[PATCH 3/5] writeback: explain why @inode is allowed to be NULL for inode_congested()

[Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

Signed-off-by: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Suggested-by: Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>
---
 fs/fs-writeback.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index f15148c..b5022c3 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -700,7 +700,7 @@ void wbc_account_io(struct writeback_control *wbc, struct page *page,
 
 /**
  * inode_congested - test whether an inode is congested
- * @inode: inode to test for congestion
+ * @inode: inode to test for congestion (may be NULL)
  * @cong_bits: mask of WB_[a]sync_congested bits to test
  *
  * Tests whether @inode is congested.  @cong_bits is the mask of congestion
@@ -710,6 +710,9 @@ void wbc_account_io(struct writeback_control *wbc, struct page *page,
  * determined by whether the cgwb (cgroup bdi_writeback) for the blkcg
  * associated with @inode is congested; otherwise, the root wb's congestion
  * state is used.
+ *
+ * @inode is allowed to be NULL as this function is often called on
+ * mapping->host which is NULL for the swapper space.
  */
 int inode_congested(struct inode *inode, int cong_bits)
 {
-- 
2.4.3