Sick VFS question

["H. Peter Anvin" <hpa@zytor.com>]

Hi everyone,

I'm considering new ideas for autofs v5, and I have a sick question:
would the VFS barf completely if autofs would take a dentry belonging
to another filesystem and replace the inode pointer with a pointer to
an inode belonging to itself (obviously incrementing the counter on
that inode appropriately)?  This would in particular be applicable to
"mount pads", i.e. let's say /auto is an autofs, and /auto/foo is an
autofs mount key, containing two filesystems: /auto/foo and
/auto/foo/bar. After mounting /auto/foo, autofs would access the "bar"
directory in the "foo" filesystem, and replace that inode with an
internal autofs inode.
This internal inode would have a follow_link(!) method which would
cause the /auto/foo/bar mount to happen and then redirect the search
to the topmost directory.

Ideally, this should not impede a umount of the /auto/foo directory if
the /auto/foo/bar directory is not mounted.

Using follow_link this way solves a whole lot of problems in autofs,
both in terms of usability and atomicity.  It solves the same problem
the dentry traps was supposed to solve, but also avoids the "mount
storm" problem by allowing lstat() without triggering a mount.

I would appreciate comments...

    -hpa
-- 
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