linux-kernel - Re: [PATCH v11 00/15] khugepaged: mTHP support

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Message-ID: <be3525a7-3d7c-4dd2-a961-e4d49d082d23@lucifer.local>
Date: Fri, 12 Sep 2025 16:45:27 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Pedro Falcato <pfalcato@...e.de>
Cc: David Hildenbrand <david@...hat.com>, Johannes Weiner <hannes@...xchg.org>,
        Kiryl Shutsemau <kas@...nel.org>, Nico Pache <npache@...hat.com>,
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Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support

On Fri, Sep 12, 2025 at 04:35:44PM +0100, Pedro Falcato wrote:
> On Fri, Sep 12, 2025 at 03:01:02PM +0100, Lorenzo Stoakes wrote:
> > Yes, but at least an eagerness parameter gets us closer to this ideal.
> >
> > Of course, I agree that max_ptes_none should simply never have been exposed like
> > this. It is emblematic of a 'just shove a parameter into a tunable/sysfs and let
> > the user decide' approach you see in the kernel sometimes.
> >
> > This is problmeatic as users have no earthly idea how to set the parameter (most
> > likely never touch it), and only start fiddling should issues arise and it looks
> > like a viable solution of some kind.
> >
> > The problem is users usually lack a great deal of context the kernel has, and
> > may make incorrect decisions that work in one situation but not another.
>
> Note that in this case we really don't have much for context. We can trivially do
> "check what number of ptes are mapped", but not anything much fancier. You can

I mean we could in theory change where we determine things, for instance doing
things in reclaim as Kiryl alluded to.

We _potentially_ have more to work with.

>
> The good news is that there are 3 or 4 separate movements for getting page
> "temperature" information with their own special infra and daemons, for their
> own special little features.

Right.

>
> >
> > TL;DR - this kind of interface is just lazy and we have to assess these kinds of
> > tunables based on the actual RoI + understanding from the user's perspective.
>
> Fully agreed.
>
> --
> Pedro

My overall point, FWIW, is that a synthetic heuristic tunable works better here
than one that maps on to an internal value that we then have no control over.

Or 'I agree with David' IOW :)

Cheers, Lorenzo