RFC Split up struct nameidata (take 3)

In several places where no nameidata object is available, we are currently passing NULL instead of constructing a temporary nameidata object. This is because struct nameidata is big, and constructing temporary nameidata objects is not efficient. Here is another shot at splitting up struct nameidata: struct vfs_lookup is similar to struct nameidata but leaves out the fields required only while walking an actual pathname. It fits in 24 bytes as opposed to 80 on x86.

This allows to construct temporary vfs_lookup objects where needed instead of the NULL passing, which gets rid of the associated NULL checking, and ensures that struct vfsmnt and the lookup intent are passed down consistently.

In addition, the dentry and vfsmnt in struct vfs_lookup (before: struct nameidata) are put inside a struct path. This allows to use path{get,put} instead of mnt{get,put} plus d{get,put}, resulting in more consistent code. The path{get,put} functions seemingly were suggested by Erez Zadok / Josef 'Jeff' Sipek; Jan Blunck has been working on this recently as well.

This proposal consists of the following patches:

nameidata-split.diff

Introduce struct vfs_lookup and change struct nameidata to embed a struct vfs_lookup. (Requires the next patch to build.)

nameidata-split-2.diff

Update all nameidata users.

permission-args.diff

Switch from struct nameidata to struct vfs_lookup in the permission inode operation and related functions.

file_permission.diff

Create a temporary vfs_lookup object in file_permission().

sys_fchdir.diff
do_path_lookup.diff

Use vfs_permission() instead of file_permission().

create-args.diff

Switch from struct nameidata to struct vfs_lookup in the create inode operation and related functions.

vfs_create-nameidata.diff

Fix all users of vfs_create() to never pass it a NULL vfs_lookup. Get rid of the NULL checks.

d_revalidate-args.diff

Switch from struct nameidata to struct vfs_lookup in the d_revalidate dentry operation and related functions.

lookup-args.diff

Switch from struct nameidata to struct vfs_lookup in the lookup inode operation and related functions.

Ultimately, it would be nice to get rid of all the NULL nameidata / vfs_lookup passing, particularly to the permission inode operation. At that point, permission() could be replaced entirely by vfs_permission(). Other vfs functions like vfs_create() can be cleaned up in a similar way.

Andreas