Re: [LSF/MM/BPF TOPIC] Swap Abstraction / Native Zswap

[Yosry Ahmed <yosryahmed@google.com>]

On Sun, Mar 26, 2023 at 6:24 PM Huang, Ying <ying.huang@intel.com> wrote:
>
> Chris Li <chrisl@kernel.org> writes:
>
> > On Fri, Mar 24, 2023 at 12:28:31AM -0700, Yosry Ahmed wrote:
> >> > In fact, I just suggest to use the minimal design on top of the current
> >> > implementation as the first step.  Then, you can improve it step by
> >> > step.
> >> >
> >> > The first step could be the minimal effort to implement indirection
> >> > layer and moving swapped pages between swap implementations.  Based on
> >> > that, you can build other optimizations, such as pulling swap counting
> >> > to the swap core.  For each step, we can evaluate the gain and cost with
> >> > data.
> >>
> >> Right, I understand that, but to implement the indirection layer on
> >> top of the current implementation, then we will need to support using
> >> zswap without a backing swap device. In order to do this without
> >
> > Agree with Ying on the minimal approach here as well.
> >
> > There are two ways to approach this.
> >
> > 1) Forget zswap, make a minimal implementation to move the page between
> > two swapfile device. It can be swapfile back to two loop back files.
> >
> > Any indirect layer you design will need to convert this usage case
> > any way.
> >
> > 2) Make zswap work without a swapfile.
> > You can implement the zswap on a fake ghosts swap file.
> >
> > If you keep the zswap as frontswap, just make zswap can work without
> > a real swapfile.
> >
> > Make that as your first minimal step. Then it does not need to touch
> > the swap count changes.
> >
> > I view make that step is independent of moving pages between swap device.
> >
> > That patch exists and I consider it has value to some users.
>
> This sounds like an even smaller approach as the first step.  Further
> improvement can be built on top of it.

I am not sure how this would be a step towards the abstraction goal we
have been discussing.

We have been discussing starting out with a minimal indirection layer,
in the shape of an xarray that maps a swap ID to a swap entry, and
that can be disabled with a config option.

For such a design to work, we have to implement swap entry management
& swap counting in zswap, right? Am I missing something?

>
> Best Regards,
> Huang, Ying
>
> >> > Anyway, I don't think you can just implement all your final solution in
> >> > one step.  And, I think the minimal design suggested could be a starting
> >> > point.
> >>
> >> I agree that's a great point, I am just afraid that we will avoid
> >> implementing that full final solution and instead do a lot of work
> >> inside zswap to make up for the difference (e.g. swap entry
> >> management, swap counting). Also, that work in zswap may end up being
> >> unacceptable due to the maintenance burden and/or complexity.
> >
> > If you do either 1) or 2), you can keep these two paths separate.
> >
> > Even if you want to move the page between zswap and swapfile.
> >
> > Idea 3)
> > You don't have to change the swap count code, you can do a
> > minimal change moves the page between zswap and another block
> > device. That way you can get two differenet swap entry with
> > existing code.
> >
> > Chris
>