Re: [PATCH v2 6/7] mm: add assertion for VMA count limit

[Kalesh Singh]

On Wed, Sep 17, 2025 at 6:44 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 15.09.25 18:36, Kalesh Singh wrote:
> > Building on the vma_count helpers, add a VM_WARN_ON_ONCE() to detect
> > cases where the VMA count exceeds the sysctl_max_map_count limit.
> >
> > This check will help catch future bugs or regressions where
> > the VMAs are allocated exceeding the limit.
> >
> > The warning is placed in the main vma_count_*() helpers, while the
> > internal *_nocheck variants bypass it. _nocheck helpers are used to
> > ensure that the assertion does not trigger a false positive in
> > the legitimate case of a temporary VMA increase past the limit
> > by a VMA split in munmap().
> >
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: David Hildenbrand <david@xxxxxxxxxx>
> > Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
> > Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > Cc: Mike Rapoport <rppt@xxxxxxxxxx>
> > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > Cc: Pedro Falcato <pfalcato@xxxxxxx>
> > Signed-off-by: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
> > ---
> >
> > Changes in v2:
> >    - Add assertions if exceeding max_vma_count limit, per Pedro
> >
> >   include/linux/mm.h               | 12 ++++++--
> >   mm/internal.h                    |  1 -
> >   mm/vma.c                         | 49 +++++++++++++++++++++++++-------
> >   tools/testing/vma/vma_internal.h |  7 ++++-
> >   4 files changed, 55 insertions(+), 14 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 8bad1454984c..3a3749d7015c 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -4219,19 +4219,27 @@ static inline bool snapshot_page_is_faithful(const struct page_snapshot *ps)
> >
> >   void snapshot_page(struct page_snapshot *ps, const struct page *page);
> >
> > +int vma_count_remaining(const struct mm_struct *mm);
> > +
> >   static inline void vma_count_init(struct mm_struct *mm)
> >   {
> >       ACCESS_PRIVATE(mm, __vma_count) = 0;
> >   }
> >
> > -static inline void vma_count_add(struct mm_struct *mm, int nr_vmas)
> > +static inline void __vma_count_add_nocheck(struct mm_struct *mm, int nr_vmas)
> >   {
> >       ACCESS_PRIVATE(mm, __vma_count) += nr_vmas;
> >   }
> >
> > +static inline void vma_count_add(struct mm_struct *mm, int nr_vmas)
> > +{
> > +     VM_WARN_ON_ONCE(!vma_count_remaining(mm));
>
> Can't that fire when changing the max count from user space at just the
> wrong time?

You are right: technically it's possible if it was raised between the
time of checking and when the new VMA is added.

>
> I assume we'll have to tolerated that and might just want to drop this
> patch from the series.
>

It is compiled out in !CONFIG_VM_DEBUG builds, would we still want to drop it?

Thanks,
Kalesh

> --
> Cheers
>
> David / dhildenb
>