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

[Yosry Ahmed]

On Thu, Nov 2, 2023 at 1:50 PM Johannes Weiner <hannes@xxxxxxxxxxx> 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@xxxxxxxxx> 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).

>
> 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.

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).