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

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Message-ID: <CAC_TJvctkyGEKv8mVE83D2DzCd-HiXh9co_DuZfwuF86FJoiJw@mail.gmail.com>
Date: Wed, 17 Sep 2025 10:22:25 -0700
From: Kalesh Singh <kaleshsingh@...gle.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, minchan@...nel.org, lorenzo.stoakes@...cle.com, 
	Liam.Howlett@...cle.com, rppt@...nel.org, pfalcato@...e.de, 
	kernel-team@...roid.com, android-mm@...gle.com, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Kees Cook <kees@...nel.org>, Vlastimil Babka <vbabka@...e.cz>, Suren Baghdasaryan <surenb@...gle.com>, 
	Michal Hocko <mhocko@...e.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Ben Segall <bsegall@...gle.com>, 
	Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>, Jann Horn <jannh@...gle.com>, 
	Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
	linux-trace-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 6/7] mm: add assertion for VMA count limit

On Wed, Sep 17, 2025 at 6:44 AM David Hildenbrand <david@...hat.com> 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@...ux-foundation.org>
> > Cc: David Hildenbrand <david@...hat.com>
> > Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> > Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > Cc: Mike Rapoport <rppt@...nel.org>
> > Cc: Minchan Kim <minchan@...nel.org>
> > Cc: Pedro Falcato <pfalcato@...e.de>
> > Signed-off-by: Kalesh Singh <kaleshsingh@...gle.com>
> > ---
> >
> > 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
>