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

[Aleksa Sarai <cyphar@cyphar.com>]

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@cyphar.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
+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).
+.\"
 .SS Extensibility
 In order to allow for future extensibility,
 .BR mount_setattr ()

-- 
2.51.0