Re: [PATCH v9 0/6] mm/swap, memcg: Introduce swap tiers for cgroup based swap control

[Youngjun Park]

On Mon, Jun 22, 2026 at 02:23:40PM -0700, Yosry Ahmed wrote:
> On Sat, Jun 20, 2026 at 11:16 AM Youngjun Park <her0gyugyu@xxxxxxxxx> wrote:
> >
> > This is the v9 series of the swap tier patchset.
> >
> > The main change in this version is the addition of selftests for the tier
> > interfaces, requested by Nhat; see the changelog below for the other changes.
> > I designed the test cases and wrote the selftests with some AI assistance.
> >
> > For context, the bulk of the series is unchanged since v8, with great thanks
> > to Shakeel Butt and Yosry for the reviews and discussions [1] that shaped it.
> > The main change in v8 was the interface change to use memory.swap.tiers.max
> > with '0' (disable) and 'max' (enable) values. This mechanism was suggested
> > by Shakeel and Yosry.
> >
> > This change allows for future extensions to control swap between tiers and
> > aligns better with existing memcg interfaces. It is confined to patch #3's
> > user-facing interface; internally, patch #3 still uses the existing mask
> > processing method, which is implementation-efficient.
> >
> > We also discussed tier extensions. Thanks to Yosry, Nhat and Shakeel for their
> > valuable feedback.
> >
> > Here is a brief summary of our tentative conclusions. Please correct me
> > if anything is misrepresented (details in references):
> >
> > * Zswap tiering [2]:
> >   Tiering applies only to the vswap + zswap combo. Zswap itself will
> >   not be tiered, as the current architecture requires a physical device
> >   for zswap allocation.
> 
> I thought we agreed that zswap should be a tier, so that proactive
> zswap writeback can be implemented as proactive swap demotion?
> 
> The only restriction we talked about is that zswap cannot be the only
> allowed tier as long as vswap isn't supported. We can lift the
> restriction when vswap support is added.

Okay, I misunderstood that part. Thanks for the clarification.

To summarize our agreement.
zswap can be the first tier regardless of vswap support.

- With vswap: zswap can be the only allowed tier, as it can operate
  independently.
- Without vswap: zswap cannot be the only allowed tier, as it cannot
  operate without a physical backing device.

I will proceed with this understanding.

Youngjun Park