[PATCH v2 01/10] s390/uv: don't call folio_wait_writeback() without a folio reference

[David Hildenbrand <david@redhat.com>]

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: 214d9bbcd3a6 ("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>
---
 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 265fea37e030..016993e9eb72 100644
--- a/arch/s390/kernel/uv.c
+++ b/arch/s390/kernel/uv.c
@@ -318,6 +318,13 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
 			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);
@@ -330,6 +337,7 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
 		 * 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
-- 
2.44.0