linux-kernel - Re: [PATCH 6/6] cxl/sysram: disallow onlining in ZONE

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Message-ID: <aWWAaDK91qAVLRLz@gourry-fedora-PF4VCD3F>
Date: Mon, 12 Jan 2026 18:14:48 -0500
From: Gregory Price <gourry@...rry.net>
To: "Cheatham, Benjamin" <benjamin.cheatham@....com>
Cc: linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-team@...a.com, dave@...olabs.net,
	jonathan.cameron@...wei.com, dave.jiang@...el.com,
	alison.schofield@...el.com, vishal.l.verma@...el.com,
	ira.weiny@...el.com, dan.j.williams@...el.com,
	David Hildenbrand <david@...nel.org>,
	Hannes Reinecke <hare@...e.de>
Subject: Re: [PATCH 6/6] cxl/sysram: disallow onlining in ZONE_NORMAL if
 state is movable only

On Mon, Jan 12, 2026 at 03:11:05PM -0600, Cheatham, Benjamin wrote:
> On 1/12/2026 10:35 AM, Gregory Price wrote:
> > If state is set to online (default to ZONE_MOVABLE), the user intends
> > for this memory to either refuse non-movable allocations, and/or intends
> > to preserve the hot-unpluggability of this memory.  However, any admin
> > can write `offline` and `online` to the memory block controller and
> > bring that memory online in ZONE_NORMAL.
> 
> Is it the expectation that the user will never want to change the zone from
> MOVABLE to NORMAL? I can't think of a reason someone would want to off the top
> of my head, but I also can't think of a reason to restrict it either.
>

It's more to restrict this pattern

echo online_movable > region0/hotplug
   -> creates:   node1/memory123/

echo offline > node1/memory123/state
echo online > node1/memory123/state

The result of this would be valid_zones=[normal movable], which would
break hot-unplug.

> > If an actor attempts to online the block into ZONE_NORMAL, it will fail,
> > but if it attempts to online into either NORMAL or MOVABLE, only MOVABLE
> > will be allowed and it will succeed.
> 
> I'm not sure you need this paragraph. I think it's a logical conclusion of the above
> that if someone attempts to online the memory as NORMAL or MOVABLE it'll only be onlined
> as MOVABLE.

in the above situation the following occurs:

echo online  > region0/hotplug
echo offline > node1/memory123/state
echo online  > node1/memory123/state
cat node1/memory123/valid_zones
   normal movable
echo offline > node1/memory123/state
echo 1  > node1/memory123/online
cat node1/memory123/valid_zones
   normal


echo online_movable  > region0/hotplug
echo offline > node1/memory123/state
echo online  > node1/memory123/state
cat node1/memory123/valid_zones
   movable
echo offline > node1/memory123/state
echo 1  > node1/memory123/online
   fail with EXXXX (i forget what code)

It's a little confusing.

> > +	switch (data->last_online_type) {
> > +	case MMOP_ONLINE_MOVABLE:
> > +		return sysfs_emit(buf, "online\n");
> > +	case MMOP_ONLINE_KERNEL:
> > +		return sysfs_emit(buf, "online_normal\n");
> > +	case MMOP_OFFLINE:
> > +	default:
> 
> You're missing the MMOP_ONLINE case. In that case the memory would be reported as "offline", which
> I doubt is the intention.
> 

Blah, i originally had all of them and just reduced to
MMOP_ONLINE_MOVABLE and MMOP_ONLINE (i don't see a good use for
MMOP_ONLINE_KERNEL), but i'll fix this up.

Thanks!
Gregory