Re: find_mount_root() issue

[Duncan <1i5t5.duncan@cox.net>]

Remco Hosman - Yerf IT.nl posted on Sun, 31 Aug 2014 09:37:38 +0200 as
excerpted:

> issue:
> on my system i have 2 entries for /, one with the type ‘rootfs’ and a
> 2nd one with the type ‘btrfs’. find_mount_root() uses the first one and
> reports a fail.
> 
> My change:
> if (longest_matchlen < len) {
> into:
> if (longest_matchlen <= len) {
> 
> i have not tested this, but in my understanding it will use the last
> longest match instead of the first.
> 
> I have no idea if this rootfs entry is normal nor if its always there
> before the ‘proper’ one.
> 
> These are the 2 entries in my mount list:
> rootfs / rootfs rw 0 0
> /dev/sda2 / btrfs rw,noatime,ssd,noacl,space_cache 0 0

AFAIK that rootfs entry is the kernel's built-in initramfs that it 
automatically mounts, even if empty, before mounting your real-root.

If you use an initramfs/initrd, the switch_root process normally hides/
unmounts the initr*, but if you don't and the kernel is using its empty 
one, nothing hides/unmounts it, so it's still there after the normal / is 
mounted over top.

At least, I always booted to root directly without an initr*, and always 
had a rootfs entry until relatively recently, presumably when I switched 
to a two-device btrfs real-rootfs, and had to create and use an initramfs 
to do so[1].  Now I don't have a rootfs entry any longer.

---
[1] Initr* required for multi-device btrfs root: Btrfs has the device= 
mount option, which would normally be passed via a kernel-command-line 
rootflags= option to btrfs, that can be used with multi-device-
filesystems in the absence of btrfs device scan.  Unfortunately, 
rootflags=device= fails for some reason, or at least did last time I 
tried it, so the only way I can get a multi-device btrfs root to mount is 
to use an initr*.

-- 
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"Every nonfree program has a lord, a master --
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