Re: [PATCH 0/9] [RFC v2] safely drop directory dentry on failed revalidate

[Ian Kent <raven@themaw.net>]

On Fri, 2013-08-30 at 07:24 +0800, Ian Kent wrote:
> On Thu, 2013-08-29 at 05:51 +0200, Miklos Szeredi wrote:
> > Ian,
> > 
> > I'm having problems fully understanding what autofs4 is trying to do
> > with have_submounts().
> 
> OK, I don't really care how I do it so I'm happy to change.
> 
> > 
> > 
> > > On Wed, 2013-08-21 at 06:40 +0100, Al Viro wrote:
> > 
> > > fs/autofs4/dev-ioctl.c:542:             err = have_submounts(path.dentry);
> > 
> > This is an ioctl() asking whether we have anything mounted on the autofs
> > mount.  Using have_submounts() and then a separate follow_down() looks
> > racy.  have_submounts() could succeed and then follow_down() could fail.
> > Or the other way round.  Shouldn't the two cases be handled separately
> > here?  If the autofs is a just a simple trigger then use follow_down().
> > If it's a multi-mount thing, then use have_submounts().
> 
> Right but IIRC I don't think I actually use the returned s_magic ATM but
> I use the return of have_submounts() a lot.
> 
> > 
> > What is the userspace automount daemon using this for?  Do we really
> > need the recursive check for submounts?
> > 
> > 
> > > fs/autofs4/root.c:381:                  if (have_submounts(dentry)) {
> > 
> > Here it explicitly says it's for v5 and for rootless mutli-mount.  So
> > for example:
> > 
> > /mnt/auto/            root of an indirect mount
> 
> or the root of direct mount for that matter.
> 
> > /mnt/auto/foo         directory with DCACHE_NEED_AUTOMOUNT
> > /mnt/auto/foo/bar     directory without DCACHE_NEED_AUTOMOUNT
> > /mnt/auto/foo/bar/baz directory with an automount trigger mounted on it
> > 
> > In this case when d_automount for "foo" is called we don't call the
> > userspace daemon because things are mounted under foo.  If there was no
> > trigger under baz, then we would try to handle "foo" as an indirect
> > mount and call userspace.
> > 
> > But it's pretty confusing.  Do we really *ever* need to call automount
> > on "foo" if it was part of a multi-mount thing? 
> 
> That's right, the directory isn't simple_empty() so there's no callback.
> 
> The problem is we can't just use the fact that the directory is empty to
> determine that there are no mounts at all underneath.
> 
> I understand your thinking, about deciding whether to callback to the
> daemon, but that's not what the ioctl above is used for.
> 
> The main use is to be able to find out if the given directory is a
> mountpoint as defined by the description in the comment above the
> function. This saves having to scan the mount table to find out and is a
> huge saving on systems with lots of mounts. In the past I've often
> needed an answer the question "is this an autofs mount or some other
> type" and that's why I stick s_magic in the return as well.
> 
> > 
> > > fs/autofs4/waitq.c:338:         if (have_submounts(dentry))
> > 
> > And here we re-validate the thing after taking another autofs4 lock.
> > Why this double checking?
> 
> This is a different case and is often not in play at times when autofs
> is checking if the directory is a "mountpoint". Such as when trying to
> re-construct a tree of mounts at startup.
> 
> The check in waitq.c above "is" used to validate the need to callback to
> the daemon to request a mount.
> 
> As I said, any suggestions how to achieve this without calling
> have_submounts() are welcome.

You know, may_umount_tree() would do this for me (I think) and would be
much less expensive ....

> 
> Ian
>