linux-kernel - Re: [RFC PATCH] memory,memory_hotplug: allow restricting memory blocks to zone movable

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Message-ID: <aV6DFJAg7yl3qXHG@gourry-fedora-PF4VCD3F>
Date: Wed, 7 Jan 2026 11:00:20 -0500
From: Gregory Price <gourry@...rry.net>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, kernel-team@...a.com,
	osalvador@...e.de, gregkh@...uxfoundation.org, rafael@...nel.org,
	dakr@...nel.org, akpm@...ux-foundation.org,
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz,
	rppt@...nel.org, surenb@...gle.com, hare@...e.de
Subject: Re: [RFC PATCH] memory,memory_hotplug: allow restricting memory
 blocks to zone movable

On Wed, Jan 07, 2026 at 04:09:34PM +0100, David Hildenbrand (Red Hat) wrote:
> > Sure, I wouldn't push for more complexity just for the sake of a
> > theoretical extensibility. And I have to admit I have't tried to a quick
> > PoC to see how complex this could grow. I was hoping this could get into
> > a simple mask for online types with default MMOP_ONLINE_KERNEL|MMOP_ONLINE_MOVABLE
> > and special cases just choosing one of the two and zone_for_pfn_range
> > checking for the compatibility with the requested online type. But I do
> > appreciate there might be some obstacles on the way to achieve that.
> 
> If we want to go down that path of failing onlining, we could likely do
> without any core-MM changes: dax can simply register a memory notifier and
> fail MEM_GOING_ONLINE of its memory with -EINVAL when it sees !ZONE_MOVABLE.
> 
> That works, because online_pages() does the move_pfn_range_to_zone() before
> calling MEM_GOING_ONLINE.
> 

This would be clean, and we could add a switch in dax-kmem for something
like hotunplug=true which limits zone eligibility to ZONE_MOVABLE.

I can look at this next week or so.

~Gregory