[PATCH v1 1/5] s390/uv: don't call wait_on_page_writeback() without a reference

[David Hildenbrand <david@redhat.com>]

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

Fixes: 214d9bbcd3a6 ("s390/mm: provide memory management functions for protected KVM guests")
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 arch/s390/kernel/uv.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
index fc07bc39e698..7401838b960b 100644
--- a/arch/s390/kernel/uv.c
+++ b/arch/s390/kernel/uv.c
@@ -314,6 +314,13 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
 			rc = make_page_secure(page, uvcb);
 			unlock_page(page);
 		}
+
+		/*
+		 * Once we drop the PTL, the page may get unmapped and
+		 * freed immediately. We need a temporary reference.
+		 */
+		if (rc == -EAGAIN)
+			get_page(page);
 	}
 	pte_unmap_unlock(ptep, ptelock);
 out:
@@ -325,6 +332,7 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
 		 * completion, this is just a useless check, but it is safe.
 		 */
 		wait_on_page_writeback(page);
+		put_page(page);
 	} else if (rc == -EBUSY) {
 		/*
 		 * If we have tried a local drain and the page refcount
-- 
2.44.0