Re: [PATCH] VFS autmounter support

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

Followup to:  <6516.1055861757@warthog.warthog>
By author:    David Howells <dhowells@redhat.com>
In newsgroup: linux.dev.kernel
>
> 
> Hi Linus, Al,
> 
> The attached patch adds automounting support and mountpount expiry support to
> the VFS.
> 
> This patch involves the adding the following features:
> 
>  (1) A new dentry operation that (a) marks a dentry as being an automount
>      point, and (b) gets called by the VFS to come up with a vfsmount
>      structure which the VFS then stitches into the mount tree fabric at the
>      appropriate place.
> 
>  (2) A new lookup flag that is used by sys_*stat() to prevent automounting of
>      the path endpoint. This means "ls -l" in an automounter directory doesn't
>      cause a mount storm, but will display all the mountpoints in that
>      directory as subdirectories (either the underlying mountpoint dir or the
>      root dir of the mounted fs if the mountpoint has been triggered already).
> 
>  (3) do_kern_mount() is now exported.
> 
>  (4) The vfsmount structure has acquired, amongst other things, a timeout
>      field. If mntput() notices a vfsmount reach a usage count of 1, then the
>      vfsmount expiry time is set and the namespace that contains the vfsmount
>      has its expiration work chitty queued.
> 
>  (5) The namespace structure has acquired a work struct that is used to
>      actually perform vfsmount expiry under process context.
> 

This seems a bit heavyweight; although some VFS support is needed for
a complex filesystem, effectively doing it all in the kernel (#3)
seems a bit... excessive.

At least #2 can be done with existing means using follow_link.

I think using a revalidation pointer like dentries might be a better
way to do #4/#5, although using the existing one in the dentries is
probably better.

#1 isn't really clear to me what you're going for, but it seems to be
to duplicate bookkeeping.

I also don't see how this solves the biggest problems with complex
automounts, which are:

a) how to guarantee that a large mount tree can be safely destroyed;
b) how to detect partial unmounts.

	-hpa
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