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

[Yosry Ahmed]

On Mon, Jul 13, 2026 at 10:03 AM Chris Li <chrisl@xxxxxxxxxx> wrote:
>
> On Mon, Jul 13, 2026 at 9:01 AM Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
> > > 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? I wonder if
>
> Zswap is not part of the tiers exactly because it sits in front of all
> swap devices (tiers) and uses a different control to enable or disable
> it.
> Let's not combine these two; let zswap use its own existing cgroup
> control interface.
>
> > 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, 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.
>
> Zswap will not be introduced as a tier. The existing user interface
> makes zswap not exactly compatible with the tier ordering because it
> sits in front of every swapfile. If we change that, we break the user
> interface. I suggest we keep zswap working as it is now.

The goal from making zswap a swap tier is to have a single framework
to configure swapping for a cgroup, instead of configuring zswap
separately. Yes, zswap currently sits in front of all swap
devices/tiers, but we are heading in the direction of changing that
such that zswap is standalone, at which point it becomes more
obviously a swap tier. If you want us to wait until that happens
before adding zswap as a tier, I don't necessarily object, but I want
to make sure that nothing will break if we add zswap as a tier later.

An advantage of adding zswap as a tier right away is the proactive
writeback use case. It naturally fits in the tiering framework as
proactive demotion between swap tiers, which I expect may be useful in
non-zswap use cases as well. Without zswap as a tier, we'll have to
use a different interface for proactive writeback, and then if/when
zswap becomes a tier, we'll have multiple ways to do proactive
writeback which isn't ideal.