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 occurrs 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 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 occurrs 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@xxxxxxxxxx>
---
mm/vmscan.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index d55e8d07ffc4..73f3718b33f7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3548,7 +3548,7 @@ 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;
--
2.45.1.467.gbab1589fc0-goog
