-------- Original Message --------
Subject: Re: find_mount_root() issue
From: Duncan <1i5t5.dun...@cox.net>
To: <linux-btrfs@vger.kernel.org>
Date: 2014年08月31日 17:41
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*.
Interesting, as mentioned by Duncan, the normal situation is that initr*
root should be hidden/unmounted.
But when things out of the normal track, like Remco's problem, current
find_mount_root will only use
the first longest match, any further match will be ignored.
So (longest_matchlen <= len) is OK for this case.
On the other hand, (longest_matchlen <= len) will fail under the
following situation:
/dev/sdb on /mnt/test type btrfs (rw,relatime,space_cache)
/dev/sdc on /mnt/test type ext4 (rw,relatime,data=ordered)
To fix the behavior , the correct fix seems to only return the mount
entry with all the following condition:
1) longgest match
2) fs type is btrfs
If I am wrong, please tell me.
Thanks,
Qu
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