With aliasing VIPT cache support, the ARM implementation of
clear_user_page() and copy_user_page() sets up a temporary kernel space
mapping such that we have the same cache colour as the userspace page.
This avoids having to consider any userspace aliases from this operation.

However, when highmem is enabled, kmap_atomic() have to setup mappings.
The copy_user_highpage() and clear_user_highpage() call these functions
before delegating the copies to copy_user_page() and clear_user_page().

The effect of this is that each of the *_user_highpage() functions setup
their own kmap mapping, followed by the *_user_page() functions setting
up another mapping.  This is rather wasteful.

Thankfully, copy_user_highpage() can be overriden by architectures by
defining __HAVE_ARCH_COPY_USER_HIGHPAGE.  However, replacement of
clear_user_highpage() is more difficult because its inline definition
is not conditional.  It seems that you're expected to define
__HAVE_ARCH_A...