[uml-user] Cannot move COW backing file

[<mj...@es...>]

Hi,

I cannot successfully move the backing file of a COW file. I am
following the instructions for moving the backing file given at:

http://user-mode-linux.sourceforge.net/shared_fs.html

e.g. I had
/somewhere/root_fs
/somewhere/root_fs_cow

starting um with:
./linux ubd0=/somewhere/root_fs_cow,/somewhere/root_fs
all works fine

Now move the root_fs
mv /somewhere/root_fs /new/root_fs

now start um:
./linux ubd0=/somewhere/root_fs_cow,/new/root_fs
....
Couldn't stat '/somewhere/root_fs', err = 2
Couldn't stat "/somewhere/root_fs" : err = 2
Failed to get modification time of backing file "/somewhere/root_fs", err = 2
Failed to open '/somewhere/root_fs_cow', errno = 2
VFS: Cannot open root device "98:0" or unknown-block(98,0)
Please append a correct "root=" boot option
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(98,0)

I checked that the root_fs system was not corrupted by reverting to
the original location
cp -a /new/root_fs /somewhere/root_fs

Now it works again using the first um invocation showing that the move
itself was not a problem.

The documentation implies that starting um with the new backing file
location updates the original COW file:

"UML will notice the mismatch between the command line and COW header,
check the size and mtime of the new backing file path, and update the
COW header to reflect it if it checks out."

Can anyone report if this actually works:

I am using a gentoo 2005.1 system with:
2.6.12-gentoo-r6-skas3-v8.2 host kernel
UM kernel is the same source tree (gentoo patched 2.6.12)
usermode-utilities-20040406

Martin
--
Dr. M.J. Reed                               Room:          1NW.5.3G
Dept. Electronic Systems Engineering         Tel:+44 (0)1206 872479
University of Essex, Colchester CO4 3SQ, UK  FAX:+44 (0)1206 872900
Email mjreed (non Essex users should add @essex.ac.uk)
Web: http://esewww.essex.ac.uk/~mjreed