Re: [PATCH 13/30] mm/vma: refactor vmg_adjust_set_range() for clarity

On Mon, Jun 29, 2026 at 01:23:24PM +0100, Lorenzo Stoakes wrote:
> Add comments with ASCII diagrams to describe what we're doing, avoid
> dubious use of PHYS_PFN(), and use vma_start_pgoff().
> 
> The most complicated scenario represented here is vmg->__adjust_next_start
> - when this is set, vmg->[start, end] actually indicate the range to be
> retained, so take special care to describe this accurately.
> 
> No functional change intended.
> 
> Signed-off-by: Lorenzo Stoakes <[email protected]>
> ---
>  mm/vma.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 47 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/vma.c b/mm/vma.c
> index 6296acecf3b7..1e99fe8aa6ef 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -704,11 +704,54 @@ static void vmg_adjust_set_range(struct vma_merge_struct *vmg)
>  	pgoff_t pgoff;
>  
>  	if (vmg->__adjust_middle_start) {
> -		adjust = vmg->middle;
> -		pgoff = adjust->vm_pgoff + PHYS_PFN(vmg->end - adjust->vm_start);
> +		/*
> +		 * vmg->start    vmg->end
> +		 * |             |
> +		 * v    merge    v
> +		 * <------------->
> +		 *         delta
> +		 *        <------>
> +		 * |------|----------------|
> +		 * | prev |    middle      |
> +		 * |------|----------------|
> +		 *        ^
> +		 *        |
> +		 *        middle->vm_start
> +		 */
> +		struct vm_area_struct *middle = vmg->middle;

FWIW this can be simplified to
		adjust = middle;
		const unsigned long delta = vmg->end - adjust->vm_start;

But I guess you're looking for explicitness here?

> +		const unsigned long delta = vmg->end - middle->vm_start;
> +
> +		pgoff = vma_start_pgoff(middle) + (delta >> PAGE_SHIFT);
> +		adjust = middle;
>  	} else if (vmg->__adjust_next_start) {
> -		adjust = vmg->next;
> -		pgoff = adjust->vm_pgoff - PHYS_PFN(adjust->vm_start - vmg->end);
> +		/*
> +		 *                Originally:
> +		 *
> +		 *            vmg->start   vmg->end
> +		 *            |            |
> +		 *            v    merge   v
> +		 *            <------------>
> +		 *            .            .
> +		 * merge_existing_range() updates to:
> +		 *            .            .
> +		 * vmg->start vmg->end     .
> +		 * |          |            .
> +		 * v  retain  v            .
> +		 * <---------->            .
> +		 *             delta       .
> +		 *            <----->      .
> +		 * |----------------|------|
> +		 * |    middle      | next |
> +		 * |----------------|------|
> +		 *                  ^
> +		 *                  |
> +		 *                  next->vm_start
> +		 */
> +		struct vm_area_struct *next = vmg->next;
> +		const unsigned long delta = next->vm_start - vmg->end;
> +
> +		pgoff = vma_start_pgoff(next) - (delta >> PAGE_SHIFT);
> +		adjust = next;
>  	} else {
>  		return;
>  	}

Reviewed-by: Pedro Falcato <[email protected]>

-- 
Pedro