lib/xarray: introduce a new helper xas_get_order

commit a4864671ca0bf51c8e78242951741df52c06766f upstream.

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.

[kasong@tencent.com: simplify comment, sparse warning fix, per Matthew Wilcox]
  Link: https://lkml.kernel.org/r/20240416071722.45997-4-ryncsn@gmail.com
Link: https://lkml.kernel.org/r/20240415171857.19244-4-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Stable-dep-of: 6758c1128ceb ("mm/filemap: optimize filemap folio adding")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/xarray.h

@@ -1530,6 +1530,7 @@ void xas_create_range(struct xa_state *);
 
 #ifdef CONFIG_XARRAY_MULTI
 int xa_get_order(struct xarray *, unsigned long index);
+int xas_get_order(struct xa_state *xas);
 void xas_split(struct xa_state *, void *entry, unsigned int order);
 void xas_split_alloc(struct xa_state *, void *entry, unsigned int order, gfp_t);
 #else
@@ -1538,6 +1539,11 @@ static inline int xa_get_order(struct xarray *xa, unsigned long index)
 	return 0;
 }
 
+static inline int xas_get_order(struct xa_state *xas)
+{
+	return 0;
+}
+
 static inline void xas_split(struct xa_state *xas, void *entry,
 		unsigned int order)
 {

lib/test_xarray.c

@@ -1756,6 +1756,39 @@ static noinline void check_get_order(struct xarray *xa)
 	}
 }
 
+static noinline void check_xas_get_order(struct xarray *xa)
+{
+	XA_STATE(xas, xa, 0);
+
+	unsigned int max_order = IS_ENABLED(CONFIG_XARRAY_MULTI) ? 20 : 1;
+	unsigned int order;
+	unsigned long i, j;
+
+	for (order = 0; order < max_order; order++) {
+		for (i = 0; i < 10; i++) {
+			xas_set_order(&xas, i << order, order);
+			do {
+				xas_lock(&xas);
+				xas_store(&xas, xa_mk_value(i));
+				xas_unlock(&xas);
+			} while (xas_nomem(&xas, GFP_KERNEL));
+
+			for (j = i << order; j < (i + 1) << order; j++) {
+				xas_set_order(&xas, j, 0);
+				rcu_read_lock();
+				xas_load(&xas);
+				XA_BUG_ON(xa, xas_get_order(&xas) != order);
+				rcu_read_unlock();
+			}
+
+			xas_lock(&xas);
+			xas_set_order(&xas, i << order, order);
+			xas_store(&xas, NULL);
+			xas_unlock(&xas);
+		}
+	}
+}
+
 static noinline void check_destroy(struct xarray *xa)
 {
 	unsigned long index;
@@ -1805,6 +1838,7 @@ static int xarray_checks(void)
 	check_reserve(&xa0);
 	check_multi_store(&array);
 	check_get_order(&array);
+	check_xas_get_order(&array);
 	check_xa_alloc();
 	check_find(&array);
 	check_find_entry(&array);

lib/xarray.c

@@ -1751,6 +1751,36 @@ unlock:
 }
 EXPORT_SYMBOL(xa_store_range);
 
+/**
+ * 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 xas_get_order(struct xa_state *xas)
+{
+	int order = 0;
+
+	if (!xas->xa_node)
+		return 0;
+
+	for (;;) {
+		unsigned int slot = xas->xa_offset + (1 << order);
+
+		if (slot >= XA_CHUNK_SIZE)
+			break;
+		if (!xa_is_sibling(xa_entry(xas->xa, xas->xa_node, slot)))
+			break;
+		order++;
+	}
+
+	order += xas->xa_node->shift;
+	return order;
+}
+EXPORT_SYMBOL_GPL(xas_get_order);
+
 /**
  * xa_get_order() - Get the order of an entry.
  * @xa: XArray.
@@ -1761,30 +1791,13 @@ EXPORT_SYMBOL(xa_store_range);
 int xa_get_order(struct xarray *xa, unsigned long index)
 {
 	XA_STATE(xas, xa, index);
-	void *entry;
 	int order = 0;
+	void *entry;
 
 	rcu_read_lock();
 	entry = xas_load(&xas);
-
+	if (entry)
-	if (!entry)
+		order = xas_get_order(&xas);
-		goto unlock;
-
-	if (!xas.xa_node)
-		goto unlock;
-
-	for (;;) {
-		unsigned int slot = xas.xa_offset + (1 << order);
-
-		if (slot >= XA_CHUNK_SIZE)
-			break;
-		if (!xa_is_sibling(xas.xa_node->slots[slot]))
-			break;
-		order++;
-	}
-
-	order += xas.xa_node->shift;
-unlock:
 	rcu_read_unlock();
 
 	return order;