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

[Shakeel Butt]

+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?

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?