[PATCH v2] docs: filesystems: idmappings: clarify from where idmappings are taken

[Alexander Mikhalitsyn]

Let's clarify from where we take idmapping of each type:
- caller
- filesystem
- mount

Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-doc@xxxxxxxxxxxxxxx
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@xxxxxxxxxxxxx>
---
v2:
	- fixed https://lore.kernel.org/oe-kbuild-all/202306252253.qxHG1txo-lkp@xxxxxxxxx/
---
 Documentation/filesystems/idmappings.rst | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/filesystems/idmappings.rst b/Documentation/filesystems/idmappings.rst
index ad6d21640576..f3d168c9f0b9 100644
--- a/Documentation/filesystems/idmappings.rst
+++ b/Documentation/filesystems/idmappings.rst
@@ -373,6 +373,13 @@ kernel maps the caller's userspace id down into a kernel id according to the
 caller's idmapping and then maps that kernel id up according to the
 filesystem's idmapping.
 
+From the implementation point it's worth mentioning how idmappings are represented.
+All idmappings are taken from the corresponding user namespace.
+
+    - caller's idmapping (usually taken from ``current_user_ns()``)
+    - filesystem's idmapping (``sb->s_user_ns``)
+    - mount's idmapping (``mnt_idmap(vfsmnt)``)
+
 Let's see some examples with caller/filesystem idmapping but without mount
 idmappings. This will exhibit some problems we can hit. After that we will
 revisit/reconsider these examples, this time using mount idmappings, to see how
-- 
2.34.1