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

[Youngjun Park]

On Mon, Jul 13, 2026 at 09:01:20AM -0700, Yosry Ahmed wrote:
> On Mon, Jul 13, 2026 at 8:57 AM Youngjun Park <youngjun.park@xxxxxxx> wrote:
> >
> > On Mon, Jul 13, 2026 at 08:50:36AM -0700, Yosry Ahmed wrote:
> > > On Sun, Jul 12, 2026 at 7:57 PM Youngjun Park <youngjun.park@xxxxxxx> wrote:
> > > >
> > > > This is the v10 series of the swap tier patchset.
> > > >
> > > > v10 folds in the Sashiko review fixes for the selftests added in v9 and
> > > > rebases onto the current mm-new. There are no functional changes to the
> > > > core swap or memcg code since v9; see the changelog for details.
> > > >
> > > > 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]:
> > > >   Zswap can itself be a tier (typically the fastest one). But, until vswap lands,
> > > >   zswap cannot be the only allowed tier,
> > > >   since it still needs a physical device for allocation;
> > > >   that restriction can be lifted once vswap is supported.
> > >
> > > Does this series support zswap being a tier? I cannot find any mention
> > > of zswap in the patches.
> >
> > Hello Yosry!
> >
> > This series does not cover zswap as a tier yet.
> >
> > My plan is to land the swap tier infrastructure together with the
> > first use case (cgroup-based swap control) first, and then follow
> > up with zswap tier support in a subsequent series, continuing the
> > discussions we've had above.
> > (I mentioned on cover letter, right above the overview section)
> >
> > Does that approach sound reasonable to you?
> 
> How does swap tiering work with zswap in the current series? I assume
> zswap is just enabled for all devices in all tiers?

Yes, that's correct.

> I wonder if introducing zswap as a tier after the fact changes user-visible
> behavior. I guess if zswap will be introduced with a default "max"
> value it will more-or-less be the same behavior,

Right, that's the plan.

> but I would check all
> user-visible behaviors related to zswap (e.g. interaction with other
> zswap interfaces) to make sure nothing breaks or changes in a
> meaningful way when zswap is introduced as a tier later.

Fair point. Let me review this more and get back to you!

Thanks,
Youngjun