Message-ID: <aPfWePBq8d3v979f@gourry-fedora-PF4VCD3F>
Date: Tue, 21 Oct 2025 14:52:40 -0400
From: Gregory Price <gourry@...rry.net>
To: Jonathan Cameron <jonathan.cameron@...wei.com>
Cc: Yiannis Nikolakopoulos <yiannis.nikolakop@...il.com>,
	Wei Xu <weixugc@...gle.com>, David Rientjes <rientjes@...gle.com>,
	Matthew Wilcox <willy@...radead.org>,
	Bharata B Rao <bharata@....com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, dave.hansen@...el.com, hannes@...xchg.org,
	mgorman@...hsingularity.net, mingo@...hat.com, peterz@...radead.org,
	raghavendra.kt@....com, riel@...riel.com, sj@...nel.org,
	ying.huang@...ux.alibaba.com, ziy@...dia.com, dave@...olabs.net,
	nifan.cxl@...il.com, xuezhengchu@...wei.com,
	akpm@...ux-foundation.org, david@...hat.com, byungchul@...com,
	kinseyho@...gle.com, joshua.hahnjy@...il.com, yuanchu@...gle.com,
	balbirs@...dia.com, alok.rathore@...sung.com, yiannis@...corp.com,
	Adam Manzanares <a.manzanares@...sung.com>
Subject: Re: [RFC PATCH v2 0/8] mm: Hot page tracking and promotion
 infrastructure

On Mon, Oct 20, 2025 at 03:05:26PM +0100, Jonathan Cameron wrote:
> > More generally could have a "Not-for-general-consumption bit" instead
> > of specifically a compressed bit.  Maybe both a "No-Consume" and a
> > "Special Node" bit would be useful separately.
> > 
> > Of course then platforms need to be made to understand all these:
> > 
> > "No-Consume" -> force EFI_MEMORY_SP or leave it reserved
> > "Special Node" -> allocate its own PXM / Provide discrete CFMWS
> > 
> > Naming obviously non-instructive here, may as well call them Nancy and
> > Bob bits.
> 
> For compression specifically I think there is value in making it
> explicitly compression because the host hardware might handle that
> differently. The other bits might be worth having as well
> though. SPM was all about 'you could' use it as normal memory but
> someone put it there for something else. This more a case of
> SPOM. Specific Purpose Only Memory - eats babies if you don't know
> the extra rules for each instance of that.
> 

This is a fair point.  Something like a SPOM bit that says some other
bit-field is valid and you get to add new extensions about how the
memory should be used?  :shrug: probably sufficiently extensible but
maybe never used for anything more than compression.

> Maybe the BIOS would have a look at devices and decide to enable a
> compressed memory CFMWS if it finds devices that need it and not do
> so otherwise, though not doing so breaks hotplug of compressed memory devices.
> 
> So my guess is either we need to fix Linux to allow splitting a fixed
> memory window up into multiple NUMA nodes, or platforms have to spin
> extra fixed memory windows (host side PA ranges with a NUMA node for each).
> 

I don't think splitting a CFMW into multiple nodes is feasible, but I
also haven't looked at that region of ACPI code since i finished the
docs.  I can look into that.

I would prefer the former, since this is already what's done for
hostbridge interleave vs non-interleave setups, where the host may
expose multiple CFMW for the same devices depending on how the OS.

> Which option depends a bit on whether we expect host hardware to either
> handle compressed differently from normal ram, or at least separate it
> for QoS reasons.
> 

There's only a handful of folks discussing this at the moment, but so
far we've all be consistent in our gut telling us it should be handled
differently for reliability reasons.  But also, more opinions always
welcome :]

~Gregory

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