Re: Overlayfs with writable lower layer

[Hao Luo <haoluo@google.com>]

On Wed, Sep 14, 2022 at 12:23 PM Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Wed, Sep 14, 2022 at 9:00 PM Hao Luo <haoluo@google.com> wrote:
> >
> > On Tue, Sep 13, 2022 at 8:46 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > >
> > > On Tue, Sep 13, 2022 at 11:33 PM Hao Luo <haoluo@google.com> wrote:
> > > >
> > > > On Tue, Sep 13, 2022 at 11:54 AM Amir Goldstein <amir73il@gmail.com> wrote:
[...]
> > > There are probably some other limitations at the moment
> > > related to pseudo filesystems that prevent them from being
> > > used as upper and/or lower fs in overlayfs.
> > >
> > > We will need to check what those limitations are and whether
> > > those limitations could be lifted for your specific use case.
> > >
> >
> > How can we approach this? Maybe I can send my patch that adds tmp dir,
> > tmp files and xattr, attr to upstream as RFC, so you can take a look?
> >
>
> I don't think I need your fs to test.
> The only thing special in this setup as far as I can tell is the dynamic
> cgroupfs (or cgroup2?) lower dirs.
>
> IIUC, everything worked for you except for oddities related to
> lower directories not appearing and not disappearing from the union.
> Is that correct? is that the only thing that you need a fix for?
>

Yes, that's correct.

> > > > Further, directory B could disappear from lower. When that happens, I
> > > > think there are two possible behaviors:
> > > >  - make 'file' disappear from union as well;
> > > >  - make 'file' and its directory accessible as well.
> > > >
> > > > In behavior 1, it will look like
> > > > $ tree union
> > > > .
> > > > └── A
> > > >     └── lower1
> > > >
> > > > In behavior 2, it will look like
> > > > $ tree union
> > > > .
> > > > └── A
> > > >     ├── B
> > > >     │   └── file
> > > >     └── lower1
> > > >
> > > > IMHO, behavior 1 works better in my use case. But if the FS experts
> > > > think behavior 2 makes more sense, I can work around.
> > > >
> > >
> > > Something that I always wanted to try is to get rid of the duplicated
> > > upper fs hierarchy.
> > >
> > > It's a bit complicated to explain the details, but if your use case
> > > does not involve any directory renames(?), then the upper path
> > > for the merge directories can be index based and not hierarchical.
> > >
> >
> > Yeah, I don't expect directory renaming. But I can't say if there is
> > anyone trying to do that by accident, or by bad intention.
> >
>
> Your fs will return an error for rename if you did not implement it.
>
> Anyway, if you can accept behavior 2, it is much more simple.
> This other idea is very vague and not simple, so better not risk it.
>
> If you confirm that you only need to get uptodate view of
> lower dirs in union, then I will look for the patches that I have
> and see if they can help you.
>

Yes, I acknowledge that behavior 2 works for me.