Re: [PATCH v6 07/12] vfs: add O_CREAT|O_DIRECTORY to open*(2)

[Amir Goldstein <amir73il@gmail.com>]

On Thu, Oct 1, 2026 at 6:23 PM Jori Koolstra <jkoolstra@xs4all.nl> wrote:
>
>
> > Op 01-10-2026 17:59 CEST schreef Amir Goldstein <amir73il@gmail.com>:
> >
> >
> > > > Maybe I am missing something, but I think you misunderstand me.
> > > > What I mean is - if directory inode has a ->atomic_open() op,
> > > > bail early with -EINVAL/-EOPNOTSUPP, because this is a network
> > > > filesystem that does not support atomic O_CREATE|O_DIRECTORY
> > > > and in most likelihood never will support it.
> > > >
> > > > This gating criteria is not dependent on cache state,
> > > > which is what we wanted.
> > > >
> > >
> > > No in that case I think I've understood you (or maybe still not?) My
> > > point is that we can't do that if we want to be able to individually
> > > support O_CREAT|O_DIRECTORY for some ->atomic_open fs. If we bail early
> > > the how can say only NFS support it at some point? You get into the
> > > situation where everybody needs to have support or no one.
> > >
> > > Does that make sense, or do I still misunderstand you point?
> > >
> >
> > If we gate on an existing criteria now like ->atomic_open()
> > and/or ->d_revalidate(), we keep semantics simple and it
> > does not limit us to change the criteria in the future.
> >
> > But I don't insist on this criteria - it's just a suggestion.
> >
>
> Oh no, I don't think it's a bad idea.

It probably is though..

> I was just trying to understand.
> And like I mentioned, I don't understand the detail of ->d_revalidate()
> usage.
>
> What do you think about Neil's suggestion to keep ->atomic_open out
> of directory creation for the time being with:
>
> 1. use ->atomic_open(O_DIRECTORY) for lookup, no O_CREAT bit passed in.
> 2. if positive then done, otherwise (on -ENOENT from atomic_open()) we
>    call ->mkdir
> 3. open that dentry in the regular way, WARN_ON negative dentry return
>

I don't know enough about network filesystems to say, but it sounds fishy.
I think we want consistent defined behavior for O_CREAT|O_DIRECTORY
for filesystems that support it and my understanding is that this
suggestion is not going in that direction?

> But I don't know if the kind of atomicity I care about is affected by
> ->d_revalidate() filesystems.

It is not directly related. It is circumstantially related.
->d_revalidate() *could* mean, in the worst case, that holding the ref
on the dentry gives no guarantee on keeping the object behind it alive.
In the worst case, maybe a new object with same inode number could
be created in the same name.

I am not sure this can happen in many filesystems for real, but I
am pretty sure that it can happen with FUSE.

> With O_CREAT|O_DIRECTORY we just want provide
> the mechanism to ensure that the process actually did the create you get
> the fd for.

Yes. And I don't know if you can get this guarantee from all network
filesystems. Requires an audit.

Thanks,
Amir.