Re: [PATCH v3 2/3] mm: zswap: add zswap_never_enabled()

[Yosry Ahmed <yosryahmed@google.com>]

On Wed, Jun 12, 2024 at 10:19:58AM +1200, Barry Song wrote:
> On Wed, Jun 12, 2024 at 9:55 AM Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> > On Tue, Jun 11, 2024 at 2:53 PM Barry Song <21cnbao@gmail.com> wrote:
> > >
> > > On Tue, Jun 11, 2024 at 2:45 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> > > >
> > > > Add zswap_never_enabled() to skip the xarray lookup in zswap_load() if
> > > > zswap was never enabled on the system. It is implemented using static
> > > > branches for efficiency, as enabling zswap should be a rare event. This
> > > > could shave some cycles off zswap_load() when CONFIG_ZSWAP is used but
> > > > 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 could
> > > >   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 if
> > > >   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_enabled);
> > > >  static bool zswap_enabled = 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_ever_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 might
> come shortly. And this patchset serves the clear purpose of drawing attention
> from mm-core to fallback to small folios.

Sure. Andrew, unless anyone objects, could you please squash the
following diff and add the following sentence to the commit log:

"Expose zswap_never_enabled() in the header for the swapin work to use
it later."

diff --git a/include/linux/zswap.h b/include/linux/zswap.h
index ce5e7bfe8f1ec..bf83ae5e285d4 100644
--- a/include/linux/zswap.h
+++ b/include/linux/zswap.h
@@ -36,6 +36,7 @@ void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg);
 void zswap_lruvec_state_init(struct lruvec *lruvec);
 void zswap_folio_swapin(struct folio *folio);
 bool zswap_is_enabled(void);
+bool zswap_never_enabled(void);
 #else
 
 struct zswap_lruvec_state {};
@@ -65,6 +66,11 @@ static inline bool zswap_is_enabled(void)
 	return false;
 }
 
+static inline bool zswap_never_enabled(void)
+{
+	return false;
+}
+
 #endif
 
 #endif /* _LINUX_ZSWAP_H */
diff --git a/mm/zswap.c b/mm/zswap.c
index 505f4b9812891..a546c01602aaf 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -137,7 +137,7 @@ bool zswap_is_enabled(void)
 	return zswap_enabled;
 }
 
-static bool zswap_never_enabled(void)
+bool zswap_never_enabled(void)
 {
 	return !static_branch_maybe(CONFIG_ZSWAP_DEFAULT_ON, &zswap_ever_enabled);
 }

> 
> Thanks
> Barry