Re: [PATCH v3] ovl: support layers on case-folding capable filesystems

[Amir Goldstein <amir73il@gmail.com>]

On Mon, Jun 16, 2025 at 2:28 PM Kent Overstreet
<kent.overstreet@linux.dev> wrote:
>
> On Mon, Jun 16, 2025 at 10:06:32AM +0200, Amir Goldstein wrote:
> > On Sun, Jun 15, 2025 at 9:20 PM Kent Overstreet
> > <kent.overstreet@linux.dev> wrote:
> > >
> > > On Mon, Jun 02, 2025 at 07:17:02PM +0200, Amir Goldstein wrote:
> > > > Case folding is often applied to subtrees and not on an entire
> > > > filesystem.
> > > >
> > > > Disallowing layers from filesystems that support case folding is over
> > > > limiting.
> > > >
> > > > Replace the rule that case-folding capable are not allowed as layers
> > > > with a rule that case folded directories are not allowed in a merged
> > > > directory stack.
> > > >
> > > > Should case folding be enabled on an underlying directory while
> > > > overlayfs is mounted the outcome is generally undefined.
> > > >
> > > > Specifically in ovl_lookup(), we check the base underlying directory
> > > > and fail with -ESTALE and write a warning to kmsg if an underlying
> > > > directory case folding is enabled.
> > > >
> > > > Suggested-by: Kent Overstreet <kent.overstreet@linux.dev>
> > > > Link: https://lore.kernel.org/linux-fsdevel/20250520051600.1903319-1-kent.overstreet@linux.dev/
> > > > Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> > > > ---
> > > >
> > > > Miklos,
> > > >
> > > > This is my solution to Kent's request to allow overlayfs mount on
> > > > bcachefs subtrees that do not have casefolding enabled, while other
> > > > subtrees do have casefolding enabled.
> > > >
> > > > I have written a test to cover the change of behavior [1].
> > > > This test does not run on old kernel's where the mount always fails
> > > > with casefold capable layers.
> > > >
> > > > Let me know what you think.
> > > >
> > > > Kent,
> > > >
> > > > I have tested this on ext4.
> > > > Please test on bcachefs.
> > >
> > > Where are we at with getting this in? I've got users who keep asking, so
> > > hoping we can get it backported to 6.15
> >
> > I'm planning to queue this for 6.17, but hoping to get an ACK from Miklos first.
>
> This is a regression for bcachefs users, why isn't it being considered for
> 6.16?

This is an ovl behavior change on fs like ext4 regardless of bcachefs.
This change of behavior, which is desired for your users, could expose other
users to other regressions.
I am not sure that it is a clear cut candidate for 6.16,
but I also don't feel very strongly this way or the other, so I will
let Miklos decide.

In any case, even if the change gets merged in 6.17, after the change
was exposed to the world for a bit and no regressions reported,
I have no objections backporting it to older kernels.

Thanks,
Amir.