Re: [PATCH v2 5/7] mm: harden vma_count against direct modification

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Mon, Sep 15, 2025 at 09:36:36AM -0700, Kalesh Singh wrote:
> To make VMA counting more robust, prevent direct modification of the
> mm->vma_count field. This is achieved by making the public-facing
> member const via a union and requiring all modifications to go
> through a new set of helper functions the operate on a private
> __vma_count.
>
> While there are no other invariants tied to vma_count currently, this
> structural change improves maintainability; as it creates a single,
> centralized point for any future logic, such as adding debug checks
> or updating related statistics (in subsequent patches).
>
> 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>

Hmmm I"m not sure about this one.

I think this is a 'we don't need it' situation, and it's making everything a bit
uglier.

I especially hate vma_count_add() and vma_count_sub(). You're essentially
overridding the whole concept in these cases to make stuff that's already in
place work in those cases

I don't think this really adds much honestly.

(You're also clearly missing cases as the kernel bot has found issues)

> ---
>  include/linux/mm.h               | 25 +++++++++++++++++++++++++
>  include/linux/mm_types.h         |  5 ++++-
>  kernel/fork.c                    |  2 +-
>  mm/mmap.c                        |  2 +-
>  mm/vma.c                         | 12 ++++++------
>  tools/testing/vma/vma.c          |  2 +-
>  tools/testing/vma/vma_internal.h | 30 +++++++++++++++++++++++++++++-
>  7 files changed, 67 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 138bab2988f8..8bad1454984c 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4219,4 +4219,29 @@ static inline bool snapshot_page_is_faithful(const struct page_snapshot *ps)
>
>  void snapshot_page(struct page_snapshot *ps, const struct page *page);
>
> +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)
> +{
> +	ACCESS_PRIVATE(mm, __vma_count) += nr_vmas;
> +}
> +
> +static inline void vma_count_sub(struct mm_struct *mm, int nr_vmas)
> +{
> +	vma_count_add(mm, -nr_vmas);
> +}
> +
> +static inline void vma_count_inc(struct mm_struct *mm)
> +{
> +	vma_count_add(mm, 1);
> +}
> +
> +static inline void vma_count_dec(struct mm_struct *mm)
> +{
> +	vma_count_sub(mm, 1);
> +}
> +
>  #endif /* _LINUX_MM_H */
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 4343be2f9e85..2ea8fc722aa2 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -1020,7 +1020,10 @@ struct mm_struct {
>  #ifdef CONFIG_MMU
>  		atomic_long_t pgtables_bytes;	/* size of all page tables */
>  #endif
> -		int vma_count;			/* number of VMAs */
> +		union {
> +			const int vma_count;		/* number of VMAs */
> +			int __private __vma_count;
> +		};
>
>  		spinlock_t page_table_lock; /* Protects page tables and some
>  					     * counters
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 8fcbbf947579..ea9eff416e51 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1037,7 +1037,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>  	mmap_init_lock(mm);
>  	INIT_LIST_HEAD(&mm->mmlist);
>  	mm_pgtables_bytes_init(mm);
> -	mm->vma_count = 0;
> +	vma_count_init(mm);
>  	mm->locked_vm = 0;
>  	atomic64_set(&mm->pinned_vm, 0);
>  	memset(&mm->rss_stat, 0, sizeof(mm->rss_stat));
> diff --git a/mm/mmap.c b/mm/mmap.c
> index c6769394a174..30ddd550197e 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1828,7 +1828,7 @@ __latent_entropy int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
>  		 */
>  		vma_iter_bulk_store(&vmi, tmp);
>
> -		mm->vma_count++;
> +		vma_count_inc(mm);
>
