[PATCH 7/7] Documentation/mm: Add details about kmap_local_page() and preemption

["Fabio M. De Francesco" <fmdefrancesco@gmail.com>]

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/20201118204007.468533059@linutronix.de/

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Suggested-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
---
 Documentation/vm/highmem.rst | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/Documentation/vm/highmem.rst b/Documentation/vm/highmem.rst
index 564017b447b1..8ce43965ddef 100644
--- a/Documentation/vm/highmem.rst
+++ b/Documentation/vm/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.
 
-- 
2.37.1