Re: [RFC PATCH v2] zswap: memcontrol: implement zswap writeback disabling

[Nhat Pham <nphamcs@gmail.com>]

On Thu, Nov 2, 2023 at 1:54 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Thu, Nov 2, 2023 at 1:50 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > On Thu, Nov 02, 2023 at 01:27:24PM -0700, Yosry Ahmed wrote:
> > > On Thu, Nov 2, 2023 at 1:02 PM Nhat Pham <nphamcs@gmail.com> wrote:
> > > > @@ -201,6 +201,12 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
> > > >                 folio_end_writeback(folio);
> > > >                 return 0;
> > > >         }
> > > > +
> > > > +       if (!mem_cgroup_zswap_writeback_enabled(folio_memcg(folio))) {
> > > > +               folio_mark_dirty(folio);
> > > > +               return AOP_WRITEPAGE_ACTIVATE;
> > > > +       }
> > > > +
> > >
> > > I am not a fan of this, because it will disable using disk swap if
> > > "zswap_writeback" is disabled, even if zswap is disabled or the page
> > > was never in zswap. The term zswap_writeback makes no sense here tbh.
> > >
> > > I am still hoping someone else will suggest better semantics, because
> > > honestly I can't think of anything. Perhaps something like
> > > memory.swap.zswap_only or memory.swap.types which accepts a string
> > > (e.g. "zswap"/"all",..).
> >
> > I had suggested the writeback name.
> >
> > My thinking was this: from a user pov, the way zswap is presented and
> > described, is a fast writeback cache that sits on top of swap. It's
> > implemented as this lookaside thing right now, but that's never how it
> > was sold. And frankly, that's not how it's expected to work, either.
> >
> > From the docs:
> >
> >   Zswap is a lightweight compressed cache for swap pages.
> >
> >   Zswap evicts pages from compressed cache on an LRU basis to the
> >   backing swap device when the compressed pool reaches its size limit.
> >
> > When zswap is enabled, IO to the swap device is expected to come from
> > zswap. Everything else would be a cache failure. There are a few cases
> > now where zswap rejects and bypasses to swap. It's not a stretch to
> > call them accelerated writeback or writethrough. But also, these
> > represent failures and LRU inversions, we're working on fixing them.
> >
> > So from a user POV it's reasonable to say if I have zswap enabled and
> > disable writeback, I don't expect swapfile IO.
>
> Makes sense (although now you had me thinking whether
> memory.zswap.writethrough is a better name).

Hmmm I lean towards writeback because it's already used in zswap
context. Users not familiar with the writethrough concept might be
confused by the naming.

>
> >
> > But yes, if zswap isn't enabled at all, this shouldn't prevent pages
> > from going to swap.
>
> Right, at least mem_cgroup_zswap_writeback_enabled() should always
> return true if zswap is disabled.

Sure enough! In the next version, I'll always return true in this case.

>
> One more unrelated thing,  I think we should drop the
> cgroup_subsys_on_dfl() check there. I understand the interface is only
> exposed in v2, but I don't see any reason why it wouldn't work in v1.
> Let's not make it harder for anyone who tries to expose this in v1
> (whether upstream or in an internal patch).

I don't have anything against cgroup v1 per se :) I just happened to
notice that zswap charging is not available in v1, so I just played it
safe here.

If nobody objects I can unguard this feature from v1. Seems
reasonable to me tbh.