linux-kernel - Re: [RFC] Disable auto_movable

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Message-ID: <2f24e725-cddb-41c5-ba87-783930efb2aa@redhat.com>
Date: Mon, 28 Jul 2025 10:53:08 +0200
From: David Hildenbrand <david@...hat.com>
To: Michal Hocko <mhocko@...e.com>, Oscar Salvador <osalvador@...e.de>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Hannes Reinecke <hare@...nel.org>
Subject: Re: [RFC] Disable auto_movable_ratio for selfhosted memmap

On 28.07.25 10:48, Michal Hocko wrote:
> On Mon 28-07-25 10:15:47, Oscar Salvador wrote:
>> Hi,
>>
>> Currently, we have several mechanisms to pick a zone for the new memory we are
>> onlining.
>> Eventually, we will land on zone_for_pfn_range() which will pick the zone.
>>
>> Two of these mechanisms are 'movable_node' and 'auto-movable' policy.
>> The former will put every single hotpluggled memory in ZONE_MOVABLE
>> (unless we can keep zones contiguous by not doing so), while the latter
>> will put it in ZONA_MOVABLE IFF we are within the established ratio
>> MOVABLE:KERNEL.
>>
>> It seems, the later doesn't play well with CXL memory where CXL cards hold really
>> large amounts of memory, making the ratio fail, and since CXL cards must be removed
>> as a unit, it can't be done if any memory block fell within
>> !ZONE_MOVABLE zone.
> 
> I suspect this is just an example of how our existing memory hotplug
> interface based on memory blocks is just suoptimal and it doesn't fit
> new usecases. We should start thinking about how a new v2 api should
> look like. I am not sure how that should look like but I believe we
> should be able to express a "device" as whole rather than having a very
> loosely bound generic memblocks. Anyway this is likely for a longer
> discussion and a long term plan rather than addressing this particular
> issue.

We have that concept with memory groups in the kernel already.

In dax/kmem we register a static memory group. It will be considered one 
union.

>   
>> One way to tackle this would be update the ratio every time a new CXL
>> card gets inserted, but this seems suboptimal.
> 
> I do not think this is a usable interface.
> 
>> Another way is that since CXL memory works with selfhosted memmap, we could relax
>> the check when 'auto-movable' and only look at the ratio if we aren't
>> working with selfhosted memmap.
> 
> This is likely the only choice we have with the current interface. We
> either need a way to disable the ratio altogether or make it more
> automagic and treat self hosted memory differently because that memory
> doesn't eat up ZONE_NORMAL memory and therefore cannot deplete it for
> ZONE_MOVABLE.

daxctl wants to online memory itself. We want to keep that memory 
offline from a kernel perspective and let daxctl handle it in this case.

We have that problem in RHEL where we currently require user space to 
disable udev rules so daxctl "can win".

-- 
Cheers,

David / dhildenb