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

[Yosry Ahmed]

> > > 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.
>
> I'm afraid your zswap user interface will have to break. I don't see a
> way around breaking your zswap user interface to fit the swap tiering.
> Once we move to the swap tier world, I don't think we should continue
> using zswap.writeback to control the tier write back behavior. We will
> need to rethink this new world.

I wasn't talking about the existing zswap interfaces. I want to make
sure that if we introduce tiering initially without zswap as a tier,
then add zswap as a tier, the semantics of tiering and user-visible
zswap behavior doesn't break.

That being said, the existing zswap interfaces don't have to break
with tiering, why do they? We may end up with redundant interfaces,
which is unfortunate, and we can work to deprecate some of them over
time. But I don't see why we have to break them?

>
> > 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.
>
> I am looking forward to abstracting a more common write back behavior
> in the swap tier world. The classic zswap behavior will be preserved.

Right, but this requires zswap being a tier, which you seem to be opposed to :)