linux-kernel - Re: [PATCH 7/8] dax/kmem: add sysfs interface for runtime hotplug state control

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Message-ID: <684f11a3-1958-4562-880c-7e0afd3ae3b5@kernel.org>
Date: Wed, 28 Jan 2026 00:06:56 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Gregory Price <gourry@...rry.net>
Cc: linux-mm@...ck.org, linux-cxl@...r.kernel.org, nvdimm@...ts.linux.dev,
 linux-kernel@...r.kernel.org, virtualization@...ts.linux.dev,
 kernel-team@...a.com, dan.j.williams@...el.com, vishal.l.verma@...el.com,
 dave.jiang@...el.com, mst@...hat.com, jasowang@...hat.com,
 xuanzhuo@...ux.alibaba.com, eperezma@...hat.com, osalvador@...e.de,
 akpm@...ux-foundation.org
Subject: Re: [PATCH 7/8] dax/kmem: add sysfs interface for runtime hotplug
 state control

On 1/23/26 01:43, Gregory Price wrote:
> On Thu, Jan 22, 2026 at 11:49:48PM +0100, David Hildenbrand (Red Hat) wrote:
>>
>> I'm merely wondering why, in the new world, you would even want the offline
>> state.
>>
>> So what are the use cases for that?
>>
> 
> I don't have one, and in the 5-patch series I killed it.  You are right,
> it makes no sense.
> 
> However:
> 
>> Why would user space possibly want that? [plugged-in offline blocks]
>>
> 
> I don't think anyone does.
> 
> This is baggage.
> 
> The CXL driver auto-creates dax_kmem w/ offline memory blocks
> 
> Changing this behavior breaks existing systems :[
> 
>> Can't ndctl just use the old (existing) interface if such an operation is
>> requested, and the new one (you want to add) when we want to do something
>> reasonable (actually use system ram? :) ).
> 
> I think we're in agreement, I think I'm doing a poor job of explaining
> the interconnected issues.
> 
> summarizing the long email:
> 
>     cxl/region + dax/cxl.c + dax/bus.c auto-probe baggage for
>     BIOS-configured regions prevents any userland policy from
>     from being plumbed from cxl to dax.  There's no interposition step.
> 
> So yes - new interfaces would resolve this and the old interfaces
> could be left for compat.

Great!

-- 
Cheers

David