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

[Michal Hocko <mhocko@suse.com>]

On Wed 07-01-26 16:09:34, David Hildenbrand 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.
> > > > 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.

Yes, that makes sense as well and it seems rather elegand way to go
about that.

-- 
Michal Hocko
SUSE Labs