Re: [PATCH] mm: zswap: support exclusive loads

[Yosry Ahmed]

On Wed, Jun 7, 2023 at 7:13 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> On Tue, May 30, 2023 at 09:02:51PM +0000, Yosry Ahmed wrote:
> > @@ -46,6 +46,19 @@ config ZSWAP_DEFAULT_ON
> >         The selection made here can be overridden by using the kernel
> >         command line 'zswap.enabled=' option.
> >
> > +config ZSWAP_EXCLUSIVE_LOADS
> > +     bool "Invalidate zswap entries when pages are loaded"
> > +     depends on ZSWAP
> > +     help
> > +       If selected, when a page is loaded from zswap, the zswap entry is
> > +       invalidated at once, as opposed to leaving it in zswap until the
> > +       swap entry is freed.
> > +
> > +       This avoids having two copies of the same page in memory
> > +       (compressed and uncompressed) after faulting in a page from zswap.
> > +       The cost is that if the page was never dirtied and needs to be
> > +       swapped out again, it will be re-compressed.
> > +
> >  choice
> >       prompt "Default compressor"
> >       depends on ZSWAP
> > diff --git a/mm/frontswap.c b/mm/frontswap.c
> > index 279e55b4ed87..e5d6825110f4 100644
> > --- a/mm/frontswap.c
> > +++ b/mm/frontswap.c
> > @@ -216,8 +216,13 @@ int __frontswap_load(struct page *page)
> >
> >       /* Try loading from each implementation, until one succeeds. */
> >       ret = frontswap_ops->load(type, offset, page);
> > -     if (ret == 0)
> > +     if (ret == 0) {
> >               inc_frontswap_loads();
> > +             if (frontswap_ops->exclusive_loads) {
> > +                     SetPageDirty(page);
> > +                     __frontswap_clear(sis, offset);
> > +             }
> > +     }
> >       return ret;
>
> This would be a much more accessible feature (distro kernels,
> experimenting, adapting to different workloads) if it were runtime
> switchable.
>
> That should be possible, right? As long as frontswap and zswap are
> coordinated, this can be done on a per-entry basis:
>
>         exclusive = READ_ONCE(frontswap_ops->exclusive_loads);
>         ret = frontswap_ops->load(type, offset, page, exclusive);
>         if (ret == 0) {
>                 if (exclusive) {
>                         SetPageDirty(page);
>                         __frontswap_clear(sis, offset);
>                 }
>         }

Sure, I can do that. My thought was that this isn't something that
someone usually wants to change at runtime, but at least for
experimentation, it does make things a lot easier. I can add a zswap
module parameter.

I am guessing for consistency with other zswap settings and for
convenience, I can leave the config option to set the default value
with, aka CONFIG_ZSWAP_EXCLUSIVE_LOADS_DEFAULT_ON or something. Does
this make sense?