[PATCH 2/9] mm/damon/ops-common: align MMU notifier range when clearing PTE young bit

[SJ Park]

From: "Shubhang Kaushik (Ampere)" <sh@xxxxxxxxxx>

DAMON selects a byte-granular sampling address.  damon_ptep_mkold()
clears the young bit of the PTE covering that address, but passes the
unaligned range [addr, addr + PAGE_SIZE) to MMU notifiers.

For an unaligned address, that range extends into the following page.
KVM ages every guest page intersecting the notifier range, so it can age
a secondary mapping for the following page although DAMON cleared only
the sampled page's primary PTE.

Align the address before both the primary PTE operation and the
MMU notifier call so that they cover the same page.

Fixes: 3f49584b262c ("mm/damon: implement primitives for the virtual memory address spaces")
Signed-off-by: Shubhang Kaushik (Ampere) <sh@xxxxxxxxxx>
Reviewed-by: Aveline Noir <jm5905938@xxxxxxxxx>
Tested-by: Aveline Noir <jm5905938@xxxxxxxxx>
Reviewed-by: SJ Park <sj@xxxxxxxxxx>
Signed-off-by: SJ Park <sj@xxxxxxxxxx>
---
Changes from v1
- v1: https://lore.kernel.org/20261021-damon-align-pte-young-notifier-v1-1-d0365ac45b66@xxxxxxxxxx
- Collect R-b: from SJ.
- Collect R-b: and T-b: from Aveline.
- Update subject prefix.
- Rebase to latest mm-new.

 mm/damon/ops-common.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
index 2f3bf86a221be..f88c6fd189fca 100644
--- a/mm/damon/ops-common.c
+++ b/mm/damon/ops-common.c
@@ -64,6 +64,8 @@ void damon_ptep_mkold(pte_t *pte, struct vm_area_struct *vma, unsigned long addr
 	bool young = false;
 	unsigned long pfn;
 
+	addr = PAGE_ALIGN_DOWN(addr);
+
 	if (likely(pte_present(pteval)))
 		pfn = pte_pfn(pteval);
 	else
@@ -83,8 +85,7 @@ void damon_ptep_mkold(pte_t *pte, struct vm_area_struct *vma, unsigned long addr
 		 * Arch implementation of ptep_test_and_clear_young() may
 		 * require aligned @addr
 		 */
-		young |= ptep_test_and_clear_young(vma, PAGE_ALIGN_DOWN(addr),
-				pte);
+		young |= ptep_test_and_clear_young(vma, addr, pte);
 	young |= mmu_notifier_clear_young(vma->vm_mm, addr, addr + PAGE_SIZE);
 	if (young)
 		folio_set_young(folio);
-- 
2.47.3