Re: [swap tier discussion] Re: [PATCH v3 2/4] mm/zswap: Implement proactive writeback

[Nhat Pham]

On Tue, Jun 16, 2026 at 4:27 PM Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
>
> On Tue, Jun 16, 2026 at 1:24 PM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
>
> Ohh I thought you meant we shouldn't allow zswap to be a tier at all,
> not the *only* tier.
>
> > Or are you suggesting that if we set zswap as the only tier then we
> > can allocate from any swapfile (since we're not doing any IO anyway)?
>
> Hmm, technically having zswap as the only tier should be equivalent to
> disabling writeback, but you're right that if zswap is the only tier
> than the memcg is not allowed to use swap slots from any swapfile, so
> zswap cannot be used. Very good point :)

Yeah the coupling of swap/zswap makes reasoning about these kinds of
things so annoying. :)

If anything, with vswap, I'll stop having to explain to folks why they
have to provision on-disk swapfile when they only want to use
in-memory compressed swap, and that's a win in my book.

>
> In this case I think yes, we need vswap to be enabled to allow making
> zswap the only tier. That's one gap between zswap being the only tier
> and disabling zswap writeback, the former requires vswap while the
> latter doesn't.

Yup! Anyway, I think Youngjun sent out v8 - let's take a look.