[RFC PATCH v3 1/8] mm: multi-gen LRU: ignore non-leaf pmd_young for force_scan=true

[Yuanchu Xie]

When non-leaf pmd accessed bits are available, MGLRU page table walks
can clear the accessed bit and promptly ignore the accessed bit on the
pte because it's on a different node, so the walk does not update the
generation of said page. When the next scan comes around on the right
node, 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 measuring the
working set size of a node/memcg.

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 4f9c854ce6cc..1a7c7d537db6 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3522,7 +3522,7 @@ 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;
 
-- 
2.44.0.396.g6e790dbe36-goog