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

[Yang Shi]

On Fri, Dec 4, 2020 at 10:54 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> 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@xxxxxx> 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@xxxxxx> 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@xxxxxxxxx>
> > > > > > ---
> > > > > >  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?

No. It is definitely not a big deal.