linux-kernel - Re: [RFC] Disable auto_movable

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Message-ID: <1f8d924d-3554-43a6-a75e-66a08d1ce7b9@redhat.com>
Date: Mon, 28 Jul 2025 14:27:01 +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 14:17, 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.

-- 
Cheers,

David / dhildenb