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

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Message-ID: <aV6VKo5gRphgFQc-@tiehlicka>
Date: Wed, 7 Jan 2026 18:17:30 +0100
From: Michal Hocko <mhocko@...e.com>
To: Hannes Reinecke <hare@...e.de>
Cc: Gregory Price <gourry@...rry.net>, 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
Subject: Re: [RFC PATCH] memory,memory_hotplug: allow restricting memory
 blocks to zone movable

On Wed 07-01-26 13:47:41, Hannes Reinecke wrote:
> On 1/6/26 20:49, Michal Hocko wrote:
> > 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.
> It's not a policy per-se, but rather a udev rule (which one could
> argue _is_ a policy, mind). There is a rather long-running SLES bug
> around this if you are interested...

I am aware of udev rules which automatically online memory that is hot
added. But I am not aware of any rules to re-online memory that has been
offlined. The former makes some sense while the latter makes very little
sense to me.

-- 
Michal Hocko
SUSE Labs