Re: [PATCH 2/8] mm/rmap: skip unfaulted VMAs on anon_vma clone, unlink

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Dec 19, 2025 at 10:28 AM Liam R. Howlett
<Liam.Howlett@oracle.com> wrote:
>
> * Lorenzo Stoakes <lorenzo.stoakes@oracle.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@oracle.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);

check_anon_vma_clone() is used only here and contains a couple of
warnings with "!src->anon_vma && ..." conditions, which now will never
be triggered even if the second part of those conditions was true.
This seems like a regression (you are not checking conditions you were
checking before this change). To avoid that, you can place this early
exit after the call to 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?
>
> >
> >       /*
> >        * Unlink each anon_vma chained to the VMA.  This list is ordered
> > @@ -465,15 +469,13 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
> >               list_del(&avc->same_vma);
> >               anon_vma_chain_free(avc);
> >       }
> > -     if (vma->anon_vma) {
> > -             vma->anon_vma->num_active_vmas--;
> >
> > -             /*
> > -              * vma would still be needed after unlink, and anon_vma will be prepared
> > -              * when handle fault.
> > -              */
> > -             vma->anon_vma = NULL;
> > -     }
> > +     vma->anon_vma->num_active_vmas--;
> > +     /*
> > +      * vma would still be needed after unlink, and anon_vma will be prepared
> > +      * when handle fault.
> > +      */
> > +     vma->anon_vma = NULL;
> >       unlock_anon_vma_root(root);
> >
> >       /*
> > --
> > 2.52.0
> >