linux-kernel - Re: [PATCH 2/5] mm/mseal: move madvise() logic to mm/madvise.c

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Message-ID: <c57a0048-17dd-445b-8d92-4c8cc6f47d17@lucifer.local>
Date: Mon, 14 Jul 2025 16:18:40 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Jeff Xu <jeffxu@...omium.org>
Subject: Re: [PATCH 2/5] mm/mseal: move madvise() logic to mm/madvise.c

On Mon, Jul 14, 2025 at 05:03:03PM +0200, David Hildenbrand wrote:
> > > or sth like that would surely clean that up further.
> >
> > Well, I plan to make this not a thing soon so I'd rather not.
> >
> > The intent is to make _all_ VMA flags work on 32-bit kernels. I have done some
> > preparatory work and next cycle intend to do more on this.
> >
> > So I'd rather avoid any config changes on this until I've given this a shot.
>
> Sure, if that is in sight.

Yes :)

> > > > + * only do so via an appropriate madvise() call.
> > > > + */
> > > > +static bool can_madvise_modify(struct madvise_behavior *madv_behavior)
> > > > +{
> > > > +	struct vm_area_struct *vma = madv_behavior->vma;
> > > > +
> > > > +	/* If the operation won't discard, we're good. */
> > > > +	if (!is_discard(madv_behavior->behavior))
> > > > +		return true;
> > >
> > >
> > > Conceptually, I would do this first and then handle all the discard cases /
> > > exceptions.
> >
> > Hm I'm confused :P we do do this first? I think the idea with this is we can
> > very cheaply ignore any MADV_ that isn't applicable.
> >
> > Did you mean to put this comment under line below?
> >
> > I mean it's not exactly a perf hotspot so don't mind moving them around.
>
> I was thinking of this (start with sealed, then go into details about
> discards):
>
> /* If the VMA isn't sealed, we're all good. */
> if (can_modify_vma(vma))
> 	return true;
>
> /* In a sealed VMA, we only care about discard operations. */
> if (!is_discard(madv_behavior->behavior))
> 	return true;
>
> /* But discards of file-backed mappings are fine. */
> if (!vma_is_anonymous(vma))
> 	return true;

Right yeah.

>
> ...
>
>
> But now I wonder, why is it okay to discard anon pages in a MAP_PRIVATE file
> mapping?

I'm duplicating existing logic here (well updating from the vma->vm_file check
and a seemingly pointless !vma->vm_file && vma->vm_flags & VM_SHARED check), but
this is a good point...

For the purposes of the refactoring I guess best to keep the logic ostensibly
the same given the 'no functional change intended', but we do need to fix this
yes.

That change would probably be better as a follow-up with a test change added
too.

But I agree this is an oversight here afaict.

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