[RFC PATCH 4/6] mm/mglru: report hot PUDs from the rmap feedback path

lru_gen_look_around() marks the PMD of a young PTE found during the
eviction rmap walk, feeding hot regions back to the aging walker.
With the PUD-level filter in place, that PMD marking alone is not
enough: the next aging walk would test the containing PUD first and
skip the whole 1GB subtree if the PUD is unmarked, never reaching the
PMD. Mark the covering PUD as well, so regions whose hotness is only
observed by eviction keep getting re-scanned by aging.

The PUD entry is re-derived from the mm page tables via pgd_offset()/
p4d_offset()/pud_offset(); it is only hashed, never dereferenced, and
the mmap lock held by the rmap walk keeps the table chain valid.

Signed-off-by: Baoquan He <[email protected]>
---
 mm/vmscan.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 74edfe2a747d..ca0f06641adc 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4416,8 +4416,14 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw, unsigned int nr)
 	lazy_mmu_mode_disable();
 
 	/* feedback from rmap walkers to page table walkers */
-	if (mm_state && suitable_to_scan(i, young))
+	if (mm_state && suitable_to_scan(i, young)) {
+		/* the PUD entry covering the young PTEs scanned above */
+		pud_t *pud_p = pud_offset(p4d_offset(pgd_offset(vma->vm_mm, pvmw->address),
+						     pvmw->address), pvmw->address);
+
 		update_bloom_filter(mm_state, max_seq, pvmw->pmd);
+		update_pud_bloom_filter(mm_state, max_seq, pud_p);
+	}
 
 	mem_cgroup_put(memcg);
 
-- 
2.54.0