mm/vma: refactor vmg_adjust_set_range() for clarity

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.

Link: https://lore.kernel.org/20260710-b4-pre-scalable-cow-v2-15-2a5aa403d977@kernel.org
Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: Ackerley Tng <ackerleytng@google.com>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Kai Huang <kai.huang@intel.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: SJ Park <sj@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Liam R. Howlett (Oracle) <liam@infradead.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 47 insertions, 4 deletions

diff --git a/mm/vma.c b/mm/vma.c
index 084afdf37385..7265a054cfa3 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;
 	}