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

[Yosry Ahmed]

On Tue, Jun 16, 2026 at 11:33 AM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
>
> On Tue, Jun 16, 2026 at 1:31 PM Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
> >
> > On Mon, Jun 15, 2026 at 8:08 PM YoungJun Park <youngjun.park@xxxxxxx> wrote:
> > >
> > > ...
> > > > - "zswap tier only": Only zswap is allowed. Fallback to other swap is
> > > >   blocked.
> > > > - "zswap writeback disabled": zswap is allowed, but if zswap_store()
> > > >   fails, pages can still fall back to other swap devices.
> > >
> > > Upon double-checking the code, my previous clarification was wrong.
> > > You are right. Sorry for the confusion. "zswap tier only" is indeed
> > > equivalent to "zswap writeback disabled".
> > > (I'm not sure why I read the code that way...)
> > >
> > > As I initially thought, it might be possible to replace the zswap writeback
> > > control with the tiering mechanism.
> > >
> > > If we need to keep the existing interface, we can integrate or share the
> > > underlying logic (though the specific details need more thought anyway).
> > >
> > > It can be summarized as follows:
> > >
> > > - "zswap tier only" + "zswap writeback disable" -> meaningless (noop)
> > > - "zswap tier only" + "zswap writeback enable" -> meaningless (no writabck backend exist)
> > > - "zswap tier with other tiers" + "zswap writeback disable" -> uses only zswap
> > >   (can be replaced by "zswap tier only". This code could be intergrated, modified or something.)
> > > - "zswap tier with other tiers" + "zswap writeback enable" -> works as is
>
> 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?