[PATCH v3 2/3] mm/damon/ops-common: handle hugetlb folios in folio mkold/young rmap walkers

[SJ Park <sj@kernel.org>]

From: Krishna Iyer <kiyer@crusoe.ai>

damon_folio_mkold_one() and damon_folio_young_one() assume the folios
they walk are mapped by normal PTEs or THP PMDs.  When the folio is a
hugetlb folio, page_vma_mapped_walk() returns the huge PTE in pvmw.pte
with its page table lock held, but the walkers treat it as a normal
PTE: they read and age it with PAGE_SIZE-granularity helpers, which is
wrong for huge PTEs (up to PUD level), and notify secondary MMUs for
only PAGE_SIZE of the mapping.

Add hugetlb branches to both walkers.  The mkold walker reuses
damon_hugetlb_mkold(), which the virtual address space operations set
has been using for hugetlb aging: it clears the young bit of the huge
PTE via set_huge_pte_at() and calls mmu_notifier_clear_young() spanning
the whole huge page size.  The young walker gets an equivalent new
helper, damon_hugetlb_young(), which reads the huge PTE with
huge_ptep_get() and consults the page idle flag and
mmu_notifier_test_young() like the existing PTE branch.

Locking mirrors what page_vma_mapped_walk() provides: the huge PTE's
page table lock is held inside the walk, and for shared hugetlb
mappings (the only ones subject to huge PMD sharing), rmap_walk_file()
already holds i_mmap_rwsem, satisfying hugetlb_walk()'s locking
requirements.

This is currently dead code: both rmap walkers are only reachable
through damon_get_folio(), which rejects hugetlb folios since they are
not on the LRU lists.  A following commit will let the physical address
space monitoring primitives opt in to hugetlb folios.

Link: https://lore.kernel.org/20260902025700.17975-3-kiyer@crusoe.ai
Cc: Andrew Morton <akpm@linux-foundation.org>
Assisted-by: Claude:claude-fable-5
Signed-off-by: Krishna Iyer <kiyer@crusoe.ai>
Reviewed-by: SJ Park <sj@kernel.org>
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/ops-common.c | 61 +++++++++++++++++++++++++++++++++----------
 1 file changed, 47 insertions(+), 14 deletions(-)

diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
index 995cc1f3b9f32..349e1604cc1b1 100644
--- a/mm/damon/ops-common.c
+++ b/mm/damon/ops-common.c
@@ -205,10 +205,15 @@ static bool damon_folio_mkold_one(struct folio *folio,
 
 	while (page_vma_mapped_walk(&pvmw)) {
 		addr = pvmw.address;
-		if (pvmw.pte)
-			damon_ptep_mkold(pvmw.pte, vma, addr);
-		else
+		if (pvmw.pte) {
+			if (folio_test_hugetlb(folio))
+				damon_hugetlb_mkold(pvmw.pte, vma->vm_mm, vma,
+						addr);
+			else
+				damon_ptep_mkold(pvmw.pte, vma, addr);
+		} else {
 			damon_pmdp_mkold(pvmw.pmd, vma, addr);
+		}
 	}
 	return true;
 }
@@ -233,27 +238,55 @@ void damon_folio_mkold(struct folio *folio)
 
 }
 
+#ifdef CONFIG_HUGETLB_PAGE
+static bool damon_hugetlb_young(pte_t *pte, struct vm_area_struct *vma,
+		unsigned long addr, struct folio *folio)
+{
+	pte_t entry = huge_ptep_get(vma->vm_mm, addr, pte);
+
+	return (pte_present(entry) && pte_young(entry)) ||
+		!folio_test_idle(folio) ||
+		mmu_notifier_test_young(vma->vm_mm, addr);
+}
+#else
+static bool damon_hugetlb_young(pte_t *pte, struct vm_area_struct *vma,
+		unsigned long addr, struct folio *folio)
+{
+	return false;
+}
+#endif	/* CONFIG_HUGETLB_PAGE */
+
+static bool damon_pte_young(pte_t *pte, struct vm_area_struct *vma,
+		unsigned long addr, struct folio *folio)
+{
+	pte_t entry = ptep_get(pte);
+
+	/*
+	 * PFN swap PTEs, such as device-exclusive ones, that actually map
+	 * pages are "old" from a CPU perspective. The MMU notifier takes care
+	 * of any device aspects.
+	 */
+	return (pte_present(entry) && pte_young(entry)) ||
+		!folio_test_idle(folio) ||
+		mmu_notifier_test_young(vma->vm_mm, addr);
+}
+
 static bool damon_folio_young_one(struct folio *folio,
 		struct vm_area_struct *vma, unsigned long addr, void *arg)
 {
 	bool *accessed = arg;
 	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, addr, 0);
-	pte_t pte;
 
 	*accessed = false;
 	while (page_vma_mapped_walk(&pvmw)) {
 		addr = pvmw.address;
 		if (pvmw.pte) {
-			pte = ptep_get(pvmw.pte);
-
-			/*
-			 * PFN swap PTEs, such as device-exclusive ones, that
-			 * actually map pages are "old" from a CPU perspective.
-			 * The MMU notifier takes care of any device aspects.
-			 */
-			*accessed = (pte_present(pte) && pte_young(pte)) ||
-				!folio_test_idle(folio) ||
-				mmu_notifier_test_young(vma->vm_mm, addr);
+			if (folio_test_hugetlb(folio))
+				*accessed = damon_hugetlb_young(pvmw.pte, vma,
+						addr, folio);
+			else
+				*accessed = damon_pte_young(pvmw.pte, vma,
+						addr, folio);
 		} else {
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 			pmd_t pmd = pmdp_get(pvmw.pmd);
-- 
2.47.3