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

[Lorenzo Stoakes]

On Thu, Jul 02, 2026 at 11:37:11AM +0100, Pedro Falcato wrote:
> 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>
> > ---
> >  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?

Yeah I'm intentionally trying to make that explicit as this code is very
confusing, so people don't have to think 'oh what was adjust again?'.

>
> > +		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 <pfalcato@xxxxxxx>

Thanks!

>
> --
> Pedro

Cheers, Lorenzo