[v3,05/10] docs: update mapping documentation

Commit Message

Christian Brauner Dec. 3, 2021, 11:17 a.m. UTC

From: Christian Brauner <[e-mail]>

Now that we implement the full remapping algorithms described in our
documentation remove the section about shortcircuting them.

Link: https://lore.kernel.org/r/[e-mail] (v1)
Link: https://lore.kernel.org/r/[e-mail] (v2)
Cc: Seth Forshee <[e-mail]>
Cc: Amir Goldstein <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: Al Viro <[e-mail]>
CC: [e-mail]
Reviewed-by: Seth Forshee <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>
---
/* v2 */
unchanged

/* v3 */
unchanged
---
 Documentation/filesystems/idmappings.rst | 72 ------------------------
 1 file changed, 72 deletions(-)

Patch

diff --git a/Documentation/filesystems/idmappings.rst b/Documentation/filesystems/idmappings.rst
index 1229a75ec75d..7a879ec3b6bf 100644
--- a/Documentation/filesystems/idmappings.rst
+++ b/Documentation/filesystems/idmappings.rst
@@ -952,75 +952,3 @@  The raw userspace id that is put on disk is ``u1000`` so when the user takes
 their home directory back to their home computer where they are assigned
 ``u1000`` using the initial idmapping and mount the filesystem with the initial
 idmapping they will see all those files owned by ``u1000``.
-
-Shortcircuting
---------------
-
-Currently, the implementation of idmapped mounts enforces that the filesystem
-is mounted with the initial idmapping. The reason is simply that none of the
-filesystems that we targeted were mountable with a non-initial idmapping. But
-that might change soon enough. As we've seen above, thanks to the properties of
-idmappings the translation works for both filesystems mounted with the initial
-idmapping and filesystem with non-initial idmappings.
-
-Based on this current restriction to filesystem mounted with the initial
-idmapping two noticeable shortcuts have been taken:
-
-1. We always stash a reference to the initial user namespace in ``struct
-   vfsmount``. Idmapped mounts are thus mounts that have a non-initial user
-   namespace attached to them.
-
-   In order to support idmapped mounts this needs to be changed. Instead of
-   stashing the initial user namespace the user namespace the filesystem was
-   mounted with must be stashed. An idmapped mount is then any mount that has
-   a different user namespace attached then the filesystem was mounted with.
-   This has no user-visible consequences.
-
-2. The translation algorithms in ``mapped_fs*id()`` and ``i_*id_into_mnt()``
-   are simplified.
-
-   Let's consider ``mapped_fs*id()`` first. This function translates the
-   caller's kernel id into a kernel id in the filesystem's idmapping via
-   a mount's idmapping. The full algorithm is::
-
-    mapped_fsuid(kid):
-      /* Map the kernel id up into a userspace id in the mount's idmapping. */
-      from_kuid(mount-idmapping, kid) = uid
-
-      /* Map the userspace id down into a kernel id in the filesystem's idmapping. */
-      make_kuid(filesystem-idmapping, uid) = kuid
-
-   We know that the filesystem is always mounted with the initial idmapping as
-   we enforce this in ``mount_setattr()``. So this can be shortened to::
-
-    mapped_fsuid(kid):
-      /* Map the kernel id up into a userspace id in the mount's idmapping. */
-      from_kuid(mount-idmapping, kid) = uid
-
-      /* Map the userspace id down into a kernel id in the filesystem's idmapping. */
-      KUIDT_INIT(uid) = kuid
-
-   Similarly, for ``i_*id_into_mnt()`` which translated the filesystem's kernel
-   id into a mount's kernel id::
-
-    i_uid_into_mnt(kid):
-      /* Map the kernel id up into a userspace id in the filesystem's idmapping. */
-      from_kuid(filesystem-idmapping, kid) = uid
-
-      /* Map the userspace id down into a kernel id in the mounts's idmapping. */
-      make_kuid(mount-idmapping, uid) = kuid
-
-   Again, we know that the filesystem is always mounted with the initial
-   idmapping as we enforce this in ``mount_setattr()``. So this can be
-   shortened to::
-
-    i_uid_into_mnt(kid):
-      /* Map the kernel id up into a userspace id in the filesystem's idmapping. */
-      __kuid_val(kid) = uid
-
-      /* Map the userspace id down into a kernel id in the mounts's idmapping. */
-      make_kuid(mount-idmapping, uid) = kuid
-
-Handling filesystems mounted with non-initial idmappings requires that the
-translation functions be converted to their full form. They can still be
-shortcircuited on non-idmapped mounts. This has no user-visible consequences.