Re: [RFC PATCH v2 0/4] mm/swap: reserve swap areas for deliberate offload

[Youngjun Park]

On 2026-09-28 23:19, Matthias Goergens wrote:

Hi Matthias !

> > Just like we already have a "swappiness=" parameter in the reclaim
> > interface, perhaps adding a "swap.tier=" would be better and much
> > cleaner to achieve the same goal? Based on Youngjun's work here:
> > https://lore.kernel.org/all/20260916183437.2946306-1-youngjun.park@xxxxxxx/
>
> Thanks, I think that's the right base.  I tried it: on top of
> Youngjun's v11, a "swap.tier=" argument to memory.reclaim is about 40
> lines.  In a VM with zram at a higher priority than a disk swap
> device, and a cgroup whose tier mask allowed only the disk, ordinary
> reclaim in that cgroup stayed on the disk, while memory.reclaim with
> "swap.tier=" pointing at zram went to zram only.

There is an earlier attempt that may be worth a look.
https://lore.kernel.org/all/20260618044857.69439-1-jiahao.kernel@xxxxxxxxx/

If you go this way, the discussion in that thread worth refering too.

> What it doesn't cover yet is reclaim outside any configured cgroup:
> under global pressure, a cgroup nobody configured still reached the

Right, so for now every cgroup has to be set to exclude zram, unless
another idea or more code covers it.

> zram tier.  So for v3 I'd like to add a system-wide limit on which
> tiers pressure reclaim may use, on top of the per-cgroup settings.
> I'll wait for Youngjun's v12 and build on that.

I have looked through the whole series, and I will keep this use case
in mind while working on v12. :)

Also, to share what I had in mind, I was planning a sysfs interface
for each swap tier, /sys/kernel/mm/swap/tiers/<tier name> (exact
naming TBD). Maybe the system-wide limit could be handled there as one
of its use cases?

Let's discuss it in more detail after v12.

Thanks,
Youngjun