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

[Lorenzo Stoakes]

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;
+		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;
 	}
-- 
2.54.0