Incorrect Overlayfs documentation

Incorrect Overlayfs documentation

[nerdopolis]

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?

Thanks

Re: Incorrect Overlayfs documentation

[Amir Goldstein]

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.

Re: Incorrect Overlayfs documentation

[nerdopolis]

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"
?

Thanks

Re: Incorrect Overlayfs documentation

[Amir Goldstein]

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.

Re: Incorrect Overlayfs documentation

[nerdopolis]

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.