linux-kernel - Re: Since sysfs_mount is static and used only in sysfs_init function, it could be just an automatic variable.

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Date:	Sat, 1 Dec 2007 23:54:37 -0800
From:	Greg KH <gregkh@...e.de>
To:	rae l <crquan@...il.com>
Cc:	linux-kernel@...r.kernel.org, ebiederm@...ssion.com
Subject: Re: Since sysfs_mount is static and used only in sysfs_init
	function, it could be just an automatic variable.

On Sun, Dec 02, 2007 at 02:52:17PM +0800, rae l wrote:
> On Dec 2, 2007 12:48 PM, Greg KH <gregkh@...e.de> wrote:
> ...
> > > and where is a detailed explaination on kern_mount? could someone give
> > > some comments or documentation pointers on this?
> >
> > See the patches that Eric Biederman just posted to lkml for why this
> > structure is a static pointer this way right now, it's in preparation
> > for future patches.
> I have checked commit 7d0c7d676cc066413e1583b5af9fba8011972d41 by Eric
> W. Biederman,
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=7d0c7d676cc066413e1583b5af9fba8011972d41
> 
> which just make sysfs_mount from externally visible to static that
> could be only used in one c file,
> 
> but I mean that the static variable is still on kernel bss section,
> this consumes a pointer (4 or 8 bytes) memory,
> 
> through a grep from fs/sysfs/, it appears that the variable
> sysfs_mount is only used in the sysfs_init function,
> 
> $ grep -RsInw sysfs_mount fs/sysfs/
> fs/sysfs/mount.c:25:static struct vfsmount *sysfs_mount;
> fs/sysfs/mount.c:101:           sysfs_mount = kern_mount(&sysfs_fs_type);
> fs/sysfs/mount.c:102:           if (IS_ERR(sysfs_mount)) {
> fs/sysfs/mount.c:104:                   err = PTR_ERR(sysfs_mount);
> fs/sysfs/mount.c:105:                   sysfs_mount = NULL;
> 
> we could mark this variable an automatic one, which scope is just in
> this function, thus created and destroyed with the stack,
> this approach does not consume a pointer on kernel bss section,
> 
> Why not do this?

Again, see the patches he _just_ posted to lkml, the specific message
you are looking for is:
	Message-ID: <m11wa693t0.fsf@...ederm.dsl.xmission.com>
	Subject: [PATCH 01/10] sysfs: Make sysfs_mount static again.

Also see the whole long thread for more details.

If you have further questions about this, please ask Eric.

thanks,

greg k-h
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