Re: [RFC PATCH v2 0/5] mm/swap, memcg: Introduce swap tiers for cgroup based swap control

[Nhat Pham <nphamcs@gmail.com>]

On Sun, Jan 25, 2026 at 10:53 PM Youngjun Park <youngjun.park@lge.com> wrote:
>
> This is the second version of the RFC for the "Swap Tiers" concept.
> Link to v1: https://lore.kernel.org/linux-mm/20251109124947.1101520-1-youngjun.park@lge.com/
>
> This version incorporates feedback received during LPC 2025 and addresses
> comments from the previous review. We have also included experimental
> results based on usage scenarios intended for our internal platforms.
>
> Motivation & Concept recap
> ==========================
> Current Linux swap allocation is global, limiting the ability to assign
> faster devices to specific cgroups. Our initial attempt at per-cgroup
> priorities proved over-engineered and caused LRU inversion.
>
> Following Chris Li's suggestion, we pivoted to "Swap Tiers." A tier is
> simply a user-named group of swap devices sharing the same priority range.
> This abstraction facilitates swap device selection based on speed, allowing
> users to configure specific tiers for cgroups.
>
> For more details, please refer to the LPC 2025 presentation
> https://lpc.events/event/19/contributions/2141/attachments/1857/3998/LPC2025Finalss.pdf
> or v1 patch.
>
> Changes in v2
> =============
> 1. Respect cgroup hierarchy principle (LPC 2025 feedback)
> - The logic now strictly follows standard cgroup hierarchy principles.
>
> Previous: Children could select any tier using "+" regardless of the
> parent's configuration. "+" tier is referenced. (could not be silently disappeared)
>
> Current: The explicit selection ("+") concept is removed. By
> default, all tiers are selected. Users now use "-" to exclude specific
> tiers. Excluded tier could disappeared silently.
> A child cgroup is always a subset of its parent. Even if a child
> re-enables a tier with "+" that was excluded by the parent, the effective
> tier list is limited to the parent's allowed subset.

This comment seems a bit clunky to me. The "+" is removed, as noted
above, but then why are we saying "even if a child re-enables a tier
with "+"" here? Am I missing something?

>
> Example:
> Global Tiers: SSD, HDD, NET
> Parent: SSD, NET (HDD excluded)
> Child: HDD, NET (SSD excluded)
> -> Effective Child Tier: NET (Intersection of Parent and Child)

But otherwise, I assume you mean to restrict child's allowed swap
tiers to be a subset of children and its ancestors? That seems more
straightforward to me than the last system :)