Re: [PATCH v3 30/40] mm/madvise: update is_valid_guard_vma() to use vma_can_merge()

[David Hildenbrand (Arm)]

On 9/17/26 18:22, Lorenzo Stoakes (ARM) wrote:
> We currently disallow the installation of lightweight guard regions in VMAs
> whose flags intersect VMA_SPECIAL_FLAGS or VMA_HUGETLB_BIT, or
> VMA_LOCKED_BIT unless allow_locked is set.
> 
> hugetlb VMAs set VMA_DONTEXPAND_BIT so this was already redundant,
> VMA_SPECIAL_FLAGS already sufficed.
> 
> However, now that VMA_IO_BIT is only set if VMA_PFNMAP or VMA_MIXEDMAP_BIT
> is set, this check collapses to being the equivalent of
> !vma_can_merge().
> 
> Update is_valid_guard_vma() to reflect this.
> 
> No functional change intended.
> 
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> ---
>  mm/madvise.c | 20 +++++++++++++-------
>  1 file changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 467601a8525b..0922d5f07a12 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1221,19 +1221,25 @@ static long madvise_remove(struct madvise_behavior *madv_behavior)
>  	return error;
>  }
>  
> -static bool is_valid_guard_vma(struct vm_area_struct *vma, bool allow_locked)
> +static bool is_valid_guard_vma(const struct vm_area_struct *vma,
> +			       bool allow_locked)
>  {
> -	vm_flags_t disallowed = VM_SPECIAL | VM_HUGETLB;
> -
>  	/*
> -	 * A user could lock after setting a guard range but that's fine, as
> +	 * A user could lock after setting a guard range but that's fine as
>  	 * they'd not be able to fault in. The issue arises when we try to zap
>  	 * existing locked VMAs. We don't want to do that.
>  	 */
> -	if (!allow_locked)
> -		disallowed |= VM_LOCKED;
> +	if (!allow_locked && vma_test(vma, VMA_LOCKED_BIT))
> +		return false;
> +	/*
> +	 * Guard regions require a VMA whose page tables are managed solely by
> +	 * the core, which is also what merging requires, so disallow any flags
> +	 * that would prevent a merge.
> +	 */
> +	if (!vma_can_merge(vma))
> +		return false;

Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

-- 
Cheers,

David