[RFC PATCH 5/6] mm/damon/vaddr: implement DAMOS_MTHP_SPLIT handler

[Wang Lian]

Implement the DAMOS_MTHP_SPLIT action for vaddr-based DAMON operations.
Walk the region in PMD-sized aligned chunks, use folio_walk_start() to
locate THP folios, and call split_folio_to_order() when the folio order
exceeds the target_order.

Unlike COLLAPSE which is limited to anonymous memory via
collapse_huge_page(), split_folio_to_order() supports both anon and
shmem folios.  This is critical for tmpfs THP-backed KVM guest memory,
where cold and hot pages bundled together in a single PMD THP cause
DAMON to overestimate hot regions.

The handler holds mmap_read_lock per chunk for VMA lookup and
folio_walk_start(), then releases it before the next iteration.
split_folio_to_order() does not reacquire mmap locks internally,
so this pattern is safe.

Co-developed-by: Kunwu Chan <kunwu.chan@xxxxxxxxx>
Signed-off-by: Kunwu Chan <kunwu.chan@xxxxxxxxx>
Signed-off-by: Wang Lian <lianux.mm@xxxxxxxxx>
---
 mm/damon/vaddr.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)

diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
index 2a3757c13bf0..1957e390a277 100644
--- a/mm/damon/vaddr.c
+++ b/mm/damon/vaddr.c
@@ -934,6 +934,71 @@ static unsigned long damos_va_collapse(struct damon_target *target,
 	return applied;
 }
 
+static unsigned long damos_va_mthp_split(struct damon_target *target,
+		struct damon_region *r, struct damos *s,
+		unsigned long *sz_filter_passed)
+{
+	unsigned long addr, end, chunk_sz;
+	unsigned int target_order = s->target_order;
+	unsigned long applied = 0;
+	struct mm_struct *mm;
+	struct vm_area_struct *vma;
+	struct folio *folio;
+	struct folio_walk fw;
+
+	mm = damon_get_mm(target);
+	if (!mm)
+		return 0;
+
+	chunk_sz = PAGE_SIZE << HPAGE_PMD_ORDER;
+	addr = ALIGN_DOWN(r->ar.start, chunk_sz);
+	end = ALIGN(r->ar.end, chunk_sz);
+
+	while (addr < end) {
+		mmap_read_lock(mm);
+		vma = find_vma(mm, addr);
+		/*
+		 * split_folio_to_order() supports both anon and shmem
+		 * folios, so we accept any VMA that has a folio at @addr.
+		 * This covers important use cases like tmpfs THP-backed
+		 * KVM guest memory where cold and hot pages are bundled
+		 * together in a single PMD THP.
+		 */
+		if (!vma || addr < vma->vm_start)
+			goto unlock;
+
+		folio = folio_walk_start(&fw, vma, addr, 0);
+		if (!folio)
+			goto unlock;
+
+		if (folio_order(folio) > target_order) {
+			if (!folio_trylock(folio)) {
+				folio_walk_end(&fw, vma);
+				goto unlock;
+			}
+			folio_get(folio);
+			folio_walk_end(&fw, vma);
+
+			if (!split_folio_to_order(folio, target_order))
+				applied += chunk_sz;
+
+			folio_unlock(folio);
+			folio_put(folio);
+		} else {
+			folio_walk_end(&fw, vma);
+		}
+
+unlock:
+		*sz_filter_passed += chunk_sz;
+		addr += chunk_sz;
+		mmap_read_unlock(mm);
+		cond_resched();
+	}
+
+	mmput(mm);
+	return applied;
+}
+
 static unsigned long damon_va_apply_scheme(struct damon_ctx *ctx,
 		struct damon_target *t, struct damon_region *r,
 		struct damos *scheme, unsigned long *sz_filter_passed)
@@ -967,6 +1032,9 @@ static unsigned long damon_va_apply_scheme(struct damon_ctx *ctx,
 		return damos_va_migrate(t, r, scheme, sz_filter_passed);
 	case DAMOS_STAT:
 		return damos_va_stat(t, r, scheme, sz_filter_passed);
+	case DAMOS_MTHP_SPLIT:
+		return damos_va_mthp_split(t, r, scheme,
+					  sz_filter_passed);
 	default:
 		/*
 		 * DAMOS actions that are not yet supported by 'vaddr'.
-- 
2.50.1 (Apple Git-155)