Re: Incorrect Overlayfs documentation

[nerdopolis <bluescreen_avenger@verizon.net>]

On Friday, January 1, 2021 10:17:08 PM EST Marco Nelissen wrote:
> On Tue, Jul 28, 2020 at 10:52 PM Amir Goldstein <amir73il@gmail.com> wrote:
> 
> > On Thu, Jul 23, 2020 at 6:51 AM nerdopolis
> > <bluescreen_avenger@verizon.net> wrote:
> > >
> > > On Thursday, July 16, 2020 1:03:52 PM EDT Amir Goldstein wrote:
> > > > On Thu, Jul 16, 2020 at 6:09 AM nerdopolis
> > > > <bluescreen_avenger@verizon.net> wrote:
> > > > >
> > > > > Hi
> > > > >
> > > > > A while back I opened up
> > https://bugzilla.kernel.org/show_bug.cgi?id=195113 describing a
> > documentation problem in
> > > > > https://www.kernel.org/doc/Documentation/filesystems/overlayfs.txt
> > but for whatever reason, it hasn't been seen.
> > > > >
> > > > >
> > > > > The problem is that it says "The lower filesystem can be any
> > filesystem supported by Linux"
> > > > > however, this is not the case, as Linux supports vfat, and vfat
> > doesn't work as a lower filesystem
> > > > >
> > > > > So there's no way to tell what filesystems are applicable for an
> > overlay lowerfs,
> > > > > and I don't think any existing userspace utilities can detect it.
> > > > >
> > > > > Could it be possible for the .txt file to be updated?
> > > > >
> > > >
> > > > The way it works usually in this project is you can submit a patch to
> > fix the
> > > > problem:
> > > > https://www.kernel.org/doc/html/v4.17/process/submitting-patches.html
> > > >
> > > > But if you don't want to go through that process, you can offer a text
> > to
> > > > fix documentation.
> > > >
> > > > But I myself cannot offer anything better than:
> > > > "The lower filesystem can be one of many filesystem supported by
> > Linux".
> > > >
> > > > I don't think that we want to start listing the supported filesystems
> > in
> > > > documentation.
> > > >
> > > > FWIW the description of upper fs isn't uptodate either.
> > > >
> > > > Thanks,
> > > > Amir.
> > > >
> > > >
> > > Hi.
> > >
> > > Yeah, that process might be cumbersome. I guess what you have is good.
> > > Maybe "A wide range of filesystems supported by Linux can be the lower
> > > filesystem, however, not all filesystems that are mountable by Linux
> > have the
> > > features needed for OverlayFS to work"
> > > ?
> > >
> >
> > That sounds good to me.
> >
> > CC the maintainer in case he wants to apply this documentation "patch"
> > himself.
> >
> > Thanks,
> > Amir.
> >
> 
> A documentation patch was submitted for this a few months ago (
> https://github.com/torvalds/linux/commit/58afaf5d605f091abf7491774e34fa29d4a1994c)
> however I think the sentence "A read-only overlay of two read-only
> filesystems may use any filesystem type." should also be removed or
> rewritten, since a read-only vfat fs still can't be used in an overlay as
> far as I can tell.
> 
Yeah, testing on a read-only vfat filesystem as the lowerdir and it doesn't 
work Something to do with dentry's and ovl_dentry_weird. I don't think 
there's any way to determine that with a userspace utility or not.