linux-kernel - Re: [RFC] Disable auto_movable

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Message-ID: <41526d97-b3bc-423b-87f4-7e0ec6cd8292@suse.de>
Date: Tue, 29 Jul 2025 11:33:58 +0200
From: Hannes Reinecke <hare@...e.de>
To: Michal Hocko <mhocko@...e.com>
Cc: David Hildenbrand <david@...hat.com>, 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 7/29/25 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.
> 
Huh?
I thought udev was _all_ about userspace preferences...
We can easily have udev rules onlining memory with whatever policy
the user want; the whole point of udev rules is that they are dynamic
and can include policy decisions.

>> 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.

Sorry, I don't get that.
udev rules can easily parse any user-space policy, and you can have a
policy as detailed as you want.
And each installation can have its own udev rules.
Why wouldn't that work?

(Excluding main memory, obviously. We need memory to execute userspace
processes after all).

I do think we're talking past each other...

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@...e.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich