mm/migrate: don't call folio_putback_active_hugetlb() on dst hugetlb folio

We replaced a simple put_page() by a putback_active_hugepage() call in
commit 3aaa76e ("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 handle the migratable flag and the active list flag in
move_hugetlb_state(), where we know that allocation succeeded and already
handle the temporary flag; use a simple folio_put() to return our
reference.

Link: https://lkml.kernel.org/r/20250113131611.2554758-4-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: davidhildenbrand

mm/hugetlb.c

@@ -7533,6 +7533,16 @@ 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"
+	 * and add it to the active list.
+	 */
+	spin_lock_irq(&hugetlb_lock);
+	folio_set_hugetlb_migratable(new_folio);
+	list_move_tail(&new_folio->lru, &(folio_hstate(new_folio))->hugepage_activelist);
+	spin_unlock_irq(&hugetlb_lock);
 }
 
 static void hugetlb_unshare_pmds(struct vm_area_struct *vma,

mm/migrate.c

@@ -1539,14 +1539,14 @@ 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;
 }