fs: add f_pipe

Only regular files with FMODE_ATOMIC_POS and directories need
f_pos_lock. Place a new f_pipe member in a union with f_pos_lock
that they can use and make them stop abusing f_version in follow-up
patches.

Link: https://lore.kernel.org/r/20240830-vfs-file-f_version-v1-18-6d3e4816aa7b@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/file_table.c

@@ -156,6 +156,13 @@ static int init_file(struct file *f, int flags, const struct cred *cred)
 	}
 
 	spin_lock_init(&f->f_lock);
+	/*
+	 * Note that f_pos_lock is only used for files raising
+	 * FMODE_ATOMIC_POS and directories. Other files such as pipes
+	 * don't need it and since f_pos_lock is in a union may reuse
+	 * the space for other purposes. They are expected to initialize
+	 * the respective member when opening the file.
+	 */
 	mutex_init(&f->f_pos_lock);
 	f->f_flags = flags;
 	f->f_mode = OPEN_FMODE(flags);

include/linux/fs.h

@@ -1001,6 +1001,7 @@ static inline int ra_has_index(struct file_ra_state *ra, pgoff_t index)
  * @f_cred: stashed credentials of creator/opener
  * @f_path: path of the file
  * @f_pos_lock: lock protecting file position
+ * @f_pipe: specific to pipes
  * @f_pos: file position
  * @f_version: file version
  * @f_security: LSM security context of this file
@@ -1026,7 +1027,12 @@ struct file {
 	const struct cred		*f_cred;
 	/* --- cacheline 1 boundary (64 bytes) --- */
 	struct path			f_path;
-	struct mutex			f_pos_lock;
+	union {
+		/* regular files (with FMODE_ATOMIC_POS) and directories */
+		struct mutex		f_pos_lock;
+		/* pipes */
+		u64			f_pipe;
+	};
 	loff_t				f_pos;
 	u64				f_version;
 	/* --- cacheline 2 boundary (128 bytes) --- */