Re: [RFC] Union mounts/writable overlays design

[hooanon05@yahoo.co.jp]

Hi,

Valerie Aurora:
> Writable overlays (formerly union mounts)
> =========================================
> 
> In this document:
>  - Overview of writable overlays
>  - Terminology
>  - VFS implementation
	:::

While I don't remember exactly when I first read the source files of
UnionMount, I think it is promising. And I have written to Val and Jan
some of my comments or reviews about UnionMount.
Recently I noticed another issue about stat(2) and mountpoint(1). The
latter is a part of 'initscripts' package.

For example,
- you have a union-ed directory, /u = /rw + /ro
- /ro/usr dir exists
- /rw/usr dir does NOT exist
- of course, /u/usr exists

As far as I know, UnionMount is expected to handle /u/usr directory
as if it exists under /u dir.
(I may be wrong since it totally depends upon the design of UnionMount)

In this case, stat(2) for /u and /u/usr will return different st_dev
from each other. eg. stat(/u/usr) returns the st_dev value of /ro,
stat(/u) returns the one for /rw.
This behaviour may make /bin/mountpoint confused, particulary in the
chroot/switch_root-ed environment.
/bin/mountpoint issues stat(2) for the specified dir and its parent, and
compares their st_dev. If they differ from each other, the utility
handles the specified dir as a "mountpoint". I am afraid it will make
some init-scripts crazy because /u/usr is NOT a mountpoint actually.

One possible solution will be setting a hook to vfs_stat(), which
handles the vfsmount set UNION flag differently and returns the pseudo
st_dev for the entires in UnionMount. But it may lead to the duplicated
inode number situation which may make applications crazy.
For instance,
- /ro/fileA is hardlinked to /ro/fileB.
- the inode number of them is i100.
- /rw/fileC is handlinked to /rw/fileD.
- the inode number of them is i100 too.

Since /ro and /rw are different, the same inode number is not a
problem natively. But if UnionMount takes an approach above, they all
have the same st_dev value. And I am afraid some applications may
handle them as a single hardlink unexpectedly.

So UnionMount should maintain its inode numbers by itself?
No, it goes to the filesystem-type implementation. It should not be the
way of UnionMount.
Are there any ideas to solve this problem?


J. R. Okajima