Both __xa_store() and xa_erase() use xas_store() under the hood, but when
the entry being stored is NULL (as in the case of xa_erase()),
xas->xa_sibs (and max) is only checked if the next entry is not a sibling,
hence allowing xas_store() to keep iterating, hence updating
node->nr_values correctly.

Add xa_erase() to check_split tests that verify functionality, with the
added intent to illustrate the usage differences between __xa_store(),
xas_store() and xa_erase() with regard to multi-index XArrays.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 lib/test_xarray.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/lib/test_xarray.c b/lib/test_xarray.c
index e71e8ff76900..bb9471a3df65 100644
--- a/lib/test_xarray.c
+++ b/lib/test_xarray.c
@@ -1874,6 +1874,10 @@ static void check_split_1(struct xarray *xa, unsigned long index,
 	rcu_read_unlock();
 	XA_BUG_ON(xa, found != 1 << (order - new_order));
 
+	for (i = 0; i < (1 << order); i += (1 << new_order))
+		xa_erase(xa, index + i);
+	XA_BUG_ON(xa, !xa_empty(xa));
+
 	xa_destroy(xa);
 }
 
@@ -1926,6 +1930,10 @@ static void check_split_2(struct xarray *xa, unsigned long index,
 	}
 	rcu_read_unlock();
 	XA_BUG_ON(xa, found != 1 << (order - new_order));
+
+	for (i = 0; i < (1 << order); i += (1 << new_order))
+		xa_erase(xa, index + i);
+	XA_BUG_ON(xa, !xa_empty(xa));
 out:
 	xas_destroy(&xas);
 	xa_destroy(xa);
-- 
2.53.0.rc1.225.gd81095ad13-goog