linux-kernel - Re: [PATCH 2/8] mm/rmap: skip unfaulted VMAs on anon

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Message-ID: <a078e419-9b22-42e9-b4df-654841550449@lucifer.local>
Date: Tue, 6 Jan 2026 13:42:33 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Shakeel Butt <shakeel.butt@...ux.dev>,
        David Hildenbrand <david@...nel.org>, Rik van Riel <riel@...riel.com>,
        Harry Yoo <harry.yoo@...cle.com>, Jann Horn <jannh@...gle.com>,
        Mike Rapoport <rppt@...nel.org>, Michal Hocko <mhocko@...e.com>,
        Pedro Falcato <pfalcato@...e.de>, Chris Li <chriscli@...gle.com>,
        Barry Song <v-songbaohua@...o.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/8] mm/rmap: skip unfaulted VMAs on anon_vma clone,
 unlink

On Tue, Jan 06, 2026 at 01:14:10PM +0000, Lorenzo Stoakes wrote:
> On Fri, Dec 19, 2025 at 01:28:03PM -0500, Liam R. Howlett wrote:
> > * Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [251217 07:27]:
> > > For both anon_vma_clone() and unlink_anon_vmas(), if the source VMA or the
> > > VMA to be linked are unfaulted (e.g. !vma->anon_vma), then the functions do
> > > nothing. Simply exit early in these cases.
> > >
> > > In the unlink_anon_vmas() case we can also remove a conditional that checks
> > > whether vma->anon_vma is set.
> > >
> > > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > > ---
> > >  mm/rmap.c | 20 +++++++++++---------
> > >  1 file changed, 11 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/mm/rmap.c b/mm/rmap.c
> > > index 0e34c0a69fbc..9332d1cbc643 100644
> > > --- a/mm/rmap.c
> > > +++ b/mm/rmap.c
> > > @@ -309,6 +309,9 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
> > >  	struct anon_vma_chain *avc, *pavc;
> > >  	struct anon_vma *root = NULL;
> > >
> > > +	if (!src->anon_vma)
> > > +		return 0;
> > > +
> > >  	check_anon_vma_clone(dst, src);
> > >
> > >  	list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
> > > @@ -441,7 +444,8 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
> > >  	mmap_assert_locked(vma->vm_mm);
> > >
> > >  	/* Unfaulted is a no-op. */
> > > -	VM_WARN_ON_ONCE(!vma->anon_vma && !list_empty(&vma->anon_vma_chain));
> > > +	if (!vma->anon_vma)
> > > +		return;
> >
> > I guess it doesn't matter because you just added the !list_empty()
> > check, but did you mean to drop that part?
>
> I did mean to. Really this doesn't happen in reality, the assert was more of a
> place holder I suppose.
>
> I don't think we should be falling over ourselves to assert impossible things,
> really the debug-only asserts are intended to essentially document what's going
> on.
>
> Anyway it's moot, as I've had to drop both the assert and the condition here
> sadly, because of the fact we (of course) use unlink_anon_vmas() to clean up
> incompletely set up anon_vma's on a destination VMA.
>
> When has doing things on incompletely setup up VMAs ever gone wrong :)
>
> As ever with anon_vma, there are always deeper depths of horror to find.
>

OK scratch that see 1/8 thread... I will add a specific partial cleanup
path to deal with the anon_vma_clone() horror show and then we get to keep
all this.

In which case I guess I'll reinstate the check just to be super careful :)