tty: shrink the size of struct tty_struct by 40 bytes

It's been a long time since anyone has looked at what struct tty_struct
looks like in memory, turns out there was a ton of holes.

So move things around a bit, change one variable (closing) from being an
int to a bool (it is only being tested for 0/1), and we end up saving 40
bytes per structure overall on x86-64 systems.

Before this patch:
	/* size: 696, cachelines: 11, members: 37 */
	/* sum members: 665, holes: 8, sum holes: 31 */
	/* forced alignments: 2, forced holes: 1, sum forced holes: 4 */
	/* last cacheline: 56 bytes */

After this change:
	/* size: 656, cachelines: 11, members: 37 */
	/* sum members: 654, holes: 1, sum holes: 2 */
	/* forced alignments: 2 */
	/* last cacheline: 16 bytes */

Cc: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/2023082519-cobbler-unholy-8d1f@gregkh
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/include/linux/tty.h b/include/linux/tty.h
index e8d5d99..f002d0f 100644 (file)
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -192,13 +192,14 @@ struct tty_operations;
  */
 struct tty_struct {
        struct kref kref;
+       int index;
        struct device *dev;
        struct tty_driver *driver;
+       struct tty_port *port;
        const struct tty_operations *ops;
-       int index;
 
-       struct ld_semaphore ldisc_sem;
        struct tty_ldisc *ldisc;
+       struct ld_semaphore ldisc_sem;
 
        struct mutex atomic_write_lock;
        struct mutex legacy_mutex;
@@ -209,6 +210,7 @@ struct tty_struct {
        char name[64];
        unsigned long flags;
        int count;
+       unsigned int receive_room;
        struct winsize winsize;
 
        struct {
@@ -219,16 +221,16 @@ struct tty_struct {
        } __aligned(sizeof(unsigned long)) flow;
 
        struct {
-               spinlock_t lock;
                struct pid *pgrp;
                struct pid *session;
+               spinlock_t lock;
                unsigned char pktstatus;
                bool packet;
                unsigned long unused[0];
        } __aligned(sizeof(unsigned long)) ctrl;
 
        bool hw_stopped;
-       unsigned int receive_room;
+       bool closing;
        int flow_change;
 
        struct tty_struct *link;
@@ -239,15 +241,13 @@ struct tty_struct {
        void *disc_data;
        void *driver_data;
        spinlock_t files_lock;
+       int write_cnt;
+       unsigned char *write_buf;
+
        struct list_head tty_files;
 
 #define N_TTY_BUF_SIZE 4096
-
-       int closing;
-       unsigned char *write_buf;
-       int write_cnt;
        struct work_struct SAK_work;
-       struct tty_port *port;
 } __randomize_layout;
 
 /* Each of a tty's open files has private_data pointing to tty_file_private */