Re: [PATCH v10 0/6] mm/swap, memcg: Introduce swap tiers for cgroup based swap control

[Yosry Ahmed]

On Tue, Jul 14, 2026 at 3:26 PM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote:
>
> On Tue, Jul 14, 2026 at 01:52:14PM -0700, Yosry Ahmed wrote:
> [...]
> > >
> > > Yosry, what is needed to enable zswap as a swap tier? What will be the minimum
> > > requirements for that?
> >
> > From zswap's perspective, we just need to skip zswap is zswap as a
> > tier is disallowed. Could just be a check in zswap_store() similar to
> > the check if zswap is enabled. I am assuming that if a swap tier is
> > disabled, nothing happens to the existing swapped out pages in this
> > tier, but new pages do not get swapped out to it. This is the same
> > behavior that happens if zswap is disabled at runtime.
> >
> > From the tiering perspective, we need to accept "zswap" as a possible
> > tier, or maybe creating it as a tier by default if zswap is configured
> > would be better to avoid handling the case where the user doesn't
> > create a tier for zswap.
>
> Default tier if zswap is configured makes sense. Should zswap be treated as
> having 32767 (or maybe 32768) as priority as it sits infront of all swap
> devices today? Also whichever swap tier has priority range containing 32767,
> will have zswap in it.
>
> > We also need to disallow zswap being the only
> > tier as that combination cannot work without vswap.
>
> Do we need to do anything explicitly for this? I am assuming in a kernel with
> swap tier support, there always exist a swap tier if there is even a single swap
> device configured i.e. a tier with the full priority range.

At least on the memcg side we need to disallow writing 0 for all tiers
except zswap. Not sure about the global tier configurations.