linux-kernel - Re: [PATCH 0/6] overlayfs + casefolding

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Message-ID: <CAOQ4uxg9sKC_8PLARkN6aB3E_U62_S3kfnBuRbAvho9BNzGAsQ@mail.gmail.com>
Date: Tue, 20 May 2025 16:03:27 +0200
From: Amir Goldstein <amir73il@...il.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: linux-fsdevel@...r.kernel.org, linux-bcachefs@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-unionfs@...r.kernel.org, 
	Miklos Szeredi <miklos@...redi.hu>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH 0/6] overlayfs + casefolding

On Tue, May 20, 2025 at 2:43 PM Kent Overstreet
<kent.overstreet@...ux.dev> wrote:
>
> On Tue, May 20, 2025 at 02:40:07PM +0200, Amir Goldstein wrote:
> > On Tue, May 20, 2025 at 2:25 PM Kent Overstreet
> > <kent.overstreet@...ux.dev> wrote:
> > >
> > > On Tue, May 20, 2025 at 10:05:14AM +0200, Amir Goldstein wrote:
> > > > On Tue, May 20, 2025 at 7:16 AM Kent Overstreet
> > > > <kent.overstreet@...ux.dev> wrote:
> > > > >
> > > > > This series allows overlayfs and casefolding to safely be used on the
> > > > > same filesystem by providing exclusion to ensure that overlayfs never
> > > > > has to deal with casefolded directories.
> > > > >
> > > > > Currently, overlayfs can't be used _at all_ if a filesystem even
> > > > > supports casefolding, which is really nasty for users.
> > > > >
> > > > > Components:
> > > > >
> > > > > - filesystem has to track, for each directory, "does any _descendent_
> > > > >   have casefolding enabled"
> > > > >
> > > > > - new inode flag to pass this to VFS layer
> > > > >
> > > > > - new dcache methods for providing refs for overlayfs, and filesystem
> > > > >   methods for safely clearing this flag
> > > > >
> > > > > - new superblock flag for indicating to overlayfs & dcache "filesystem
> > > > >   supports casefolding, it's safe to use provided new dcache methods are
> > > > >   used"
> > > > >
> > > >
> > > > I don't think that this is really needed.
> > > >
> > > > Too bad you did not ask before going through the trouble of this implementation.
> > > >
> > > > I think it is enough for overlayfs to know the THIS directory has no
> > > > casefolding.
> > >
> > > overlayfs works on trees, not directories...
> >
> > I know how overlayfs works...
> >
> > I've explained why I don't think that sanitizing the entire tree is needed
> > for creating overlayfs over a filesystem that may enable casefolding
> > on some of its directories.
>
> So, you want to move error checking from mount time, where we _just_
> did a massive API rework so that we can return errors in a way that
> users will actually see them - to open/lookup, where all we have are a
> small fixed set of error codes?

That's one way of putting it.

Please explain the use case.

When is overlayfs created over a subtree that is only partially case folded?
Is that really so common that a mount time error justifies all the vfs
infrastructure involved?

Thanks,
Amir.