Re: [PATCH] mm: zswap: fix zswap_never_enabled() for CONFIG_ZSWAP==N

[Yosry Ahmed <yosryahmed@google.com>]

On Mon, Jul 1, 2024 at 11:52 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Sun, 30 Jun 2024 11:22:31 +1200 Barry Song <21cnbao@gmail.com> wrote:
>
> > From: Barry Song <v-songbaohua@oppo.com>
> >
> > If CONFIG_ZSWAP is set to N, it means zswap cannot be enabled.
> > zswap_never_enabled() should return true.
> >
> > ...
> >
> > --- a/include/linux/zswap.h
> > +++ b/include/linux/zswap.h
> > @@ -68,7 +68,7 @@ static inline bool zswap_is_enabled(void)
> >
> >  static inline bool zswap_never_enabled(void)
> >  {
> > -     return false;
> > +     return true;
> >  }
>
> Well, that code was as wrong as it's possible to get.
>
> But what effect does this have?  Seems "not much"?  Perhaps we'll
> attempt a zswap_load() which later fails for other reasons?

Actually zswap_load() is a noop with !CONFIG_ZSWAP, so it doesn't have
an effect there. The only effect is that with Barry's latest large
folio swapin patches for zram, we will always fallback to order-0
swapin, even mistakenly when !CONFIG_ZSWAP.

Basically the bug just makes Barry's in progress patches not work at all.