[RESEND RFC PATCH v2 5/5] mm/damon/vaddr: implement DAMOS_SPLIT handler

[Lian Wang]

Implement the vaddr operations layer handler for DAMOS_SPLIT.
For each folio in the target region that is larger than the
scheme's target_order, split it via split_folio_to_order().

This supports both anonymous and file-backed (e.g. tmpfs/shmem)
folios, covering KVM guest memory backed by THP tmpfs.

Signed-off-by: Lian Wang <lianux.mm@xxxxxxxxx>
Signed-off-by: Lian Wang <lianux.wang@xxxxxxxxxxxxxxxxxx>
---
 mm/damon/vaddr.c | 80 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 80 insertions(+)

diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
index 98a87609376b..a177703f7e0a 100644
--- a/mm/damon/vaddr.c
+++ b/mm/damon/vaddr.c
@@ -944,6 +944,83 @@ static unsigned long damos_va_collapse(struct damon_target *target,
 	return applied;
 }
 
+static unsigned long damos_va_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);
+	if (end < addr)
+		goto out_mmput;
+
+	while (addr < end) {
+		unsigned long folio_sz;
+
+		if (addr + chunk_sz < addr)
+			break;
+
+		mmap_read_lock(mm);
+		vma = find_vma(mm, addr);
+
+		if (!vma || addr < vma->vm_start ||
+		    vma->vm_flags & (VM_HUGETLB | VM_MIXEDMAP))
+			goto unlock;
+
+		folio = folio_walk_start(&fw, vma, addr, 0);
+		if (!folio)
+			goto unlock;
+
+		folio_sz = PAGE_SIZE << folio_order(folio);
+
+		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 += folio_sz;
+
+			folio_unlock(folio);
+			folio_put(folio);
+			*sz_filter_passed += folio_sz;
+			addr += folio_sz;
+		} else {
+			folio_walk_end(&fw, vma);
+			*sz_filter_passed += chunk_sz;
+			addr += chunk_sz;
+		}
+		mmap_read_unlock(mm);
+		cond_resched();
+		continue;
+
+unlock:
+		*sz_filter_passed += chunk_sz;
+		addr += chunk_sz;
+		mmap_read_unlock(mm);
+		cond_resched();
+	}
+
+out_mmput:
+	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)
@@ -977,6 +1054,9 @@ 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_SPLIT:
+		return damos_va_split(t, r, scheme,
+					  sz_filter_passed);
 	default:
 		/*
 		 * DAMOS actions that are not yet supported by 'vaddr'.