 
> Please use kvfree_rcu(old, rcu) instead of kvfree_rcu(old). The single
> param can call synchronize_rcu().

Especially given that you already have the ->rcu field that the
two-argument form requires.

The reason for using the single-argument form is when you have lots of
little data structures, such that getting rid of that rcu_head structure
is valuable enough to be worth the occasional call to synchronize_rcu().
However, please note that this call to synchronize_rcu() happens only
under OOM conditions.

							Thanx, Paul