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From: David Howells on 26 Jul 2010 10:30

Ian Kent <raven(a)themaw.net> wrote:

> > (4) Stops pathwalk at the automount point and returns that point in the fs
> > tree if it decides not to automount rather than reporting ELOOP (see its
> > use of EXDEV for this).

Does it make autofs easier if d_op->d_automount() is allowed to return -EXDEV
to request this? Then you can return it in Oz mode to allow the daemon to
see/use the underlying mountpoint without recursing back into d_automount().

Ideally, the daemon would use AT_NO_AUTOMOUNT, but there's no way to pass that
to sys_mount() or sys_umount().

David
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From: David Howells on 26 Jul 2010 12:00
Ian Kent <raven(a)themaw.net> wrote:

> > Does it make autofs easier if d_op->d_automount() is allowed to return
> > -EXDEV to request this? Then you can return it in Oz mode to allow the
> > daemon to see/use the underlying mountpoint without recursing back into
> > d_automount().
>
> Yes, it's really useful.

I think what's required, then, is if d_automount() returns -EXDEV then:

(1) If the dentry is terminal in the lookup path, then we just return -EXDEV
to indicate to __follow_mount() that we really do want to stop there.

(2) If the dentry is not terminal, then we convert the error to -EREMOTE to
indicate that we can't complete the pathwalk as one of the earlier
components is inaccessible.

See the attached patch.

David
---
diff --git a/fs/namei.c b/fs/namei.c
index c154112..6c385d4 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -653,8 +653,20 @@ static int follow_automount(struct path *path, unsigned flags, int res)
return -ELOOP;

mnt = path->dentry->d_op->d_automount(path);
- if (IS_ERR(mnt))
+ if (IS_ERR(mnt)) {
+ /*
+ * The filesystem is allowed to return -EXDEV here to indicate
+ * they don't want to automount. For instance, autofs would do
+ * this so that its userspace daemon can mount on this dentry.
+ *
+ * However, we can only permit this if it's a terminal point in
+ * the path being looked up; if it wasn't then the remainder of
+ * the path is inaccessible and we should say so.
+ */
+ if (PTR_ERR(mnt) == -EXDEV && (flags & LOOKUP_CONTINUE))
+ return -EREMOTE;
return PTR_ERR(mnt);
+ }
if (!mnt) /* mount collision */
return 0;


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From: David Howells on 27 Jul 2010 04:50
Ian Kent <raven(a)themaw.net> wrote:

> Is this something others need?

Not as far as I know... I think autofs is the only one doing out-of-kernel
automounting.

That doesn't mean it shouldn't be provided, though...

> Again, the exists vs not yet exists case for paths within indirect
> autofs mounts. At the moment I can just set the flag on all dentrys in
> the autofs fs and return EXDEV for non-empty directories in order to
> return the dentry as a path component. OTOH if the dentry is a mount
> embeded in the path and the mount fails we get a error return.

Seems redundant, but I'd say go with it for now. Maybe we can offload
S_AUTOMOUNT to the dentry.

> I could clear the flag on non-root parent dentrys during mkdir if this
> is needed by others.

I'm not sure that would actually matter, since it would come to
follow_automount() at the same place.

Note that someone who tries to stat() with AT_NO_AUTOMOUNT will cause the call
to d_automount() to be suppressed and will see the negative or non-mounted
directory. That might be okay for you.

David
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