Re: [swap tier discussion] Re: [PATCH v3 2/4] mm/zswap: Implement proactive writeback

[Yosry Ahmed]

On Tue, Jun 16, 2026 at 1:24 PM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
>
> On Tue, Jun 16, 2026 at 4:10 PM Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
> >
> > On Tue, Jun 16, 2026 at 1:09 PM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
> > >
> > > On Tue, Jun 16, 2026 at 3:54 PM Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
> > > >
> > > > On Tue, Jun 16, 2026 at 11:33 AM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
> > > > >
> > > > > TBH, without vswap, we should not allow setting zswap as its own tier.
> > > > > It's meaningless. Maybe makes it a no-op, and warn users what they're
> > > > > setting is gibberish?
> > > >
> > > > Why? vswap is transparent to the user. Why can't zswap be its own tier?
> > >
> > > Without vswap, if you set zswap as its own tier, which phys swap
> > > device should we allocate from for the backing slot? :)
> >
> > Today we just allocate a swap slot in a swapfile during reclaim,
> > before swapout, and zswap will just writeback to that one. I assume
> > the same will work with swap tiering, except that maybe the way that
> > swap slot will respect the allowed swap tiers?
>
> Yep! So if we set zswap as the only tier, then it wouldn't be able to
> allocate a swap slot in swapfile right?

Ohh I thought you meant we shouldn't allow zswap to be a tier at all,
not the *only* tier.

> Or are you suggesting that if we set zswap as the only tier then we
> can allocate from any swapfile (since we're not doing any IO anyway)?

Hmm, technically having zswap as the only tier should be equivalent to
disabling writeback, but you're right that if zswap is the only tier
than the memcg is not allowed to use swap slots from any swapfile, so
zswap cannot be used. Very good point :)

In this case I think yes, we need vswap to be enabled to allow making
zswap the only tier. That's one gap between zswap being the only tier
and disabling zswap writeback, the former requires vswap while the
latter doesn't.