s390/uv: Don't call folio_wait_writeback() without a folio reference

[ Upstream commit 3f29f6537f54d74e64bac0a390fb2e26da25800d ]

folio_wait_writeback() requires that no spinlocks are held and that
a folio reference is held, as documented. After we dropped the PTL, the
folio could get freed concurrently. So grab a temporary reference.

Fixes: 214d9bbcd3 ("s390/mm: provide memory management functions for protected KVM guests")
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20240508182955.358628-2-david@redhat.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

arch/s390/kernel/uv.c

@@ -317,6 +317,13 @@ again:
 			rc = make_folio_secure(folio, uvcb);
 			folio_unlock(folio);
 		}
+
+		/*
+		 * Once we drop the PTL, the folio may get unmapped and
+		 * freed immediately. We need a temporary reference.
+		 */
+		if (rc == -EAGAIN)
+			folio_get(folio);
 	}
 unlock:
 	pte_unmap_unlock(ptep, ptelock);
@@ -329,6 +336,7 @@ out:
 		 * completion, this is just a useless check, but it is safe.
 		 */
 		folio_wait_writeback(folio);
+		folio_put(folio);
 	} else if (rc == -EBUSY) {
 		/*
 		 * If we have tried a local drain and the folio refcount