Re: [RFC PATCH 6/6] mm/memcontrol: Make memory.high tier-aware

[Gregory Price <gourry@gourry.net>]

On Tue, Mar 24, 2026 at 04:21:06PM +0530, Donet Tom wrote:
> 
> IIUC The intent of this patch is to partition cgroup memory such that
> 0 → toptier_high is backed by higher-tier memory, and
> toptier_high → max is backed by lower-tier memory.
> 
> Based on this:
> 
> 1.If top-tier usage exceeds toptier_high, pages should be
>   demoted to the lower tier.
> 
> 2. If lower-tier usage exceeds (max - toptier_high), pages
>   should be swapped out.
> 

This is not accurate and an incorrect heuristic.

Transiently, lower-tier usage may exceed (max - toptier_high) for any
number of reasons which should not be used as signal for pushing swap.

driving swap usage is a function of (usage > memory.high) without regard
for toptier / lowtier.

> 3. If total memory usage exceeds max, demotion should be
>   avoided and reclaim should directly swap out pages.
> 

This is also incorrect, as it would drive agingin inversions.
Demotion is a natural extension of the LRU infrastructure:

toptier active -> toptier inactive -> lowtier inactive -> swap

if you do (toptier inactive -> swap) you have inverted the LRU.

As far as I know, from testing, we retain all the existing behavior - we
are just managing a limited resource (top tier memory) to manage the
noisy-neighbor issue.  So...


> Should we also handle cases (2) and (3) in this patch?

No, I don't think we should

~Gregory