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

[Shakeel Butt <shakeel.butt@linux.dev>]

Hi Youngjun,

On Mon, Jan 26, 2026 at 03:52:37PM +0900, Youngjun Park 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.
> 

One of the LPC feedback you missed is to not add memcg interface for
this functionality and explore BPF way instead.

We are normally very conservative to add new interfaces to cgroup.
However I am not even convinced that memcg interface is the right way to
expose this functionality. Swap is currently global and the idea to
limit or assign specific swap devices to specific cgroups makes sense
but that is the decision for the job orchestator or node controller.
Allowing workloads to pick and choose swap devices do not make sense to
me.

Shakeel