linux-kernel - Re: [v5 PATCH 04/11] mm: vmscan: remove memcg_shrinker_map

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Date:   Fri, 29 Jan 2021 09:05:30 -0800
From:   Yang Shi <shy828301@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Roman Gushchin <guro@...com>, Kirill Tkhai <ktkhai@...tuozzo.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        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 FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [v5 PATCH 04/11] mm: vmscan: remove memcg_shrinker_map_size

On Fri, Jan 29, 2021 at 3:22 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 1/28/21 10:22 PM, Yang Shi wrote:
> >> > @@ -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.
>
> IMHO it's not just taste. DIV_ROUND_UP() says what it does and you don't need to
> guess it from the math expression. Also your expression is shorter as it simply
> adds + 1, so if shrinker_nr_max is a multiple of BITS_PER_LONG, there's an extra
> unsigned long that shouldn't be needed. People reading that code will wonder
> whether there was some non-obvious intention behind that, and possibly send
> cleanup patches.

OK, will replace back to DIV_ROUND_UP(). And, a helper macro is
introduced in patch #6, will add that helper in this patch and use
DIV_ROUND_UP() in the helper.

>
> >>
> >> >       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.
>
> Ah, good point.