[PATCH v2 1/2] mm: introduce memalloc_flags_move() for transferring allocation scopes

[Yun Zhou]

Add memalloc_flags_move() to transfer a saved memalloc scope from one
tracking variable to another.  The source is zeroed so that a
subsequent memalloc_flags_restore() on it becomes a no-op, effectively
transferring ownership of the scope to the destination.

This is needed when a subsystem hands off an allocation context from
one structure to another (e.g. during transaction rolling in
filesystems), and needs to ensure the scope remains active without an
extra save/restore cycle.

Suggested-by: Darrick J. Wong <djwong@xxxxxxxxxx>
Signed-off-by: Yun Zhou <yun.zhou@xxxxxxxxxxxxx>
---
 include/linux/sched/mm.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 95d0040df584..a6de84b70c0a 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -342,6 +342,23 @@ static inline void memalloc_flags_restore(unsigned flags)
 	current->flags &= ~flags;
 }
 
+/**
+ * memalloc_flags_move - transfer a memalloc scope from one tracking
+ * variable to another.
+ * @old_flags: pointer to the source flags (will be zeroed)
+ *
+ * Returns the flags value to store in the destination.  The source is
+ * set to zero so that a subsequent memalloc_flags_restore() on it is
+ * a no-op.
+ */
+static inline unsigned int memalloc_flags_move(unsigned int *old_flags)
+{
+	unsigned int ret = *old_flags;
+
+	*old_flags = 0;
+	return ret;
+}
+
 /**
  * memalloc_noio_save - Marks implicit GFP_NOIO allocation scope.
  *
-- 
2.43.0