linux-kernel - Re: [PATCH 1/2] mm: memory-tiers, numa_emu: enable to create memory tiers using fake numa nodes

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Message-Id: <20251216122441.2810043bbc51621315e3862c@linux-foundation.org>
Date: Tue, 16 Dec 2025 12:24:41 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Akinobu Mita <akinobu.mita@...il.com>
Cc: linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, axelrasmussen@...gle.com, yuanchu@...gle.com,
 weixugc@...gle.com, hannes@...xchg.org, david@...nel.org,
 mhocko@...nel.org, zhengqi.arch@...edance.com, shakeel.butt@...ux.dev,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz,
 rppt@...nel.org, surenb@...gle.com
Subject: Re: [PATCH 1/2] mm: memory-tiers, numa_emu: enable to create memory
 tiers using fake numa nodes

On Mon,  8 Dec 2025 18:40:27 +0900 Akinobu Mita <akinobu.mita@...il.com> wrote:

> This makes it possible to create memory tiers using fake numa nodes
> generated by numa emulation.
> 
> The new "numa_emulation.adistance" kernel parameter allows you to set the
> abstract distance for each NUMA node.
> 
> For example, if the system is booted with the parameters
> "numa=fake=2 numa_emulation.adistance=576,704", it will configure memory
> tiers with node0 having the default DRAM adistance value and node1 having
> a lower adistance value.

Confusing.  I'd have thought that this commandline would gave node0 a
distance of 576 and node1 a distance of 704?  But the text talks about
some third "default" distance, of unknown value.

Can we please clear all this up?

Also, we have little documentation for this stuff. 
fake-numa-for-cpusets.rst and kernel-parameters.txt.  Can you please
find somewhere appropriate to document this new user-facing feature? 
Maybe a new Documentation file?