The NUMA interleave index was computed as two separate terms:

    *ilx += vma->vm_pgoff >> order;
    *ilx += (addr - vma->vm_start) >> (PAGE_SHIFT + order);

This has two problems:

1. When vm_start is not aligned to the folio size, the
   subtraction before the shift lets low bits affect the
   result via borrows.

2. For file-backed VMAs, shifting vm_pgoff and the VMA
   offset independently loses carries between them, giving
   wrong chunk indices when vm_pgoff is not aligned to order.

Combine into a single expression that adds vm_pgoff and
the page-granularity VMA offset first, then shifts once:

    *ilx += (vma->vm_pgoff +
            (addr >> PAGE_SHIFT) -
            (vma->vm_start >> PAGE_SHIFT)) >> order;

For anonymous VMAs, vm_pgoff equals vm_start >> PAGE_SHIFT,
so the vm_pgoff and vm_start terms cancel and the result
reduces to addr >> (PAGE_SHIFT + order), same as before.

For file-backed VMAs, the sum vm_pgoff + (addr >> PAGE_SHIFT)
- (vm_start >> PAGE_SHIFT) gives the file page offset of addr.
Shifting by order gives the correct file chunk index.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Assisted-by: Claude:claude-opus-4-6
Reviewed-by: Gregory Price <gourry@gourry.net>
---
 mm/mempolicy.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 4e4421b22b59..d139b074a599 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2048,8 +2048,9 @@ struct mempolicy *get_vma_policy(struct vm_area_struct *vma,
 		pol = get_task_policy(current);
 	if (pol->mode == MPOL_INTERLEAVE ||
 	    pol->mode == MPOL_WEIGHTED_INTERLEAVE) {
-		*ilx += vma->vm_pgoff >> order;
-		*ilx += (addr - vma->vm_start) >> (PAGE_SHIFT + order);
+		*ilx += (vma->vm_pgoff +
+			(addr >> PAGE_SHIFT) -
+			(vma->vm_start >> PAGE_SHIFT)) >> order;
 	}
 	return pol;
 }
-- 
MST