Documentation/mm: add details about kmap_local_page() and preemption

What happens if a thread is preempted after mapping pages with
kmap_local_page() was questioned recently.[1]

Commit f3ba3c7 ("mm/highmem: Provide kmap_local*") from Thomas
Gleixner explains clearly that on context switch, the maps of an outgoing
task are removed and the map of the incoming task are restored and that
kmap_local_page() can be invoked from both preemptible and atomic
contexts.[2]

Therefore, for the purpose to make it clearer that users can call
kmap_local_page() from contexts that allow preemption, rework a couple of
sentences and add further information in highmem.rst.

[1] https://lore.kernel.org/lkml/5303077.Sb9uPGUboI@opensuse/
[2] https://lore.kernel.org/all/[email protected]/

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Fabio M. De Francesco <[email protected]>
Suggested-by: Ira Weiny <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Peter Collingbourne <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: xp4ns3

Documentation/mm/highmem.rst

@@ -60,14 +60,19 @@ list shows them in order of preference of use.
   This function should be preferred, where feasible, over all the others.
 
   These mappings are thread-local and CPU-local, meaning that the mapping
-  can only be accessed from within this thread and the thread is bound the
-  CPU while the mapping is active. Even if the thread is preempted (since
-  preemption is never disabled by the function) the CPU can not be
-  unplugged from the system via CPU-hotplug until the mapping is disposed.
+  can only be accessed from within this thread and the thread is bound to the
+  CPU while the mapping is active. Although preemption is never disabled by
+  this function, the CPU can not be unplugged from the system via
+  CPU-hotplug until the mapping is disposed.
 
   It's valid to take pagefaults in a local kmap region, unless the context
   in which the local mapping is acquired does not allow it for other reasons.
 
+  As said, pagefaults and preemption are never disabled. There is no need to
+  disable preemption because, when context switches to a different task, the
+  maps of the outgoing task are saved and those of the incoming one are
+  restored.
+
   kmap_local_page() always returns a valid virtual address and it is assumed
   that kunmap_local() will never fail.