linux-kernel - Re: [RFC] Disable auto_movable

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Message-ID: <a9e524af-baf4-4da9-938f-5da71cfbd769@redhat.com>
Date: Tue, 29 Jul 2025 11:29:28 +0200
From: David Hildenbrand <david@...hat.com>
To: Michal Hocko <mhocko@...e.com>, Hannes Reinecke <hare@...e.de>
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 29.07.25 11:19, Michal Hocko wrote:
> On Tue 29-07-25 09:24:37, Hannes Reinecke wrote:
>> On 7/28/25 15:08, David Hildenbrand wrote:
>>> On 28.07.25 15:06, Michal Hocko wrote:
>>>> On Mon 28-07-25 11:37:46, Hannes Reinecke wrote:
>>>>> On 7/28/25 11:10, David Hildenbrand wrote:
>>>>> And to make matters worse, we have two competing user-space programs:
>>>>> - udev
>>>>> - daxctl
>>>>> neither of which is (or can be made) aware of each other.
>>>>> This leads to races and/or inconsistencies.
>>>>
>>>> Would it help if generic udev memory hotplug rule exclude anything that
>>>> is dax backed? Is there a way to check for that? Sorry if this is a
>>>> stupid question.
>>> Parsing /proc/iomem, it's indicated as "System RAM (kmem)".
>>>
>> I would rather do it the other way round, and make daxctl aware of
>> udev. In the end, even 'daxctl' uses the sysfs interface to online
>> memory, which really is the territory of udev and can easily be
>> done via udev rules (for static configuration).
> 
> udev doesn't really have any context what user space wants to do with
> the memory and therefore how to online it. Therefore we have (arguably)
> ugly hacks like auto onlining and movable_ration etc. daxctl can take
> information from the admin directly and therfore it can do what is
> needed without further hacks.

Really the only difference between daxctl and everything else is the way 
the memory is added.

daxctl triggers hotplug of memory synchronously, everything else is 
asynchronous.

On most systems, the admin (the same one that triggers onlining) could 
just set the auto-onlining policy accordingly instead of manually 
onlining memory blocks from user space.

> 
>> Note, we do a similar thing on s/390; the configuration tool there
>> just spits out udev rules.
> 
> Those were easy times when you just need to online memory without any
> more requirements where it should land.

Again, I don't think udev is the future for that.

What I think we (Red Hat) want is a better and easier way to configure 
the kernel policy.

If you want to control onlining manually, then disable the auto-online 
policy.

-- 
Cheers,

David / dhildenb