Re: [PATCH] reduce sizeof(struct file)

["Paul E. McKenney" <paulmck@us.ibm.com>]

On Sat, Sep 24, 2005 at 05:43:05AM +0200, Eric Dumazet wrote:
> Christoph Hellwig a écrit :
> >
> >Please just change all callers to use the union, there's not very many
> >of them.
> 
> Yes it's better, thanks Christoph.
> 
> What about this version then ?

After a momentary panic attack where I was worried that f_list might
be accessed by one CPU while another was sending the same struct file
to call_rcu(), I realized that all accesses to f_list do file_list_lock()
first, thus preventing any other CPU from doing call_rcu() concurrently
on that struct file.

So it looks OK to me.

But you did have me going there for a bit!  ;-)

							Thanx, Paul

> Hi all
> 
> Now that RCU applied on 'struct file' seems stable, we can place f_rcuhead 
> in a memory location that is not anymore used at call_rcu(&f->f_rcuhead, 
> file_free_rcu) time, to reduce the size of this critical kernel object.
> 
> The trick I used is to move f_rcuhead and f_list in an union called f_u
> 
> The callers are changed so that f_rcuhead becomes f_u.fu_rcuhead and f_list 
> becomes f_u.f_list
> 
> Tested on allyesconfig, diffed against 2.6.14-rc2
> 
>  drivers/char/tty_io.c        |    2 +-
>  fs/dquot.c                   |    2 +-
>  fs/file_table.c              |   14 +++++++-------
>  fs/proc/generic.c            |    2 +-
>  fs/super.c                   |    2 +-
>  include/linux/fs.h           |    9 +++++++--
>  security/selinux/hooks.c     |    2 +-
>  security/selinux/selinuxfs.c |    2 +-
>  8 files changed, 20 insertions(+), 15 deletions(-)
> 
> 
> Thank you
> 
> Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
> 

