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

[Christian Brauner <brauner@kernel.org>]

On Sun, Jun 22, 2025 at 09:20:24AM +0200, Amir Goldstein wrote:
> On Mon, Jun 16, 2025 at 10:06 AM Amir Goldstein <amir73il@gmail.com> 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.
> >
> 
> Hi Christian,
> 
> I would like to let this change soak in next for 6.17.
> I can push to overlayfs-next, but since you have some changes on vfs.file,
> I wanted to consult with you first.
> 
> The changes are independent so they could go through different trees,
> but I don't like that so much, so I propose a few options.
> 
> 1. make vfs.file a stable branch, so I can base overlayfs-next on it
> 2. rename to vfs.backing_file and make stable
> 3. take this single ovl patch via your tree, as I don't currently have
>     any other ovl patches queued to 6.17

Let's start with 3. and switch to a stable branch on demand?