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

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Message-ID: <aV1nPG1BgBQDWkjE@tiehlicka>
Date: Tue, 6 Jan 2026 20:49:16 +0100
From: Michal Hocko <mhocko@...e.com>
To: Gregory Price <gourry@...rry.net>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	david@...hat.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 Tue 06-01-26 11:53:30, Gregory Price wrote:
> On Tue, Jan 06, 2026 at 04:05:48PM +0100, Michal Hocko wrote:
> > On Mon 05-01-26 15:36:11, Gregory Price wrote:
> > > It was reported (LPC 2025) that userland services which monitor memory
> > > blocks can cause hot-unplug to fail permanently.
> > >
> > > This can occur when drivers attempt to hot-remove memory in two phases
> > > (offline, remove), while a userland service detects the memory offline
> > > and re-onlines the memory into a zone which may prevent removal.
> > 
> > Are there more details about this?
> 
> The details are with Hannes, I was just recapping what was described in
> his devmem talk at LPC ("To online or not online").

I know of policies to online newly added memory blocks but I am not
aware of policies to re-online something that has been made offline.
 
> > That being said, rather than movable_only, should we have a mask of
> > online types supported for the mem block?
> > 
> 
> I briefly considered this.  I went with this for RFC-v1 since it's
> fairly simple and because movable is really the only zone with hotplug
> guarantees (any other zone makes no hotplug guarantees).
> 
> It's also significantly more complex of a change for questionable value,
> but if people see this as the way to go i'll happily pivot to that.

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.
-- 
Michal Hocko
SUSE Labs