lock. The unmap path, instead,
+	 * takes the mmu_lock in write mode first, and then potentially
+	 * calls this function, which takes the ptl lock. This can lead to a
+	 * deadlock.
+	 * The unused page mechanism is only an optimization, if the
+	 * _PAGE_UNUSED bit is not set, the unused page is swapped as normal
+	 * instead of being discarded.
+	 * If the lock is contended the bit is not set and the deadlock is
+	 * avoided.
+	 */
+	if (spin_trylock(ptl)) {
+		/*
+		 * Make sure the pte we are touching is still the correct
+		 * one. In theory this check should not be needed, but
+		 * better safe than sorry.
+		 * Disabling interrupts or holding the mmap lock is enough to
+		 * guarantee that no concurrent updates to the page tables
+		 * are possible.
+		 */
+		if (likely(pmd_same(pmdval, pmdp_get_lockless(pmdp))))
+			__atomic64_or(_PAGE_UNUSED, (long *)ptep);
+		spin_unlock(ptl);
+	}
+
+	pte_unmap(ptep);
+}
+EXPORT_SYMBOL_GPL(gmap_helper_try_set_pte_unused);
+
 static int find_zeropage_pte_entry(pte_t *pte, unsigned long addr,
 				   unsigned long end, struct mm_walk *walk)
 {
-- 
2.52.0[PATCH v6 05/28] KVM: s390: Add gmap_helper_set_unused()Claudio Imbrenda <imbrenda@linux.ibm.com> undefinedkvm@vger.kernel.org undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined¸:„×