fs: pack struct file

Now that we shrunk struct file to 192 bytes aka 3 cachelines reorder
struct file to not leave any holes or have members cross cachelines.

Add a short comment to each of the fields and mark the cachelines.
It's possible that we may have to tweak this based on profiling in the
future. So far I had Jens test this comparing io_uring with non-fixed
and fixed files and it improved performance. The layout is a combination
of Jens' and my changes.

Link: https: //lore.kernel.org/r/20240824-peinigen-hocken-7384b977c643@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

include/linux/fs.h

@@ -987,52 +987,63 @@ static inline int ra_has_index(struct file_ra_state *ra, pgoff_t index)
 		index <  ra->start + ra->size);
 }
 
-/*
- * f_{lock,count,pos_lock} members can be highly contended and share
- * the same cacheline. f_{lock,mode} are very frequently used together
- * and so share the same cacheline as well. The read-mostly
- * f_{path,inode,op} are kept on a separate cacheline.
+/**
+ * struct file - Represents a file
+ * @f_count: reference count
+ * @f_lock: Protects f_ep, f_flags. Must not be taken from IRQ context.
+ * @f_mode: FMODE_* flags often used in hotpaths
+ * @f_op: file operations
+ * @f_mapping: Contents of a cacheable, mappable object.
+ * @private_data: filesystem or driver specific data
+ * @f_inode: cached inode
+ * @f_flags: file flags
+ * @f_iocb_flags: iocb flags
+ * @f_cred: stashed credentials of creator/opener
+ * @f_path: path of the file
+ * @f_pos_lock: lock protecting file position
+ * @f_pos: file position
+ * @f_version: file version
+ * @f_security: LSM security context of this file
+ * @f_owner: file owner
+ * @f_wb_err: writeback error
+ * @f_sb_err: per sb writeback errors
+ * @f_ep: link of all epoll hooks for this file
+ * @f_task_work: task work entry point
+ * @f_llist: work queue entrypoint
+ * @f_ra: file's readahead state
  */
 struct file {
-	union {
-		/* fput() uses task work when closing and freeing file (default). */
-		struct callback_head 	f_task_work;
-		/* fput() must use workqueue (most kernel threads). */
-		struct llist_node	f_llist;
-		/* Invalid after last fput(). */
-		struct file_ra_state	f_ra;
-	};
-	/*
-	 * Protects f_ep, f_flags.
-	 * Must not be taken from IRQ context.
-	 */
-	spinlock_t		f_lock;
-	fmode_t			f_mode;
-	atomic_long_t		f_count;
-	struct mutex		f_pos_lock;
-	loff_t			f_pos;
-	unsigned int		f_flags;
-	unsigned int 		f_iocb_flags;
-	struct fown_struct	*f_owner;
-	const struct cred	*f_cred;
-	struct path		f_path;
-	struct inode		*f_inode;	/* cached value */
+	atomic_long_t			f_count;
+	spinlock_t			f_lock;
+	fmode_t				f_mode;
 	const struct file_operations	*f_op;
-
-	u64			f_version;
+	struct address_space		*f_mapping;
+	void				*private_data;
+	struct inode			*f_inode;
+	unsigned int			f_flags;
+	unsigned int			f_iocb_flags;
+	const struct cred		*f_cred;
+	/* --- cacheline 1 boundary (64 bytes) --- */
+	struct path			f_path;
+	struct mutex			f_pos_lock;
+	loff_t				f_pos;
+	u64				f_version;
+	/* --- cacheline 2 boundary (128 bytes) --- */
 #ifdef CONFIG_SECURITY
-	void			*f_security;
+	void				*f_security;
 #endif
-	/* needed for tty driver, and maybe others */
-	void			*private_data;
-
+	struct fown_struct		*f_owner;
+	errseq_t			f_wb_err;
+	errseq_t			f_sb_err;
 #ifdef CONFIG_EPOLL
-	/* Used by fs/eventpoll.c to link all the hooks to this file */
-	struct hlist_head	*f_ep;
-#endif /* #ifdef CONFIG_EPOLL */
-	struct address_space	*f_mapping;
-	errseq_t		f_wb_err;
-	errseq_t		f_sb_err; /* for syncfs */
+	struct hlist_head		*f_ep;
+#endif
+	union {
+		struct callback_head	f_task_work;
+		struct llist_node	f_llist;
+		struct file_ra_state	f_ra;
+	};
+	/* --- cacheline 3 boundary (192 bytes) --- */
 } __randomize_layout
   __attribute__((aligned(4)));	/* lest something weird decides that 2 is OK */