Message-ID: <616f97b7-24e0-4134-a08d-5abaf07a8b09@kernel.org>
Date: Tue, 6 Jan 2026 19:38:54 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Gregory Price <gourry@...rry.net>, hare@...e.de
Cc: 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,
 mhocko@...e.com
Subject: Re: [RFC PATCH] memory,memory_hotplug: allow restricting memory
 blocks to zone movable

On 1/6/26 19:06, Gregory Price wrote:
> On Tue, Jan 06, 2026 at 06:52:11PM +0100, David Hildenbrand (Red Hat) wrote:
>> On 1/6/26 17:58, Gregory Price wrote:
>>> On Tue, Jan 06, 2026 at 04:24:21PM +0100, David Hildenbrand (Red Hat) wrote:
>>>
>>> I'm not against this idea, but it also makes the sysfs a little more
>>> confusing (`echo online` now does different things based on prior
>>> state).
>>
>> Right, but only for the contig-zones policy.
>>
>> But maybe you really want the default for such memory to be "movable" even
>> when not onlined beforehand? So I am not sure if the description of the
>> problem here is accurate.
>>
>> Isn't one problem also udev racing with ndctl?
>>
> 
> Yeah there's a bunch of races, the specific ones mentioned by Hannes i
> need to go back and re-listen to the talk.
> 
>>> I preferred just failing if the block wasn't compatible with
>>> the zone (maybe making it more clear with a dmesg print?)
>>
>> The thing is that this block is compatible with the zone, no?
>>
>> In a system where you would never want to offline that memory, why should we
>> stop someone from onlining it to a kernel zone? I'm sure someone with a
>> weird use case will show up later that will complain about this.
>>
> 
> Presumably you wouldn't be setting the MHP flag that prevents the blocks
> from being onlined in a kernel zone then - in which case this all just
> works as intended today.
> 
>> But the patch is missing details on who would actually set MHP_MOVABLE_ONLY.
>> A user should be posted alongside the core change.
>>
> 
> This is fair and probably the obvious immediate user would be a dax
> device with some kind of `dax0.0/protect_unplug` feature set.
> (With a better name obviuosly).
> 
> I will defer to Hannes on his specific use case, but I could see the
> CXL-DCD (Dynamic Capacity) set wanting something like this.
> 
>>>
>>> Anyway, let me know what your preference is, happy to pivot however.
>>
>> Restricting memory to be movable-only to handle a user-space problem as
>> described here sounds like the wrong approach to me. You really want the
>> default of such memory to be "movable".
>>
>> Almost like an optimized "auto-movable" policy :)
>>
>> Or a new policy that will respect a provided default (MHP_DEFAULT_MOVABLE).
>>
> 
> Fair, I'll revist this once Hannes gets a chance to chime in.
> 
> This was effective at getting the discussion started though :P

Hehe, yes.

Another thing to look into would be to provide a way for ndctl to just 
add+online the memory in one shot, without having to go back to walking 
memory blocks to online them etc.

After all, ndctl knows exactly what it wants, as configured by user space.


Something like "dax0.0/online_mode" (or however we can make it clearer 
that this is for the system ram mode), which would default to "offline" 
(what we have right now).

When set to "online_movable", we'd online the memory to online_movable 
right during add_memory(). So no races with udev and no manual onlining 
necessary.

One could also envision a mechanism for ndctl to 
offline_and_remove_memory() memory, instead of manually offlining it, to 
then race with somebody else wanting to reonline it.

-- 
Cheers

David

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