Re: [v5 PATCH 04/11] mm: vmscan: remove memcg_shrinker_map_size

[Yang Shi <shy828301@gmail.com>]

On Thu, Jan 28, 2021 at 8:53 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 1/28/21 12:33 AM, Yang Shi wrote:
> > Both memcg_shrinker_map_size and shrinker_nr_max is maintained, but actually the
> > map size can be calculated via shrinker_nr_max, so it seems unnecessary to keep both.
> > Remove memcg_shrinker_map_size since shrinker_nr_max is also used by iterating the
> > bit map.
> >
> > Signed-off-by: Yang Shi <shy828301@gmail.com>
> > ---
> >  mm/vmscan.c | 18 ++++++++----------
> >  1 file changed, 8 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index d3f3701dfcd2..847369c19775 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -185,8 +185,7 @@ static LIST_HEAD(shrinker_list);
> >  static DECLARE_RWSEM(shrinker_rwsem);
> >
> >  #ifdef CONFIG_MEMCG
> > -
> > -static int memcg_shrinker_map_size;
> > +static int shrinker_nr_max;
> >
> >  static void free_shrinker_map_rcu(struct rcu_head *head)
> >  {
> > @@ -248,7 +247,7 @@ int alloc_shrinker_maps(struct mem_cgroup *memcg)
> >               return 0;
> >
> >       down_write(&shrinker_rwsem);
> > -     size = memcg_shrinker_map_size;
> > +     size = (shrinker_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
> >       for_each_node(nid) {
> >               map = kvzalloc_node(sizeof(*map) + size, GFP_KERNEL, nid);
> >               if (!map) {
> > @@ -266,12 +265,13 @@ int alloc_shrinker_maps(struct mem_cgroup *memcg)
> >  static int expand_shrinker_maps(int new_id)
> >  {
> >       int size, old_size, ret = 0;
> > +     int new_nr_max = new_id + 1;
> >       struct mem_cgroup *memcg;
> >
> > -     size = DIV_ROUND_UP(new_id + 1, BITS_PER_LONG) * sizeof(unsigned long);
> > -     old_size = memcg_shrinker_map_size;
> > +     size = (new_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
> > +     old_size = (shrinker_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
>
> What's wrong with using DIV_ROUND_UP() here?

I don't think there is anything wrong with DIV_ROUND_UP. Should be
just different taste and result in shorter statement.

>
> >       if (size <= old_size)
> > -             return 0;
> > +             goto out;
>
> Can this even happen? Seems to me it can't, so just remove this?

Yes, it can. The maps use unsigned long value for bitmap, so any
shrinker ID < 31 would fall into the same unsigned long, so we may see
size <= old_size, but we need increase shrinker_nr_max since
expand_shrinker_maps() is called iff id >= shrinker_nr_max.

>
> >
> >       if (!root_mem_cgroup)
> >               goto out;
> > @@ -286,9 +286,10 @@ static int expand_shrinker_maps(int new_id)
> >                       goto out;
> >               }
> >       } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
> > +
> >  out:
> >       if (!ret)
> > -             memcg_shrinker_map_size = size;
> > +             shrinker_nr_max = new_nr_max;
> >
> >       return ret;
> >  }
> > @@ -321,7 +322,6 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
> >  #define SHRINKER_REGISTERING ((struct shrinker *)~0UL)
> >
> >  static DEFINE_IDR(shrinker_idr);
> > -static int shrinker_nr_max;
> >
> >  static int prealloc_memcg_shrinker(struct shrinker *shrinker)
> >  {
> > @@ -338,8 +338,6 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker)
> >                       idr_remove(&shrinker_idr, id);
> >                       goto unlock;
> >               }
> > -
> > -             shrinker_nr_max = id + 1;
> >       }
> >       shrinker->id = id;
> >       ret = 0;
> >
>