[PATCH v1 3/6] mm/migrate: don't call folio_putback_active_hugetlb() on dst hugetlb folio

[David Hildenbrand <david@redhat.com>]

We replaced a simple put_page() by a putback_active_hugepage() call in
commit 3aaa76e125c1 ("mm: migrate: hugetlb: putback destination hugepage
to active list"), to set the "active" flag on the dst hugetlb folio.

Nowadays, we decoupled the "active" list from the flag, by calling the
flag "migratable".

Calling "putback" on something that wasn't allocated is weird and not
future proof, especially if we might reach that path when migration failed
and we just want to free the freshly allocated hugetlb folio.

Let's simply set the "migratable" flag in move_hugetlb_state(), where we
know that allocation succeeded, and use simple folio_put() to return
our reference.

Do we need the hugetlb_lock for setting that flag? Staring at other
users of folio_set_hugetlb_migratable(), it does not look like it. After
all, the dst folio should already be on the active list, and we are not
modifying that list.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/hugetlb.c | 5 +++++
 mm/migrate.c | 8 ++++----
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index da98d671088d0..b24ccf8ecbf38 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -7529,6 +7529,11 @@ void move_hugetlb_state(struct folio *old_folio, struct folio *new_folio, int re
 		}
 		spin_unlock_irq(&hugetlb_lock);
 	}
+	/*
+	 * Our old folio is isolated and has "migratable" cleared until it
+	 * is putback. As migration succeeded, set the new folio "migratable".
+	 */
+	folio_set_hugetlb_migratable(new_folio);
 }
 
 static void hugetlb_unshare_pmds(struct vm_area_struct *vma,
diff --git a/mm/migrate.c b/mm/migrate.c
index 80887cadb2774..7e23e78f1e57b 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1542,14 +1542,14 @@ static int unmap_and_move_huge_page(new_folio_t get_new_folio,
 		list_move_tail(&src->lru, ret);
 
 	/*
-	 * If migration was not successful and there's a freeing callback, use
-	 * it.  Otherwise, put_page() will drop the reference grabbed during
-	 * isolation.
+	 * If migration was not successful and there's a freeing callback,
+	 * return the folio to that special allocator. Otherwise, simply drop
+	 * our additional reference.
 	 */
 	if (put_new_folio)
 		put_new_folio(dst, private);
 	else
-		folio_putback_active_hugetlb(dst);
+		folio_put(dst);
 
 	return rc;
 }
-- 
2.47.1