Re: [PATCH 12/30] mm/vma: clean up anon_vma_compatible()

[Pedro Falcato]

On Mon, Jun 29, 2026 at 01:23:23PM +0100, Lorenzo Stoakes wrote:
> Break up the existing very large conditional, add comments and use
> vma_[start/end]_pgoff() to make clearer what we're doing here.
> 
> No functional change intended.
> 
> Signed-off-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> ---
>  mm/vma.c | 21 ++++++++++++++++-----
>  1 file changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/vma.c b/mm/vma.c
> index b60375c6c5c3..6296acecf3b7 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -1967,14 +1967,25 @@ static int anon_vma_compatible(struct vm_area_struct *a, struct vm_area_struct *
>  {
>  	vma_flags_t diff = vma_flags_diff_pair(&a->flags, &b->flags);
>  
> +	/* Ignore flags that mprotect() can change. */
>  	vma_flags_clear_mask(&diff, VMA_ACCESS_FLAGS);
> +	/* Ignore flags that do not impact merging. */
>  	vma_flags_clear_mask(&diff, VMA_IGNORE_MERGE_FLAGS);
>  
> -	return a->vm_end == b->vm_start &&
> -		mpol_equal(vma_policy(a), vma_policy(b)) &&
> -		a->vm_file == b->vm_file &&
> -		vma_flags_empty(&diff) &&
> -		b->vm_pgoff == a->vm_pgoff + ((b->vm_start - a->vm_start) >> PAGE_SHIFT);
> +	/* Must be adjacent. */
> +	if (a->vm_end != b->vm_start)
> +		return false;
> +	/* Must have matching policy. */
> +	if (!mpol_equal(vma_policy(a), vma_policy(b)))
> +		return false;
> +	/* Must both be anon or map the same file (MAP_PRIVATE case). */
> +	if (a->vm_file != b->vm_file)
> +		return false;
> +	/* Flags must be equivalent modulo mprotect(). */
> +	if (!vma_flags_empty(&diff))
> +		return false;
> +	/* Page offset must align. */
> +	return vma_end_pgoff(a) == vma_start_pgoff(b);

Very nice.

Reviewed-by: Pedro Falcato <pfalcato@xxxxxxx>

-- 
Pedro