linux-kernel - Re: [PATCH] mm: zswap: fix zswap_never_enabled() for CONFIG

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Message-ID: <CAJD7tkYJwRF5B2-vQGkmEhgw9t20OcVbB7MNrQqvmTor8Vjptw@mail.gmail.com>
Date: Tue, 2 Jul 2024 04:58:05 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Barry Song <21cnbao@...il.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Barry Song <v-songbaohua@...o.com>, Nhat Pham <nphamcs@...il.com>, 
	Chengming Zhou <chengming.zhou@...ux.dev>, Chris Li <chrisl@...nel.org>, 
	David Hildenbrand <david@...hat.com>, Johannes Weiner <hannes@...xchg.org>, 
	Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] mm: zswap: fix zswap_never_enabled() for CONFIG_ZSWAP==N

On Mon, Jul 1, 2024 at 11:52 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Sun, 30 Jun 2024 11:22:31 +1200 Barry Song <21cnbao@...il.com> wrote:
>
> > From: Barry Song <v-songbaohua@...o.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.