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From: Barry Song <21cnbao@gmail.com>
Date: Wed, 12 Jun 2024 10:19:58 +1200
Message-ID: <CAGsJ_4xAHR-fMP6c8w6Xf5cVF2OJYwChiGn5Y66qvM_qiEnEDQ@mail.gmail.com>
Subject: Re: [PATCH v3 2/3] mm: zswap: add zswap_never_enabled()
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Johannes Weiner <hannes@cmpxchg.org>, 
	Nhat Pham <nphamcs@gmail.com>, Chengming Zhou <chengming.zhou@linux.dev>, 
	Chris Li <chrisl@kernel.org>, David Hildenbrand <david@redhat.com>, 
	Matthew Wilcox <willy@infradead.org>, linux-mm@kvack.org, linux-kernel@vger.kernel.org
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On Wed, Jun 12, 2024 at 9:55=E2=80=AFAM Yosry Ahmed <yosryahmed@google.com>=
 wrote:
>
> On Tue, Jun 11, 2024 at 2:53=E2=80=AFPM Barry Song <21cnbao@gmail.com> wr=
ote:
> >
> > On Tue, Jun 11, 2024 at 2:45=E2=80=AFPM Yosry Ahmed <yosryahmed@google.=
com> wrote:
> > >
> > > Add zswap_never_enabled() to skip the xarray lookup in zswap_load() i=
f
> > > zswap was never enabled on the system. It is implemented using static
> > > branches for efficiency, as enabling zswap should be a rare event. Th=
is
> > > could shave some cycles off zswap_load() when CONFIG_ZSWAP is used bu=
t
> > > zswap is never enabled.
> > >
> > > However, the real motivation behind this patch is two-fold:
> > > - Incoming large folio swapin work will need to fallback to order-0
> > >   folios if zswap was ever enabled, because any part of the folio cou=
ld
> > >   be in zswap, until proper handling of large folios with zswap is
> > >   added.
> > >
> > > - A warning and recovery attempt will be added in a following change =
in
> > >   case the above was not done incorrectly. Zswap will fail the read i=
f
> > >   the folio is large and it was ever enabled.
> > >
> > > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> > > ---
> > >  mm/zswap.c | 10 ++++++++++
> > >  1 file changed, 10 insertions(+)
> > >
> > > diff --git a/mm/zswap.c b/mm/zswap.c
> > > index a8c8dd8cfe6f5..7fcd751e847d6 100644
> > > --- a/mm/zswap.c
> > > +++ b/mm/zswap.c
> > > @@ -83,6 +83,7 @@ static bool zswap_pool_reached_full;
> > >  static int zswap_setup(void);
> > >
> > >  /* Enable/disable zswap */
> > > +static DEFINE_STATIC_KEY_MAYBE(CONFIG_ZSWAP_DEFAULT_ON, zswap_ever_e=
nabled);
> > >  static bool zswap_enabled =3D IS_ENABLED(CONFIG_ZSWAP_DEFAULT_ON);
> > >  static int zswap_enabled_param_set(const char *,
> > >                                    const struct kernel_param *);
> > > @@ -136,6 +137,11 @@ bool zswap_is_enabled(void)
> > >         return zswap_enabled;
> > >  }
> > >
> > > +static bool zswap_never_enabled(void)
> > > +{
> > > +       return !static_branch_maybe(CONFIG_ZSWAP_DEFAULT_ON, &zswap_e=
ver_enabled);
> > > +}
> >
> > Will we "extern" this one so that mm-core can use it to fallback
> > to small folios?
> > or you prefer this to be done within the coming swapin series?
>
> My intention was to keep it static for now, and expose it in the
> header when needed (in the swapin series). If others think it's better
> to do this now to avoid the churn I am happy to do it as well.

Personally, I'd vote for exposing it now to avoid one more patch which migh=
t
come shortly. And this patchset serves the clear purpose of drawing attenti=
on
from mm-core to fallback to small folios.

Thanks
Barry