Re: [RFC PATCH v3 7/8] mm/zswap: compressed ram direct integration

[Nhat Pham <nphamcs@gmail.com>]

On Tue, Jan 13, 2026 at 4:35 PM Nhat Pham <nphamcs@gmail.com> wrote:
>
> > This part needs more thought. Zswap cannot charge a full page because
> > then from the memcg perspective reclaim is not making any progress.
> > OTOH, as you mention, from the system perspective we just consumed a
> > full page, so not charging that would be inconsistent.
> >
> > This is not a zswap-specific thing though, even with cram.c we have to
> > figure out how to charge memory on the compressed node to the memcg.
> > It's perhaps not as much of a problem as with zswap because we are not
> > dealing with reclaim not making progress.
> >
> > Maybe the memcg limits need to be "enlightened" about different tiers?
> > We did have such discussions in the past outside the context of
> > compressed memory, for memory tiering in general.
>
> What if we add a reclaim flag that says "hey, we are hitting actual
> memory limit and need to make memory reclaim forward progress".
>
> Then, we can have zswap skip compressed cxl backend and fall back to
> real compression.
>
> (Maybe also demotion, which only move memory from one node to another,
> as well as the new cram.c stuff? This will technically also save some
> wasted work, as in the status quo we will need to do a demotion pass
> first, before having to reclaiom memory from the bottom tier anyway?
> But not sure if we want this).

Some more thoughts - right now demotion is kinda similar, right? We
move pages from one node (fast tier) to another (slow tier). This
frees up space in the fast tier, but it actually doesn't change the
memcg memory usage. So we are not making "forward progress" with this
either.

I suppose this is fine-ish, because reclaim subsystem can then proceed
by reclaiming from the bottom tier, which will now go to disk swap,
zswap, etc.

Can we achieve the same effect by making pages in
zswap-backed-by-compressed-cxl reclaimable:

1. Recompression - take them off compressed cxl and store them in
zswap proper (i.e in-memory compression).

2. Just enable zswap shrinker and have memory reclaim move these pages
into disk swap. This will have a much more drastic performance
implications though :)