> --- linux-2.6.14-rc2-orig/include/linux/fs.h	2005-09-20 05:00:41.000000000 +0200
> +++ linux-2.6.14-rc2/include/linux/fs.h	2005-09-24 04:52:20.000000000 +0200
> @@ -574,7 +574,13 @@
>  #define RA_FLAG_INCACHE 0x02	/* file is already in cache */
>  
>  struct file {
> -	struct list_head	f_list;
> +/*
> + * f_list and f_rcuhead can share the same memory location
> + */
> +	union {
> +		struct list_head	fu_list;
> +		struct rcu_head 	fu_rcuhead;
> +		} f_u;
>  	struct dentry		*f_dentry;
>  	struct vfsmount         *f_vfsmnt;
>  	struct file_operations	*f_op;
> @@ -598,7 +604,6 @@
>  	spinlock_t		f_ep_lock;
>  #endif /* #ifdef CONFIG_EPOLL */
>  	struct address_space	*f_mapping;
> -	struct rcu_head 	f_rcuhead;
>  };
>  extern spinlock_t files_lock;
>  #define file_list_lock() spin_lock(&files_lock);
> --- linux-2.6.14-rc2-orig/fs/file_table.c	2005-09-20 05:00:41.000000000 +0200
> +++ linux-2.6.14-rc2/fs/file_table.c	2005-09-24 05:02:35.000000000 +0200
> @@ -56,13 +56,13 @@
>  
>  static inline void file_free_rcu(struct rcu_head *head)
>  {
> -	struct file *f =  container_of(head, struct file, f_rcuhead);
> +	struct file *f =  container_of(head, struct file, f_u.fu_rcuhead);
>  	kmem_cache_free(filp_cachep, f);
>  }
>  
>  static inline void file_free(struct file *f)
>  {
> -	call_rcu(&f->f_rcuhead, file_free_rcu);
> +	call_rcu(&f->f_u.fu_rcuhead, file_free_rcu);
>  }
>  
>  /* Find an unused file structure and return a pointer to it.
> @@ -95,7 +95,7 @@
>  	f->f_gid = current->fsgid;
>  	rwlock_init(&f->f_owner.lock);
>  	/* f->f_version: 0 */
> -	INIT_LIST_HEAD(&f->f_list);
> +	INIT_LIST_HEAD(&f->f_u.fu_list);
>  	return f;
>  
>  over:
> @@ -225,15 +225,15 @@
>  	if (!list)
>  		return;
>  	file_list_lock();
> -	list_move(&file->f_list, list);
> +	list_move(&file->f_u.fu_list, list);
>  	file_list_unlock();
>  }
>  
>  void file_kill(struct file *file)
>  {
> -	if (!list_empty(&file->f_list)) {
> +	if (!list_empty(&file->f_u.fu_list)) {
>  		file_list_lock();
> -		list_del_init(&file->f_list);
> +		list_del_init(&file->f_u.fu_list);
>  		file_list_unlock();
>  	}
>  }
> @@ -245,7 +245,7 @@
>  	/* Check that no files are currently opened for writing. */
>  	file_list_lock();
>  	list_for_each(p, &sb->s_files) {
> -		struct file *file = list_entry(p, struct file, f_list);
> +		struct file *file = list_entry(p, struct file, f_u.fu_list);
>  		struct inode *inode = file->f_dentry->d_inode;
>  
>  		/* File with pending delete? */
> --- linux-2.6.14-rc2-orig/drivers/char/tty_io.c	2005-09-20 05:00:41.000000000 +0200
> +++ linux-2.6.14-rc2/drivers/char/tty_io.c	2005-09-24 05:02:35.000000000 +0200
> @@ -809,7 +809,7 @@
>  	check_tty_count(tty, "do_tty_hangup");
>  	file_list_lock();
>  	/* This breaks for file handles being sent over AF_UNIX sockets ? */
> -	list_for_each_entry(filp, &tty->tty_files, f_list) {
> +	list_for_each_entry(filp, &tty->tty_files, f_u.fu_list) {
>  		if (filp->f_op->write == redirected_tty_write)
>  			cons_filp = filp;
>  		if (filp->f_op->write != tty_write)
> --- linux-2.6.14-rc2-orig/fs/dquot.c	2005-09-20 05:00:41.000000000 +0200
> +++ linux-2.6.14-rc2/fs/dquot.c	2005-09-24 05:02:35.000000000 +0200
> @@ -662,7 +662,7 @@
>  restart:
>  	file_list_lock();
>  	list_for_each(p, &sb->s_files) {
> -		struct file *filp = list_entry(p, struct file, f_list);
> +		struct file *filp = list_entry(p, struct file, f_u.fu_list);
>  		struct inode *inode = filp->f_dentry->d_inode;
>  		if (filp->f_mode & FMODE_WRITE && dqinit_needed(inode, type)) {
>  			struct dentry *dentry = dget(filp->f_dentry);
> --- linux-2.6.14-rc2-orig/fs/proc/generic.c	2005-09-20 05:00:41.000000000 +0200
> +++ linux-2.6.14-rc2/fs/proc/generic.c	2005-09-24 05:02:35.000000000 +0200
> @@ -533,7 +533,7 @@
>  	 */
>  	file_list_lock();
>  	list_for_each(p, &sb->s_files) {
> -		struct file * filp = list_entry(p, struct file, f_list);
> +		struct file * filp = list_entry(p, struct file, f_u.fu_list);
>  		struct dentry * dentry = filp->f_dentry;
>  		struct inode * inode;
>  		struct file_operations *fops;
> --- linux-2.6.14-rc2-orig/fs/super.c	2005-09-20 05:00:41.000000000 +0200
> +++ linux-2.6.14-rc2/fs/super.c	2005-09-24 05:02:35.000000000 +0200
> @@ -513,7 +513,7 @@
>  	struct file *f;
>  
>  	file_list_lock();
> -	list_for_each_entry(f, &sb->s_files, f_list) {
> +	list_for_each_entry(f, &sb->s_files, f_u.fu_list) {
>  		if (S_ISREG(f->f_dentry->d_inode->i_mode) && file_count(f))
>  			f->f_mode &= ~FMODE_WRITE;
>  	}
> --- linux-2.6.14-rc2-orig/security/selinux/hooks.c	2005-09-20 05:00:41.000000000 +0200
> +++ linux-2.6.14-rc2/security/selinux/hooks.c	2005-09-24 05:02:35.000000000 +0200
> @@ -1599,7 +1599,7 @@
>  
>  	if (tty) {
>  		file_list_lock();
> -		file = list_entry(tty->tty_files.next, typeof(*file), f_list);
> +		file = list_entry(tty->tty_files.next, typeof(*file), f_u.fu_list);
>  		if (file) {
>  			/* Revalidate access to controlling tty.
>  			   Use inode_has_perm on the tty inode directly rather
> --- linux-2.6.14-rc2-orig/security/selinux/selinuxfs.c	2005-09-20 05:00:41.000000000 +0200
> +++ linux-2.6.14-rc2/security/selinux/selinuxfs.c	2005-09-24 05:02:35.000000000 +0200
> @@ -924,7 +924,7 @@
>  
>  	file_list_lock();
>  	list_for_each(p, &sb->s_files) {
> -		struct file * filp = list_entry(p, struct file, f_list);
> +		struct file * filp = list_entry(p, struct file, f_u.fu_list);
>  		struct dentry * dentry = filp->f_dentry;
>  
>  		if (dentry->d_parent != de) {