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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Message-ID: <m14pf03fex.fsf@ebiederm.dsl.xmission.com>
Date:	Sun, 02 Dec 2007 15:22:46 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Greg KH <gregkh@...e.de>
Cc:	rae l <crquan@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: Since sysfs_mount is static and used only in sysfs_init function, it could be just an automatic variable.

Greg KH <gregkh@...e.de> writes:

> 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.

As long as you aren't talking about the subthread that spun off
about GPL exports, and just the rest of my patches and showing
where I am going that sounds reasonable.

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

Honestly I think there is a reasonable chance we could kill
sysfs_mount and the kern_mount entirely.  We used to need  the
internal kernel mount because of the coupling between sysfs_dirent
and the directory dentries but that is gone now.

So basically the variable sysfs_mount is static because sysfs_mount
used to be simply global I changed it to be static, and I haven't
looked at optimization opportunities beyond that.

So as long as we are using sysfs_mount less and don't get in
the way of multiple superblocks for sysfs I'm happy.

Eric
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