When non-leaf pmd accessed bits are available, MGLRU page table walks
can clear the non-leaf pmd accessed bit and ignore the accessed bit on
the pte if it's on a different node, skipping a generation update as
well. If another scan occurs on the same node as said skipped pte.
the non-leaf pmd accessed bit might remain cleared and the pte accessed
bits won't be checked. While this is sufficient for reclaim-driven
aging, where the goal is to select a reasonably cold page, the access
can be missed when aging proactively for workingset estimation of a
node/memcg.

In more detail, get_pfn_folio returns NULL if the folio's nid != node
under scanning, so the page table walk skips processing of said pte. Now
the pmd_young flag on this pmd is cleared, and if none of the pte's are
accessed before another scan occurs on the folio's node, the pmd_young
check fails and the pte accessed bit is skipped.

Since force_scan disables various other optimizations, we check
force_scan to ignore the non-leaf pmd accessed bit.

Signed-off-by: Yuanchu Xie <yuanchu@google.com>
---
 mm/vmscan.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index cfa839284b92..4a112c2d1a64 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3476,7 +3476,7 @@ static void walk_pmd_range_locked(pud_t *pud, unsigned long addr, struct vm_area
 			goto next;
 
 		if (!pmd_trans_huge(pmd[i])) {
-			if (should_clear_pmd_young())
+			if (!walk->force_scan && should_clear_pmd_young())
 				pmdp_test_and_clear_young(vma, addr, pmd + i);
 			goto next;
 		}
@@ -3563,7 +3563,7 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end,
 
 		walk->mm_stats[MM_NONLEAF_TOTAL]++;
 
-		if (should_clear_pmd_young()) {
+		if (!walk->force_scan && should_clear_pmd_young()) {
 			if (!pmd_young(val))
 				continue;
 
-- 
2.46.0.76.ge559c4bf1a-goog

On Tue, Aug 13, 2024 at 10:38 AM Yuanchu Xie <yuanchu@google.com> wrote:
>
> When non-leaf pmd accessed bits are available, MGLRU page table walks
> can clear the non-leaf pmd accessed bit and ignore the accessed bit on
> the pte if it's on a different node, skipping a generation update as
> well. If another scan occurs on the same node as said skipped pte.
> the non-leaf pmd accessed bit might remain cleared and the pte accessed
> bits won't be checked. While this is sufficient for reclaim-driven
> aging, where the goal is to select a reasonably cold page, the access
> can be missed when aging proactively for workingset estimation of a
> node/memcg.
>
> In more detail, get_pfn_folio returns NULL if the folio's nid != node
> under scanning, so the page table walk skips processing of said pte. Now
> the pmd_young flag on this pmd is cleared, and if none of the pte's are
> accessed before another scan occurs on the folio's node, the pmd_young
> check fails and the pte accessed bit is skipped.
>
> Since force_scan disables various other optimizations, we check
> force_scan to ignore the non-leaf pmd accessed bit.
>
> Signed-off-by: Yuanchu Xie <yuanchu@google.com>

Acked-by: Yu Zhao <yuzhao@google.com>