Documentation/mm/highmem.rst

18 The part of (physical) memory not covered by a permanent mapping is what we
64   These mappings are thread-local and CPU-local, meaning that the mapping
66   CPU while the mapping is active. Although preemption is never disabled by
68   CPU-hotplug until the mapping is disposed.
71   in which the local mapping is acquired does not allow it for other reasons.
82   virtual address of the direct mapping. Only real highmem pages are
95   therefore try to design their code to avoid the use of kmap() by mapping
107   NOTE: Conversions to kmap_local_page() must take care to follow the mapping
116   This permits a very short duration mapping of a single page.  Since the
117   mapping is restricted to the CPU that issued it, it performs well, but
133   NOTE: Conversions to kmap_local_page() must take care to follow the mapping
140   This should be used to make short duration mapping of a single page with no
141   restrictions on preemption or migration. It comes with an overhead as mapping
143   mapping is no longer needed, the address that the page was mapped to must be
146   Mapping changes must be propagated across all the CPUs. kmap() also
148   block when the mapping space is fully utilized until a slot becomes
151   All the above work is necessary if a mapping must last for a relatively
163 * vmap().  This can be used to make a long duration mapping of multiple
174 If CONFIG_HIGHMEM is not set, then the kernel will try and create a mapping
190   pageframes need to live in the permanent mapping, which means: