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

[David Hildenbrand <david@redhat.com>]

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@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Pedro Falcato <pfalcato@suse.de>
> Signed-off-by: Kalesh Singh <kaleshsingh@google.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?

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

-- 
Cheers

David / dhildenb