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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Mon, Jun 08, 2026 at 01:19:32PM -0700, Yosry Ahmed wrote:
> On Mon, Jun 8, 2026 at 12:50 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >
> > +Youngjun
> >
> > On Mon, Jun 08, 2026 at 11:30:30AM -0700, Shakeel Butt wrote:
> > > On Wed, Jun 03, 2026 at 10:36:07PM -0700, Yosry Ahmed wrote:
> > > > > >> But doesn't it make more sense to specify the compressed size, which is
> > > > > >> ultimately the amount of memory you actually want to reclaim.
> > > > > >>
> > > > > >
> > > > > > I personally prefer compressed size to pre-compressed size. That's
> > > > > > kinda what user cares about, no?
> > > > > >
> > > > > > One thing we can do is let users prescribe a compressed size, but
> > > > > > internally, we can multiply that by the average compression ratio.
> > > > > > That gives us a guesstimate of how many pages we need to reclaim, and
> > > > > > you can follow the rest of your implementation as is (perhaps with
> > > > > > short-circuit when we reach the goal with fewer pages reclaimed).
> > > > >
> > > > > Got it. I will change it to use the compressed size in the next version.
> > > > >
> > > > > Yosry, Nhat, should we continue using the zswap_writeback_only key to
> > > > > trigger proactive writeback?
> > > >
> > > > I *really* want the memcg maintainers to chime in here, it's
> > > > ultimately their call.
> > > >
> > > > Michal? Johannes? Shakeel? Roman? Anyone? :D
> > >
> > > Between the options of having an explicit interface (i.e.
> > > memory.zswap.writeback*) or a key (i.e. zswap_writeback_only) to memory.reclaim
> > > interface, I prefer the key option. I have not looked into how much proactively
> > > reclaiming zswap memory or proactively triggering zswap writeback makes sense
> > > but from the perspective of memcg interface, I think the key option would give a
> > > more clean solution if we decide in the future that this whole thing was a bad
> > > idea.
> > >
> > > Next regarding future proofing zswap writeback trigger, do we expect any
> > > potential additions/changes/new-features for this interface? For example do we
> > > expect in future we may want to trigger the zswap writeback only from a specific
> > > node or lowest memory tier?
> 
> The way I see it, zswap writeback is just a "special" type of
> proactive reclaim, but the goal is still proactively freeing cold
> memory. In that regard, I think it makes sense to have things like
> node-specific reclaim. Not sure about other extensions, but Hao
> initially suggested making this age-based, so I think the answer is
> yes.
> 
> For both of these examples (node-specific reclaim, age-based reclaim),
> I think the same semantics could apply to memory.reclaim in general,
> which is why I suggested making it a part of memory.reclaim. I also
> like the idea of having a single proactive reclaim interface in
> general, but maybe we don't want to overload it too much.

IMHO it is fine.

> 
> > Youngjun is working on swap tiers. At the moment he is more interested in
> > allowing a specific swap device to a memcg or not. I can imagine in future there
> > will be use-cases where there will be a need to demote data on higher tier swap
> > to lower tier swap. What would be the appropriate interface?
> >
> > BTW does zswap folks think of zswap as a top swap tier or something different?
> 
> I haven't been following the swap tiers work closely, but personally I
> do think of zswap as a top swap tier.

Same for me though I imagine swap tiers would introduce some duplication i.e.
different way (interface) to set limits for swap tiers for a given memcg.

> Things will probably get more
> blurry with memory tiers and compressed memory nodes though.

I think there will still be distinction between byte addressable and fault on
access devices.