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 f3ba3c710ac5 ("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]>

0 files changed, 0 insertions, 0 deletions