linux-kernel - Re: [PATCH v1] kernel/fork: only call untrack_pfn

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Message-ID: <7aeaa30d-3fa6-4edf-82a0-e0c494ef3df8@lucifer.local>
Date: Thu, 24 Apr 2025 10:49:33 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
        Rik van Riel <riel@...riel.com>, "H. Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v1] kernel/fork: only call untrack_pfn_clear() on VMAs
 duplicated for fork()

On Thu, Apr 24, 2025 at 10:45:36AM +0200, David Hildenbrand wrote:
> > > Probably the right way of attaching such metadata to a VMA would be
> > > remembering it alongside the VMA in a very simple way.
> > >
> > > For example, when we perform a reservation we would allocate a refcounted
> > > object and assign it to the VMA (pointer, xarray, whatever).
> > >
> > > Duplicating the VMA would increase the refcount. Freeing a VMA would
> > > decrease the refcount.
> > >
> > > Once the refcount goes to zero, we undo the reservation and free the object.
> > >
> > > We would not adjust a reservation on partial VMA unmap (split + unmap A or
> > > B), but I strongly assume that would just be fine as long as we undo the
> > > reservation once the refcount goes to 0.
> >
> > Yeah this is a really good idea actually, almost kinda what refcounts are
> > for haha...
> >
> > The problem is we talk about this idly here, but neither of us wants to
> > actually write PAT code I'd say, so this may go nowhere. But maybe one of
> > us will get so frustrated that we do this anyway but still...
> >
> > Then again - actually, is this something you are planning to tackle?
>
> I hate this much with that much passion that I'll give it a try for a couple
> of hours, as it might fix the other issues we are seeing.  So far it looks
> like it cleans up stuff *beautifully*. Even VM_PAT can go ... :)

To quote a film, let the hate flow through you :P

I mean I am very familiar with this - vma merge, anon_vma, etc. there's a
pattern... ;)

>
> ... and I think we still have space in vm_area_struct without increasing it
> beyond 192 bytes.

Hm, so you're thinking of a general field in the VMA? I thought this would
belong to the PAT object somehow?

Though getting rid of VM_PAT would be fantastic...

I wonder if a _general_ VMA ref count would be a bit much just for this
case.

But maybe I misunderstand your approach :) Happy to obviously look and if
not like some crazy thing just for PAT (you can understand why I would not
like this) will be supportive :>)


>
> --
> Cheers,
>
> David / dhildenb
>

Cheers, Lorenzo