linux-kernel - Re: [PATCH v4 10/10] man/man2/{fsconfig,mount_setattr}.2: add note about attribute-parameter distinction

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Message-ID: <fkipds352mktqx42tsqfzrbr75zghs7wayb7udmlenxxq2i3yp@tho7msrqy5sc>
Date: Sun, 21 Sep 2025 11:59:01 +0200
From: Alejandro Colomar <alx@...nel.org>
To: Aleksa Sarai <cyphar@...har.com>
Cc: "Michael T. Kerrisk" <mtk.manpages@...il.com>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, Askar Safin <safinaskar@...omail.com>, 
	"G. Branden Robinson" <g.branden.robinson@...il.com>, linux-man@...r.kernel.org, linux-api@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	David Howells <dhowells@...hat.com>, Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v4 10/10] man/man2/{fsconfig,mount_setattr}.2: add note
 about attribute-parameter distinction

Hi Aleksa,

On Fri, Sep 19, 2025 at 11:59:51AM +1000, Aleksa Sarai wrote:
> This was not particularly well documented in mount(8) nor mount(2), and
> since this is a fairly notable aspect of the new mount API, we should
> probably add some words about it.
> 
> Signed-off-by: Aleksa Sarai <cyphar@...har.com>
> ---
>  man/man2/fsconfig.2      | 12 ++++++++++++
>  man/man2/mount_setattr.2 | 40 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 52 insertions(+)
> 
> diff --git a/man/man2/fsconfig.2 b/man/man2/fsconfig.2
> index 5a18e08c700ac93aa22c341b4134944ee3c38d0b..d827a7b96e08284fb025f94c3348a4acc4571b7d 100644
> --- a/man/man2/fsconfig.2
> +++ b/man/man2/fsconfig.2
> @@ -579,6 +579,18 @@ .SS Generic filesystem parameters
>  Linux Security Modules (LSMs)
>  are also generic with respect to the underlying filesystem.
>  See the documentation for the LSM you wish to configure for more details.
> +.SS Mount attributes and filesystem parameters
> +Some filesystem parameters
> +(traditionally associated with
> +.BR mount (8)-style
> +options)
> +have a sibling mount attribute
> +with superficially similar user-facing behaviour.
> +.P
> +For a description of the distinction between
> +mount attributes and filesystem parameters,
> +see the "Mount attributes and filesystem parameters" subsection of
> +.BR mount_setattr (2).
>  .SH CAVEATS
>  .SS Filesystem parameter types
>  As a result of
> diff --git a/man/man2/mount_setattr.2 b/man/man2/mount_setattr.2
> index b27db5b96665cfb0c387bf5b60776d45e0139956..f7d0b96fddf97698e36cab020f1d695783143025 100644
> --- a/man/man2/mount_setattr.2
> +++ b/man/man2/mount_setattr.2
> @@ -790,6 +790,46 @@ .SS ID-mapped mounts
>  .BR chown (2)
>  system call changes the ownership globally and permanently.
>  .\"
> +.SS Mount attributes and filesystem parameters
> +Some mount attributes
> +(traditionally associated with
> +.BR mount (8)-style
> +options)
> +have a sibling mount attribute
> +with superficially similar user-facing behaviour.
> +For example, the
> +.I -o ro

As said in another page, this should be

	.I \-o\~ro

> +option to
> +.BR mount (8)
> +can refer to the
> +"read-only" filesystem parameter,
> +or the "read-only" mount attribute.
> +Both of these result in mount objects becoming read-only,
> +but they do have different behaviour.
> +.P
> +The distinction between these two kinds of option is that
> +mount object attributes are applied per-mount-object
> +(allowing different mount objects
> +derived from a given filesystem instance
> +to have different attributes),
> +while filesystem instance parameters
> +("superblock flags" in kernel-developer parlance)
> +apply to all mount objects
> +derived from the same filesystem instance.
> +.P
> +When using
> +.BR mount (2),
> +the line between these two types of mount options was blurred.
> +However, with
> +.BR mount_setattr ()
> +and
> +.BR fsconfig (2),
> +the distinction is made much clearer.
> +Mount attributes are configured with
> +.BR mount_setattr (),
> +while filesystem parameters can be configured using
> +.BR fsconfig (2).
> +.\"

LGTM.

I've finished with the review of the patch set.  It's quite good.  :)


Have a lovely day!
Alex

>  .SS Extensibility
>  In order to allow for future extensibility,
>  .BR mount_setattr ()
> 
> -- 
> 2.51.0
> 
> 

-- 
<https://www.alejandro-colomar.es>
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