linux-kernel - Re: [PATCH 4/9] mm: vmscan: use a new flag to indicate shrinker is registered

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Date:   Fri, 4 Dec 2020 13:52:47 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Yang Shi <shy828301@...il.com>
Cc:     Roman Gushchin <guro@...com>, Kirill Tkhai <ktkhai@...tuozzo.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Dave Chinner <david@...morbit.com>,
        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: [PATCH 4/9] mm: vmscan: use a new flag to indicate shrinker is
 registered

On Thu, Dec 03, 2020 at 02:25:20PM -0800, Yang Shi wrote:
> On Thu, Dec 3, 2020 at 12:09 PM Roman Gushchin <guro@...com> wrote:
> >
> > On Wed, Dec 02, 2020 at 08:59:40PM -0800, Yang Shi wrote:
> > > On Wed, Dec 2, 2020 at 7:01 PM Roman Gushchin <guro@...com> wrote:
> > > >
> > > > On Wed, Dec 02, 2020 at 10:27:20AM -0800, Yang Shi wrote:
> > > > > Currently registered shrinker is indicated by non-NULL shrinker->nr_deferred.
> > > > > This approach is fine with nr_deferred atthe shrinker level, but the following
> > > > > patches will move MEMCG_AWARE shrinkers' nr_deferred to memcg level, so their
> > > > > shrinker->nr_deferred would always be NULL.  This would prevent the shrinkers
> > > > > from unregistering correctly.
> > > > >
> > > > > Introduce a new "state" field to indicate if shrinker is registered or not.
> > > > > We could use the highest bit of flags, but it may be a little bit complicated to
> > > > > extract that bit and the flags is accessed frequently by vmscan (every time shrinker
> > > > > is called).  So add a new field in "struct shrinker", we may waster a little bit
> > > > > memory, but it should be very few since there should be not too many registered
> > > > > shrinkers on a normal system.
> > > > >
> > > > > Signed-off-by: Yang Shi <shy828301@...il.com>
> > > > > ---
> > > > >  include/linux/shrinker.h |  4 ++++
> > > > >  mm/vmscan.c              | 13 +++++++++----
> > > > >  2 files changed, 13 insertions(+), 4 deletions(-)
> > > > >
> > > > > diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> > > > > index 0f80123650e2..0bb5be88e41d 100644
> > > > > --- a/include/linux/shrinker.h
> > > > > +++ b/include/linux/shrinker.h
> > > > > @@ -35,6 +35,9 @@ struct shrink_control {
> > > > >
> > > > >  #define SHRINK_STOP (~0UL)
> > > > >  #define SHRINK_EMPTY (~0UL - 1)
> > > > > +
> > > > > +#define SHRINKER_REGISTERED  0x1
> > > > > +
> > > > >  /*
> > > > >   * A callback you can register to apply pressure to ageable caches.
> > > > >   *
> > > > > @@ -66,6 +69,7 @@ struct shrinker {
> > > > >       long batch;     /* reclaim batch size, 0 = default */
> > > > >       int seeks;      /* seeks to recreate an obj */
> > > > >       unsigned flags;
> > > > > +     unsigned state;
> > > >
> > > > Hm, can't it be another flag? It seems like we have a plenty of free bits.
> > >
> > > I thought about this too. But I was not convinced by myself that
> > > messing flags with state is a good practice. We may add more flags in
> > > the future, so we may end up having something like:
> > >
> > > flag
> > > flag
> > > flag
> > > state
> > > flag
> > > flag
> > > ...
> > >
> > > Maybe we could use the highest bit for state?
> >
> > Or just
> > state
> > flag
> > flag
> > flag
> > flag
> > flag
> > ...
> >
> > ?
> 
> It is fine too. We should not add more states in foreseeable future.

It's always possible to shuffle things around for cleanup later on,
too. We don't have to provide binary compatibility for existing flags,
and changing a couple of adjacent bits isn't a big deal to keep things
neat. Or am I missing something?