linux-kernel - Re: [RFC] Disable auto_movable

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Message-ID: <aIiSEpQhWqPsvaST@tiehlicka>
Date: Tue, 29 Jul 2025 11:19:14 +0200
From: Michal Hocko <mhocko@...e.com>
To: Hannes Reinecke <hare@...e.de>
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 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.

> 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.
-- 
Michal Hocko
SUSE Labs