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

[Gregory Price]

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 <ljs@xxxxxxxxxx>

Reviewed-by: Gregory Price <gourry@xxxxxxxxxx>

> +		/*
> +		 * vmg->start    vmg->end
> +		 * |             |
> +		 * v    merge    v
> +		 * <------------->
> +		 *         delta
> +		 *        <------>
> +		 * |------|----------------|
> +		 * | prev |    middle      |
> +		 * |------|----------------|
> +		 *        ^
> +		 *        |
> +		 *        middle->vm_start
> +		 */

Even with these diagrams, it's a bit difficult to understand what the
actual intent/result of this chunk is (but that may be a limitation of
me not spending enough time reading the surrounding code, not a comment
of your work here).

~Gregory

> +		/*
> +		 *                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
> +		 */