Initramfs FSID altered in 3.14

[Dave Reisner <d@falconindy.com>]

Hi,

[This is a repost of a G+ post at Tejun's request]

With Linux 3.14, you might notice in /proc/self/mountinfo that your
root's parent FSID is now 0, instead of the 1 that it's been for the
last N years. Tejun wrote the change (9e30cc9595303b27b48) that caused
this, but the change comes in a rather innocuous way. Instead of an
internal kernel mount of sysfs being assigned 0, it's now the initramfs.

So far, this has already caused switch_root and findmnt (from
util-linux) to break, cp (from coreutils) to break when using the -x
flag in early userspace, and it's also been pointed out that systemd's
readahead code makes assumptions about a device number of 0.

Are we now supposed to go and change all the assumptions in userspace
about 0 being special? I'm conflicted. The kernel isn't supposed to
break userspace, but it seems to me that FSIDs were never something to
rely on -- similar to the block device numbering scheme.

Cheers,
Dave