s390/uv: gmap_make_secure() cleanups for further changes

Let's factor out handling of LRU cache draining and convert the if-else
chain to a switch-case.
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20240508182955.358628-3-david@redhat.comSigned-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

arch/s390/kernel/uv.c

@@ -266,6 +266,36 @@ static bool should_export_before_import(struct uv_cb_header *uvcb, struct mm_str
 	return atomic_read(&mm->context.protected_count) > 1;
 }
 
+/*
+ * Drain LRU caches: the local one on first invocation and the ones of all
+ * CPUs on successive invocations. Returns "true" on the first invocation.
+ */
+static bool drain_lru(bool *drain_lru_called)
+{
+	/*
+	 * If we have tried a local drain and the folio refcount
+	 * still does not match our expected safe value, try with a
+	 * system wide drain. This is needed if the pagevecs holding
+	 * the page are on a different CPU.
+	 */
+	if (*drain_lru_called) {
+		lru_add_drain_all();
+		/* We give up here, don't retry immediately. */
+		return false;
+	}
+	/*
+	 * We are here if the folio refcount does not match the
+	 * expected safe value. The main culprits are usually
+	 * pagevecs. With lru_add_drain() we drain the pagevecs
+	 * on the local CPU so that hopefully the refcount will
+	 * reach the expected safe value.
+	 */
+	lru_add_drain();
+	*drain_lru_called = true;
+	/* The caller should try again immediately */
+	return true;
+}
+
 /*
  * Requests the Ultravisor to make a page accessible to a guest.
  * If it's brought in the first time, it will be cleared. If
@@ -275,7 +305,7 @@ static bool should_export_before_import(struct uv_cb_header *uvcb, struct mm_str
 int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
 {
 	struct vm_area_struct *vma;
-	bool local_drain = false;
+	bool drain_lru_called = false;
 	spinlock_t *ptelock;
 	unsigned long uaddr;
 	struct folio *folio;
@@ -331,37 +361,21 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
 out:
 	mmap_read_unlock(gmap->mm);
 
-	if (rc == -EAGAIN) {
+	switch (rc) {
+	case -EAGAIN:
 		/*
 		 * If we are here because the UVC returned busy or partial
 		 * 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
-		 * still does not match our expected safe value, try with a
-		 * system wide drain. This is needed if the pagevecs holding
-		 * the page are on a different CPU.
-		 */
-		if (local_drain) {
-			lru_add_drain_all();
-			/* We give up here, and let the caller try again */
 		return -EAGAIN;
-		}
-		/*
-		 * We are here if the folio refcount does not match the
-		 * expected safe value. The main culprits are usually
-		 * pagevecs. With lru_add_drain() we drain the pagevecs
-		 * on the local CPU so that hopefully the refcount will
-		 * reach the expected safe value.
-		 */
-		lru_add_drain();
-		local_drain = true;
-		/* And now we try again immediately after draining */
+	case -EBUSY:
+		/* Additional folio references. */
+		if (drain_lru(&drain_lru_called))
 			goto again;
-	} else if (rc == -ENXIO) {
+		return -EAGAIN;
+	case -ENXIO:
 		if (gmap_fault(gmap, gaddr, FAULT_FLAG_WRITE))
 			return -EFAULT;
 		return -EAGAIN;