lib/xarray: introduce a new helper xas_get_order

It can be used after xas_load to check the order of loaded entries. 
Compared to xa_get_order, it saves an XA_STATE and avoid a rewalk.

Added new test for xas_get_order, to make the test work, we have to export
xas_get_order with EXPORT_SYMBOL_GPL.

Also fix a sparse warning by checking the slot value with xa_entry instead
of accessing it directly, as suggested by Matthew Wilcox.

[[email protected]: simplify comment, sparse warning fix, per Matthew Wilcox]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Kairui Song <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 31 insertions, 18 deletions

diff --git a/lib/xarray.c b/lib/xarray.c
index 39f07bfc4dcc..da79128ad754 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -1750,39 +1750,52 @@ unlock:
 EXPORT_SYMBOL(xa_store_range);
 
 /**
- * xa_get_order() - Get the order of an entry.
- * @xa: XArray.
- * @index: Index of the entry.
+ * xas_get_order() - Get the order of an entry.
+ * @xas: XArray operation state.
+ *
+ * Called after xas_load, the xas should not be in an error state.
  *
  * Return: A number between 0 and 63 indicating the order of the entry.
  */
-int xa_get_order(struct xarray *xa, unsigned long index)
+int xas_get_order(struct xa_state *xas)
 {
-	XA_STATE(xas, xa, index);
-	void *entry;
 	int order = 0;
 
-	rcu_read_lock();
-	entry = xas_load(&xas);
-
-	if (!entry)
-		goto unlock;
-
-	if (!xas.xa_node)
-		goto unlock;
+	if (!xas->xa_node)
+		return 0;
 
 	for (;;) {
-		unsigned int slot = xas.xa_offset + (1 << order);
+		unsigned int slot = xas->xa_offset + (1 << order);
 
 		if (slot >= XA_CHUNK_SIZE)
 			break;
-		if (!xa_is_sibling(xas.xa_node->slots[slot]))
+		if (!xa_is_sibling(xa_entry(xas->xa, xas->xa_node, slot)))
 			break;
 		order++;
 	}
 
-	order += xas.xa_node->shift;
-unlock:
+	order += xas->xa_node->shift;
+	return order;
+}
+EXPORT_SYMBOL_GPL(xas_get_order);
+
+/**
+ * xa_get_order() - Get the order of an entry.
+ * @xa: XArray.
+ * @index: Index of the entry.
+ *
+ * Return: A number between 0 and 63 indicating the order of the entry.
+ */
+int xa_get_order(struct xarray *xa, unsigned long index)
+{
+	XA_STATE(xas, xa, index);
+	int order = 0;
+	void *entry;
+
+	rcu_read_lock();
+	entry = xas_load(&xas);
+	if (entry)
+		order = xas_get_order(&xas);
 	rcu_read_unlock();
 
 	return order;