Document mounting options for casefold support in tmpfs.

Signed-off-by: André Almeida <andrealmeid@igalia.com>
---
 Documentation/filesystems/tmpfs.rst | 37 +++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/Documentation/filesystems/tmpfs.rst b/Documentation/filesystems/tmpfs.rst
index 56a26c843dbe..ce24fb16979a 100644
--- a/Documentation/filesystems/tmpfs.rst
+++ b/Documentation/filesystems/tmpfs.rst
@@ -241,6 +241,41 @@ So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
 will give you tmpfs instance on /mytmpfs which can allocate 10GB
 RAM/SWAP in 10240 inodes and it is only accessible by root.
 
+tmpfs has the following mounting options for case-insesitive lookups support:
+
+================= ==============================================================
+casefold          Enable casefold support at this mount point using the given
+                  argument as the encoding standard. Currently only utf8
+                  encodings are supported.
+strict_encoding   Enable strict encoding at this mount point (disabled by
+                  default). This means that invalid sequences will be rejected
+                  by the file system.
+================= ==============================================================
+
+Note that this option doesn't enable casefold by default; one needs to set
+casefold flag per directory, setting the +F attribute in an empty directory. New
+directories within a casefolded one will inherit the flag.
+
+Example::
+
+    $ mount -t tmpfs -o casefold=utf8-12.1.0,cf_strict fs_name /mytmpfs
+    $ cd /mytmpfs # case-sensitive by default
+    $ touch a; touch A
+    $ ls
+    A  a
+    $ mkdir B
+    $ cd b
+    cd: The directory 'b' does not exist
+    $ mkdir casefold_dir
+    $ chattr +F casefold_dir/ # marking it as case-insensitive
+    $ cd
+    $ touch dir/a; touch dir/A
+    $ ls dir
+    a
+    $ mkdir B
+    $ cd b
+    $ pwd
+    /home/user/mytmpfs/casefold_dir/B
 
 :Author:
    Christoph Rohland <cr@sap.com>, 1.12.01
@@ -250,3 +285,5 @@ RAM/SWAP in 10240 inodes and it is only accessible by root.
    KOSAKI Motohiro, 16 Mar 2010
 :Updated:
    Chris Down, 13 July 2020
+:Updated:
+   André Almeida, 23 Aug 2024
-- 
2.46.0

André Almeida <andrealmeid@igalia.com> writes:

> Document mounting options for casefold support in tmpfs.
>
> Signed-off-by: André Almeida <andrealmeid@igalia.com>
> ---
>  Documentation/filesystems/tmpfs.rst | 37 +++++++++++++++++++++++++++++
>  1 file changed, 37 insertions(+)
>
> diff --git a/Documentation/filesystems/tmpfs.rst b/Documentation/filesystems/tmpfs.rst
> index 56a26c843dbe..ce24fb16979a 100644
> --- a/Documentation/filesystems/tmpfs.rst
> +++ b/Documentation/filesystems/tmpfs.rst
> @@ -241,6 +241,41 @@ So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
>  will give you tmpfs instance on /mytmpfs which can allocate 10GB
>  RAM/SWAP in 10240 inodes and it is only accessible by root.
>  
> +tmpfs has the following mounting options for case-insesitive lookups support:

insensitive

> +
> +================= ==============================================================
> +casefold          Enable casefold support at this mount point using the given
> +                  argument as the encoding standard. Currently only utf8
> +                  encodings are supported.
> +strict_encoding   Enable strict encoding at this mount point (disabled by
> +                  default). This means that invalid sequences will be rejected

Invalid sequences is not clear. Perhaps:

In this mode, the filesystem refuses to create file and directory with
names containing invalid UTF-8 characters.


> +                  by the file system.
> +================= ==============================================================
> +
> +Note that this option doesn't enable casefold by default; one needs to set
> +casefold flag per directory, setting the +F attribute in an empty directory. New
> +directories within a casefolded one will inherit the flag.
> +
> +Example::
> +
> +    $ mount -t tmpfs -o casefold=utf8-12.1.0,cf_strict fs_name /mytmpfs

cf_strict should be strict_encoding.

I keep wondering if we should accept 'casefold' without any argument to
just mean the latest encoding version available.  Sure, that is a
problem for filesystems that can be moved between systems, but for tmpfs
that is not a problem.  It is cumbersome to specify the version and most
will just want the latest.

> +    $ cd /mytmpfs # case-sensitive by default
> +    $ touch a; touch A
> +    $ ls
> +    A  a
> +    $ mkdir B
> +    $ cd b
> +    cd: The directory 'b' does not exist
> +    $ mkdir casefold_dir
> +    $ chattr +F casefold_dir/ # marking it as case-insensitive
> +    $ cd
> +    $ touch dir/a; touch dir/A
> +    $ ls dir
> +    a
> +    $ mkdir B
> +    $ cd b
> +    $ pwd
> +    /home/user/mytmpfs/casefold_dir/B

I don't think we need this example,  since it is just generic
how case-insensitiveness work.

>  
>  :Author:
>     Christoph Rohland <cr@sap.com>, 1.12.01
> @@ -250,3 +285,5 @@ RAM/SWAP in 10240 inodes and it is only accessible by root.
>     KOSAKI Motohiro, 16 Mar 2010
>  :Updated:
>     Chris Down, 13 July 2020
> +:Updated:
> +   André Almeida, 23 Aug 2024

-- 
Gabriel Krisman Bertazi