>  		if (tmp->vm_ops && tmp->vm_ops->open)
>  			tmp->vm_ops->open(tmp);
> diff --git a/mm/vma.c b/mm/vma.c
> index 64f4e7c867c3..0cd3cb472220 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -352,7 +352,7 @@ static void vma_complete(struct vma_prepare *vp, struct vma_iterator *vmi,
>  		 * (it may either follow vma or precede it).
>  		 */
>  		vma_iter_store_new(vmi, vp->insert);
> -		mm->vma_count++;
> +		vma_count_inc(mm);
>  	}
>
>  	if (vp->anon_vma) {
> @@ -383,7 +383,7 @@ static void vma_complete(struct vma_prepare *vp, struct vma_iterator *vmi,
>  		}
>  		if (vp->remove->anon_vma)
>  			anon_vma_merge(vp->vma, vp->remove);
> -		mm->vma_count--;
> +		vma_count_dec(mm);
>  		mpol_put(vma_policy(vp->remove));
>  		if (!vp->remove2)
>  			WARN_ON_ONCE(vp->vma->vm_end < vp->remove->vm_end);
> @@ -1266,7 +1266,7 @@ static void vms_complete_munmap_vmas(struct vma_munmap_struct *vms,
>  	struct mm_struct *mm;
>
>  	mm = current->mm;
> -	mm->vma_count -= vms->vma_count;
> +	vma_count_sub(mm, vms->vma_count);
>  	mm->locked_vm -= vms->locked_vm;
>  	if (vms->unlock)
>  		mmap_write_downgrade(mm);
> @@ -1795,7 +1795,7 @@ int vma_link(struct mm_struct *mm, struct vm_area_struct *vma)
>  	vma_start_write(vma);
>  	vma_iter_store_new(&vmi, vma);
>  	vma_link_file(vma);
> -	mm->vma_count++;
> +	vma_count_inc(mm);
>  	validate_mm(mm);
>  	return 0;
>  }
> @@ -2495,7 +2495,7 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap)
>  	/* Lock the VMA since it is modified after insertion into VMA tree */
>  	vma_start_write(vma);
>  	vma_iter_store_new(vmi, vma);
> -	map->mm->vma_count++;
> +	vma_count_inc(map->mm);
>  	vma_link_file(vma);
>
>  	/*
> @@ -2810,7 +2810,7 @@ int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	if (vma_iter_store_gfp(vmi, vma, GFP_KERNEL))
>  		goto mas_store_fail;
>
> -	mm->vma_count++;
> +	vma_count_inc(mm);
>  	validate_mm(mm);
>  out:
>  	perf_event_mmap(vma);
> diff --git a/tools/testing/vma/vma.c b/tools/testing/vma/vma.c
> index 69fa7d14a6c2..ee5a1e2365e0 100644
> --- a/tools/testing/vma/vma.c
> +++ b/tools/testing/vma/vma.c
> @@ -261,7 +261,7 @@ static int cleanup_mm(struct mm_struct *mm, struct vma_iterator *vmi)
>  	}
>
>  	mtree_destroy(&mm->mm_mt);
> -	mm->vma_count = 0;
> +	vma_count_init(mm);
>  	return count;
>  }
>
> diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
> index 15525b86145d..6e724ba1adf4 100644
> --- a/tools/testing/vma/vma_internal.h
> +++ b/tools/testing/vma/vma_internal.h
> @@ -251,7 +251,10 @@ struct mutex {};
>
>  struct mm_struct {
>  	struct maple_tree mm_mt;
> -	int vma_count;			/* number of VMAs */
> +	union {
> +		const int vma_count;		/* number of VMAs */
> +		int __vma_count;
> +	};
>  	unsigned long total_vm;	   /* Total pages mapped */
>  	unsigned long locked_vm;   /* Pages that have PG_mlocked set */
>  	unsigned long data_vm;	   /* VM_WRITE & ~VM_SHARED & ~VM_STACK */
> @@ -1526,4 +1529,29 @@ static int vma_count_remaining(const struct mm_struct *mm)
>  	return (max_count > vma_count) ? (max_count - vma_count) : 0;
>  }
>
> +static inline void vma_count_init(struct mm_struct *mm)
> +{
> +	mm->__vma_count = 0;
> +}
> +
> +static inline void vma_count_add(struct mm_struct *mm, int nr_vmas)
> +{
> +	mm->__vma_count += nr_vmas;
> +}
> +
> +static inline void vma_count_sub(struct mm_struct *mm, int nr_vmas)
> +{
> +	vma_count_add(mm, -nr_vmas);
> +}
> +
> +static inline void vma_count_inc(struct mm_struct *mm)
> +{
> +	vma_count_add(mm, 1);
> +}
> +
> +static inline void vma_count_dec(struct mm_struct *mm)
> +{
> +	vma_count_sub(mm, 1);
> +}
> +
>  #endif	/* __MM_VMA_INTERNAL_H */
> --
> 2.51.0.384.g4c02a37b29-goog
>