[PATCH 13/18] maple_tree: Document that erase may use GFP_KERNEL for allocations

[Liam R. Howlett (Oracle)]

State that the mas_erase() and mtree_erase() functions may use
GFP_KERNEL on allocation retry.  Don't just depend on people reading the
documentation by adding a check that will warn of the use.

Cc: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Signed-off-by: Liam R. Howlett (Oracle) <liam@xxxxxxxxxxxxx>
---
 lib/maple_tree.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 67cc480534e96..c5efa3e2f7461 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -5686,6 +5686,10 @@ EXPORT_SYMBOL_GPL(mas_find_range_rev);
  * Searches for @mas->index, sets @mas->index and @mas->last to the range and
  * erases that range.
  *
+ * Note that erase requires allocations and will use GFP_KERNEL to do so if
+ * necessary.  If the allocation fails, the internal lock will be dropped to
+ * retry.
+ *
  * Return: the entry that was erased or %NULL, @mas->index and @mas->last are updated.
  */
 void *mas_erase(struct ma_state *mas)
@@ -5694,6 +5698,14 @@ void *mas_erase(struct ma_state *mas)
 	unsigned long index = mas->index;
 	MA_WR_STATE(wr_mas, mas, NULL);
 
+	/*
+	 * In low memory situations, the allocation is retried with the gfp flag
+	 * GFP_KERNEL.  The internal spinlock is dropped in mas_nomem(), however
+	 * the external lock is not dropped.
+	 */
+	if (mt_external_lock(mas->tree))
+		might_alloc(GFP_KERNEL);
+
 	if (!mas_is_active(mas) || !mas_is_start(mas))
 		mas->status = ma_start;
 
@@ -6040,6 +6052,10 @@ EXPORT_SYMBOL(mtree_alloc_rrange);
  * Erasing is the same as a walk to an entry then a store of a NULL to that
  * ENTIRE range.  In fact, it is implemented as such using the advanced API.
  *
+ * Note that erase requires allocations and will use GFP_KERNEL to do so if
+ * necessary.  If the allocation fails, the internal lock will be dropped to
+ * retry.
+ *
  * Return: The entry stored at the @index or %NULL
  */
 void *mtree_erase(struct maple_tree *mt, unsigned long index)
-- 
2.47.3