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: <3ae72650712021646r8376bb9s73e326db5789839d@mail.gmail.com>
Date:	Mon, 3 Dec 2007 01:46:57 +0100
From:	"Kay Sievers" <kay.sievers@...y.org>
To:	"Greg KH" <gregkh@...e.de>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"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.

On Dec 2, 2007 11:53 PM, Greg KH <gregkh@...e.de> wrote:
>
> On Sun, Dec 02, 2007 at 03:22:46PM -0700, Eric W. Biederman wrote:
> > 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.
>
> Yes, that subthread really had nothing to do with your patch series :)
>
> > > 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.
>
> Hm, this does take into account the fact that we internally mount sysfs
> as part of the boot process to determine the boot disk major:minor,
> right?  As long as we don't break that, I'm happy.

That cruft is going away with the block patch in your tree.

Kay
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