Re: Is initramfs freed after kernel is booted?

[Hannes Reinecke <hare@suse.de>]

viro@parcelfarce.linux.theplanet.co.uk wrote:
> On Mon, Nov 17, 2003 at 09:33:59PM +0300, Andrey Borzenkov wrote:
> 
>>On Monday 17 November 2003 21:03, viro@parcelfarce.linux.theplanet.co.uk 
>>wrote:
>>
>>>On Mon, Nov 17, 2003 at 12:50:34PM -0500, Chris Friesen wrote:
>>>
>>>>viro@parcelfarce.linux.theplanet.co.uk 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@suse.de
SuSE Linux AG				S390 & zSeries
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