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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <39533aa8-ca78-41a8-b005-9202ce53e3ae@kernel.org>
Date: Wed, 7 Jan 2026 16:09:34 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Michal Hocko <mhocko@...e.com>, Gregory Price <gourry@...rry.net>
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,
 hare@...e.de
Subject: Re: [RFC PATCH] memory,memory_hotplug: allow restricting memory
 blocks to zone movable

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.
>   
>>> 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.

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.

-- 
Cheers

David