[PATCH] backing file: free directly

[Christian Brauner <brauner@kernel.org>]

Backing files as used by overlayfs are never installed into file
descriptor tables and are explicitly documented as such. They aren't
subject to rcu access conditions like regular files are.

Their lifetime is bound to the lifetime of the overlayfs file, i.e.,
they're stashed in ovl_file->private_data and go away otherwise. If
they're set as vma->vm_file - which is their main purpose - then they're
subject to regular refcount rules and vma->vm_file can't be installed
into an fdtable after having been set. All in all I don't see any need
for rcu delay here. So free it directly.

This all hinges on such hybrid beasts to never actually be installed
into fdtables which - as mentioned before - is not allowed. So add an
explicit WARN_ON_ONCE() so we catch any case where someone is suddenly
trying to install one of those things into a file descriptor table so we
can have nice long chat with them.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
Hey Linus,

I've put the following change on top of the rcu change. We really don't
need any rcu delayed freeing for backing files unless I'm missing
something. They should never ever appear in fdtables. So fd_install()
should yell if anyone tries to do that. I'm still off this week but
this bothered me.

Christian
---
 fs/file.c       | 3 +++
 fs/file_table.c | 9 +--------
 2 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 666514381159..2f6965848907 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -604,6 +604,9 @@ void fd_install(unsigned int fd, struct file *file)
 	struct files_struct *files = current->files;
 	struct fdtable *fdt;
 
+	if (WARN_ON_ONCE(unlikely(file->f_mode & FMODE_BACKING)))
+		return;
+
 	rcu_read_lock_sched();
 
 	if (unlikely(files->resize_in_progress)) {
diff --git a/fs/file_table.c b/fs/file_table.c
index a79a80031343..08fd1dd6d863 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -61,13 +61,6 @@ struct path *backing_file_real_path(struct file *f)
 }
 EXPORT_SYMBOL_GPL(backing_file_real_path);
 
-static void file_free_rcu(struct rcu_head *head)
-{
-	struct file *f = container_of(head, struct file, f_rcuhead);
-
-	kfree(backing_file(f));
-}
-
 static inline void file_free(struct file *f)
 {
 	security_file_free(f);
@@ -76,7 +69,7 @@ static inline void file_free(struct file *f)
 	put_cred(f->f_cred);
 	if (unlikely(f->f_mode & FMODE_BACKING)) {
 		path_put(backing_file_real_path(f));
-		call_rcu(&f->f_rcuhead, file_free_rcu);
+		kfree(backing_file(f));
 	} else {
 		kmem_cache_free(filp_cachep, f);
 	}
-- 
2.34.1