Re: [RFC PATCH v2 0/7] mm, swap: Virtual Swap Space (Swap Table Edition)

[YoungJun Park <youngjun.park@lge.com>]

On Mon, Jun 15, 2026 at 12:56:26PM -0700, Yosry Ahmed wrote:
> On Sun, Jun 14, 2026 at 7:39 PM Nhat Pham <nphamcs@gmail.com> wrote:
> >
> > On Sun, Jun 14, 2026 at 4:20 AM YoungJun Park <youngjun.park@lge.com> wrote:
> > >
> > > ...
> > > > * Integration with swap.tier by Youngjun (see [12]). For now, I'm
> > > >   leaning towards opting out the vswap device from swap.tier entirely, and
> > > >   treat it as a special device. Integrating it with swap.tiers will
> > > >   benefit the cases where you want some cgroups to skip vswap for fast
> > > >   swap devices (pmem), whereas other should go through zswap first. But
> > > >   most other use cases, either the overhead of vswap will be acceptable
> > > >   (or not the bottleneck), or we can just disable CONFIG_VSWAP entirely :)
> > > >
> > > >   Youngjun, may I ask for your thoughts on this?
> > >
> > > Hi Nhat,
> > >
> > > Tier 1: VSWAP, Tier 2: ZSWAP ...
> > >
> > > I don't see any problem applying the desired functionality with the
> > > currently proposed mechanism and interface. With this, a user would be
> > > assigned the default Virtual -> RAM swap tier, and the overall picture
> > > becomes one where swap tiers are composed according to the priority
> > > setting.
> >
> > It's more - is there a strong argument to let vswap be a tier (which
> > is not supported by just turning of vswap altogether).
> >
> > Because right now I'm not exposing vswap device to userspace in any
> > manner, pretty much. It's abstract and transparent, and minimizes
> > complexity (no vswap and swap.tier interaction) and surfaces for
> > issues.
> 
> I definitely think vswap should *not* be a tier. First of all, a vswap
> entry can be backed by zswap or an actual swap device, which would be
> two different tiers. How does that work?
> 
> I also think vswap should not be exposed to userspace in any way, at
> least not now. I still think we should aim to just make the
> redirection layer always on and eliminate "vswap devices".

After following the answers and giving it some thought, I agree that
vswap should be kept user-transparent. If there is a strict need to
disable it, relying on CONFIG_VSWAP to remove it entirely seems like
the right approach.

If a strong use case for user interaction emerges in the future, we can
revisit the design and figure out how to handle it at that time.

Thanks,
Youngjun Park