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

[Nhat Pham]

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?

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)?