Re: [PATCH 4/4] ovl: alllow remote upper

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On Fri, Jan 31, 2020 at 4:30 PM Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>
> On Fri, Jan 31, 2020 at 1:51 PM Miklos Szeredi <mszeredi@xxxxxxxxxx> wrote:
> >
> > No reason to prevent upper layer being a remote filesystem.  Do the
> > revalidation in that case, just as we already do for lower layers.
> >
>
> No reason to prevent upper layer from being a remote filesystem, but
> the !remote criteria for upper fs kept away a lot of filesystems from
> upper. Those filesystems have never been tested as upper and many
> of them are probably not fit for upper.
>
> The goal is to lift the !remote limitation, not to allow for lots of new
> types of upper fs's.
>
> What can we do to minimize damages?
>
> We can assert that is upper is remote, it must qualify for a more strict
> criteria as upper fs, that is:
> - support d_type
> - support xattr
> - support RENAME_EXCHANGE|RENAME_WHITEOUT
>
> I have a patch on branch ovl-strict which implements those restrictions.

Sounds good.  Not sure how much this is this going to be a
compatibility headache.  If it does, then we can conditionally enable
this with a config/module option.

>
> Now I know fuse doesn't support RENAME_WHITEOUT, but it does
> support RENAME_EXCHANGE, which already proves to be a very narrow
> filter for remote fs: afs, fuse, gfs2.
> Did not check if afs, gfs2 qualify for the rest of the criteria.
>
> Is it simple to implement RENAME_WHITEOUT for fuse/virtiofs?

Trivial.

> Is it not a problem to rely on an upper fs for modern systems
> that does not support RENAME_WHITEOUT?

Limited r/w overlay functionality is still available without
whiteout/xattr support, so it could turn out to be something people
already rely on.  Can't tell without trying...

Thanks,
Miklos



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