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

[Shakeel Butt]

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?