Re: Is initramfs freed after kernel is booted?

[Hannes Reinecke]

viro@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx wrote:
> On Mon, Nov 17, 2003 at 09:33:59PM +0300, Andrey Borzenkov wrote:
>
> > On Monday 17 November 2003 21:03, viro@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx 
> > wrote:
> >
> > > On Mon, Nov 17, 2003 at 12:50:34PM -0500, Chris Friesen wrote:
> > >
> > > > viro@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx wrote:
> > > >
> > > > > On Mon, Nov 17, 2003 at 11:06:48AM -0500, Chris Friesen wrote:
> > > > >
> > > > > > Anyone know why it overmounts rather than pivots?
> > > > >
> > > > > Because amount of extra code you lose that way takes more memory than
> > > > > empty roots takes.
> > > > >
> > > > > Remove whatever files you don't need and be done with that.
> > > >
> > > > How do you remove files from the old rootfs after the new one has been
> > > > mounted on top of it?
> > >
> > > You do that before ;-)
> >
> > would the following work?
> >
> > pivot_root . /initramfs
> > cd /initramfs && rm -rf *
>
>
> No.  pivot_root() will not move the absolute root of tree elsewhere.
>
>
> > ?? doing it before is rather hard ... you apparently still need something to 
> > execute your mounts :)
>
>
> You do, but you can trivially call unlink() on the executable itself.  It
> will be freed after it does exec() of final /sbin/init...
>
> Alternatively, you could
> mkdir /root
> mount final root on /root
>
> chdir("/root");
> mount("/", "initramfs", NULL, MS_BIND, NULL);
> mount(".", "/", NULL, MS_MOVE, NULL);
> chroot(".");
> execve("/sbin/init", ...)
Nope. initramfs shares the superblock with 'rootfs', which has the 
MS_NOUSER flags set. Hence graft_tree() (which is the worker function 
for MS_BIND) refuses to work.
Can't we just remove the MS_NOUSER flags if initramfs is active?
Probably not the correct way, but certainly the quickest :-)
The correct way would probably be to clone the superblock of initramfs, 
set the filesystem-type of initramfs to 'ramfs' so that initramfs 
appears to be a chroot()ed filesystem like initrd. Then we could do a 
pivot_root and we have the contents of initramfs still available.
But needs someone with deeper fs-knowledge than myself to do it.

Cheers,

Hannes
--
Dr. Hannes Reinecke			hare@xxxxxxx
SuSE Linux AG				S390 & zSeries
Deutschherrnstr. 15-19			+49 911 74053 688
90429 Nürnberg				http://www.suse.de

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