mm: multi-gen LRU: ignore non-leaf pmd_young for force_scan=true

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.

Link: https://lkml.kernel.org/r/20240813163759.742675-1-yuanchu@google.comSigned-off-by: Yuanchu Xie <yuanchu@google.com>
Acked-by: Yu Zhao <yuzhao@google.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Lance Yang <ioworker0@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmscan.c

@@ -3481,7 +3481,7 @@ 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;
 		}
@@ -3568,7 +3568,7 @@ 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;