linux-kernel - Re: [PATCH 0/3] ovl: Enable support for casefold filesystems

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Message-ID: <00854dc3-538b-4b62-953a-68d0b9ff2295@igalia.com>
Date: Thu, 10 Jul 2025 17:54:49 -0300
From: André Almeida <andrealmeid@...lia.com>
To: Amir Goldstein <amir73il@...il.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

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?

> Thanks,
> Amir.
>