linux-kernel - Re: [RFC] Disable auto_movable

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Message-ID: <a06922c2-9dad-4449-991c-913fa7765bc2@redhat.com>
Date: Mon, 28 Jul 2025 14:27:29 +0200
From: David Hildenbrand <david@...hat.com>
To: Michal Hocko <mhocko@...e.com>
Cc: Oscar Salvador <osalvador@...e.de>, 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 14:27, David Hildenbrand wrote:
> On 28.07.25 14:17, Michal Hocko wrote:
>> On Mon 28-07-25 11:10:44, David Hildenbrand wrote:
>>> On 28.07.25 11:04, Michal Hocko wrote:
>>>> On Mon 28-07-25 10:53:08, David Hildenbrand wrote:
>> [...]
>>>>> 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".
>>>>
>>>> ... this is the result. Those shouldn't really race. If udev is suppose
>>>> to see the device then only in its entirity so regular memory block
>>>> based onlining rules shouldn't even see that memory. Or am I completely
>>>> missing the picture?
>>>
>>> We can't break user space, which relies on individual memory blocks.
>>
>> We do have userspace which onlines specific memory blocks and we cannot
>> break that. But do we have any userspace that wants to online CXL like
>> memory (or in general dax like memory) that would need to operate on
>> those memory blocks with that kind of granularity?
> 
> I'm afraid that ship has sailed.
> 
>>
>> In other words what would break if we didn't expose CXL memory through
>> memory blocks in sysfs?
> 
> I think the whole libdaxctl handling for onlining memory is based on that.
> 

Sorry, forgot to add a pointer:

https://github.com/pmem/ndctl/blob/main/daxctl/lib/libdaxctl.c

-- 
Cheers,

David / dhildenb