Message-ID: <CAOQ4uxjCmZkWd8x27hiLTU0tFA1N9+Nj9T5bviiDuF+Q4ANKvg@mail.gmail.com>
Date: Fri, 11 Jul 2025 11:08:17 +0200
From: Amir Goldstein <amir73il@...il.com>
To: André Almeida <andrealmeid@...lia.com>
Cc: Miklos Szeredi <miklos@...redi.hu>, Theodore Tso <tytso@....edu>, 
	Gabriel Krisman Bertazi <krisman@...nel.org>, linux-unionfs@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	kernel-dev@...lia.com
Subject: Re: [PATCH 0/3] ovl: Enable support for casefold filesystems

On Thu, Jul 10, 2025 at 10:54 PM André Almeida <andrealmeid@...lia.com> wrote:
>
> Hi Amir,
>
> Sorry for my delay.
>
> Em 09/04/2025 14:17, Amir Goldstein escreveu:
> > On Wed, Apr 9, 2025 at 5:01 PM André Almeida <andrealmeid@...lia.com> wrote:
> >>
> >> Hi all,
> >>
> >> We would like to support the usage of casefold filesystems with
> >> overlayfs. This patchset do some of the work needed for that, but I'm
> >> sure there are more places that need to be tweaked so please share your
> >> feedback for this work.
> >>
> >> * Implementation
> >>
> >> The most obvious place that required change was the strncmp() inside of
> >> ovl_cache_entry_find(), that I managed to convert to use d_same_name(),
> >
> > That's a very niche part of overlayfs where comparison of names matter.
> >
> > Please look very closely at ovl_lookup() and how an overlay entry stack is
> > composed from several layers including the option to redirect to different names
> > via redirect xattr, so there is really very much to deal with other
> > than readdir.
> >
> > I suggest that you start with a design proposal of how you intend to tackle this
> > task and what are your requirements?
> > Any combination of casefold supported layers?
> >
>
> The intended use case here is to use overlayfs as a container layer for
> games. The lower layer will have the common libraries required for
> games, and the upper layer will be a container for the running game, so
> the game will be able to have write permission and even change the
> common libraries if needed without impacting the original libraries. For
> that, we would use case-folded enable ext4 mounting points.
>
> This use case doesn't need layers redirection, or to combine different
> layers of enabled/disable case-fold. We would have just two layers,
> upper and lower, both with case-fold enabled prior to mounting. If the
> layers doesn't agree on the casefold flags/version/status, we can refuse
> mounting it.
>
> To avoid complexity and corner cases, I propose to have this feature
> enabled only for the layout described above: one upper and one lower
> layer, with both layers with the same casefold status and to refuse
> otherwise.
>
> The implementation would be, on top of this patchset, to create
> restrictions on the mounting options if casefold is enabled in a
> mounting point.
>
> Thoughts?
>

Good plan, but I don't think it is enough.

First of all take a look at this patch already queued for next:
https://lore.kernel.org/linux-unionfs/20250602171702.1941891-1-amir73il@gmail.com/

We will now check for ovl_dentry_casefolded() per dir on every lookup,
not only at mount time, so you need to rebase your patches and adjust the logic
to per-ovl-dir logic - all entries in ovl_stack need to have same casefold.

The second thing is that from vfs POV, your patches do not mark the overlayfs
dirs as casefolded and do not define d_compare()/d_hash() methods
for overlayfs. I think this is wrong and will result in odd behavior
w.r.t overlayfs
dcache.

I think that ovl_fill_super(), in the case where all layers are on same fs and
that fs sb_has_encoding(), assign ovl sb->s_encoding = upper_sb->s_encoding
and call generic_set_sb_d_ops().

As a matter of fact, I don't think we even need to restrict to all
layers on same
sb, just to all layers use the same sb->s_encoding and I think this
will be pretty
easy to implement on-the-fly.

Then in ovl_lookup() when composing the overlayfs stack for ovl_inode,
IFF ovl has_encoding make sure that all or none dentry on stack are
ovl_dentry_casefolded() and if they are casefolded, mark also the overlay
inode S_CASEFOLDED as well.

Please make sure to write fstests for the new functionality.
You can base you test on my WIP test for the patch that is in linux-next:
https://github.com/amir73il/xfstests/commits/ovl-casefold/

Your changes are going to change the results of my test
to allow lookup in a lower casefolded subdir
and so will need to change test expectation and add file name
case insensitive lookups to test this case.

Thanks,
Amir.

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