kernfs: Skip kernfs_drain_open_files() more aggressively

Track the number of mmapped files and files that need to be released and
skip kernfs_drain_open_file() if both are zero, which are the precise
conditions which require draining open_files. The early exit test is
factored into kernfs_should_drain_open_files() which is now tested by
kernfs_drain_open_files()'s caller - kernfs_drain().

This isn't a meaningful optimization on its own but will enable future
stand-alone kernfs_deactivate() implementation.

v2: Chengming noticed that on->nr_to_release was leaking after ->open()
    failure. Fix it by telling kernfs_unlink_open_file() that it's called
    from the ->open() fail path and should dec the counter. Use kzalloc() to
    allocate kernfs_open_node so that the tracking fields are correctly
    initialized.

Cc: Chengming Zhou <zhouchengming@bytedance.com>
Tested-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220828050440.734579-5-tj@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 1cc88ba6de90..8ae44db920d4 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -489,7 +489,8 @@ static void kernfs_drain(struct kernfs_node *kn)
 		rwsem_release(&kn->dep_map, _RET_IP_);
 	}
 
-	kernfs_drain_open_files(kn);
+	if (kernfs_should_drain_open_files(kn))
+		kernfs_drain_open_files(kn);
 
 	down_write(&root->kernfs_rwsem);
 }