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

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Message-ID: <00bb2d76-4d52-40ec-ac75-546311b8ddcb@lucifer.local>
Date: Mon, 14 Jul 2025 16:27:48 +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 05:24:14PM +0200, David Hildenbrand wrote:
> On 14.07.25 17:18, Lorenzo Stoakes wrote:
> > 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...
>
> Yeah, not blaming you, just scratching my head :)
>
> >
> > 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.
>
>
> Likely a fix should be pulled in early? Not sure ... but it sure sounds
> broken.

Once this lands in mm-new I can send a fix (like litearlly tomorrow if it lands
:P). I just don't think a functional change belongs as part of a refactoring.

I worry that we'll get catch-22 caught on the (numerous) problems with the mseal
implementation and thus not provide a foundation upon which to fix them...

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