Ian Kent wrote:
I am a little unclear about this. Which directory are you talking about? The
mount-over directory or the directory being mounted? My picture of things
(and I haven't verified this by doing anything mundane like reading the code)
is that There is a directory (such as /net ) which is of type autofs and as
needed new directories of type autofs are created under it for the various
hosts/filenames. Over these subdirectories (the mount-over directories) get
mounted the remote filesystems, usually of type NFS. Is this a correct
understanding?
Yep. That's an indirect mount.
OK. Got it.
The directories I'm refering to are the ones created inside the autofs
mount point /net or other autofs mount point. Creating the directories
makes them browsable without necessarily mounting them (as long as the
module knows when to trigger a mount request). This lazyness is what
causes all the fus.
Case 2 needs special attention. To achieve this I would need to have two
types of unhashed dentry, valid and invalid (perhaps because of a ENOENT
return from the daemon on mount).
If my understanding above is correct, I don't think you need to hide the
dentry for the autofs mount-over directory. If there is an active mount, then
the dentries d_mounted flag will be set and the normal mountpoint traversal
will work. If nothing is mounted here then the autofs mount-over directory
lookup functions will be called. This is where the actual mount request gets
triggered. The dentry created here should not be added to the dentry cache
until the dentry is actually ready to use. It has to be kept in a way that
can be found by subsequent calls to lookup in case there are multiple requests
for it. The trick is that the first lookup to succeed has to be the one for
the mount request. But once it is on the dentry hash chain, revalidate has to
be careful because if the revalidate fails then the dentry will be
invalidated. And if revalidate succeeds then everything needs to be setup so
that folow_down will work. Hmm. I will have to think about this some more.
That was how things used to work but late mounting is what's needed to
provide the function expected of an automounter when people start using
this in an enterprise environment. Basically autofs tells lies until it's
forced to tell the truth.
Point is people expect to be able to see the mount points without causing
them to mount until they actually try and access something inside the
mount.
Hence "browseable".
OK. This is a whole other kettle of fish.
This means that readdir lies and reports about the existance of
directories but doesn't mount them until someone is actually foolish
enough to include them in a pathname. That's the problem. Too many
fools. :^)
And thanks for the pointer to the paper.
Will
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