Re: [PATCH v5 00/36] Private Memory NUMA Nodes

[Balbir Singh]

On 7/22/26 4:16 AM, Gregory Price wrote:
> On Tue, Jul 21, 2026 at 01:46:01PM +1000, Balbir Singh wrote:
>> On 7/21/26 5:33 AM, Gregory Price wrote:
>>>
>>> Patch Layout
>>> ============
>>> The series is broken into two sections:
>>>
>>> 1) N_MEMORY_PRIVATE Introduction.
>>>    Introduce the node state.
>>>    Opt those nodes out of mm/ services.
>>>
>>> 2) NODE_PRIVATE_CAP_* features
>>>    A set of mm/ service opt-in flags that augment private
>>>    nodes to make them more useful (i.e. reclaim = overcommit).
>>>
>>>    NODE_PRIVATE_CAP_LTPIN for private node folio pinning
>>>    NODE_PRIVATE_CAP_NUMA_BALANCING for private-node NUMA balancing
>>>    NODE_PRIVATE_CAP_DEMOTION for private-node tiering demotion
>>>    NODE_PRIVATE_CAP_HOTUNPLUG for opted-in private nodes
>>>    NODE_PRIVATE_CAP_USER_NUMA for userland numa controls
>>>    NODE_PRIVATE_CAP_RECLAIM for opted-in private node reclaim
>>>
>>
>> Looks reasonable, I wonder why USER_NUMA/HOTUNPUG is an opt-in?
>>
> 
> Ideologically: Private nodes default to fully isolated, why should any
> given feature be special?
> 
> Concretely: 
> 
> User-numa
> 
>    Some devices don't want the user to have control over placement.
>    I have been working on compressed memory, for example, which only
>    ever wants to be used as a reclaim-demotion target.
>    
>    (The reasoning for this is another thread, i plan on publishing
>     my research on this this year)
> 

I don't fully understand, how do we allocate memory on these devices then? Is
the driver expected to allocate and map via vm_insert_page()?

> Hot-unplug:
> 
>    Some devices can't necessarily handle unexpected migration, and
>    hot-unplug is fundamentally a migration.  So the HOTUNPLUG cap
>    actually means "hot-unplug can execute migrations".
> 
>    If the entire device has pre-drained the memory (all memory is free)
>    then unplug works - it's just not very hot (no migrations) :]
> 
>    Maybe a naming issue?
> 

Yes, I would prefer MIGRATION in the name, HOTUNPLUG made me wonder how these
devices come online and because it is a device, it can go offline while the
system is still online

> ~Gregory
> 

Balbir