There is no need to kmap*() pages which are guaranteed to come from
ZONE_NORMAL (or lower). Linux has currently several call sites of
kmap{,_atomic,_local_page}() on pages which are clearly known which
can't come from ZONE_HIGHMEM.
Therefore, add a paragraph to highmem.rst, to explain better that a
plain page_address() may be used for getting the address of pages
which cannot come from ZONE_HIGHMEM, although it is always safe to use
kmap_local_page() / kunmap_local() also on those pages.
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>
---
Documentation/vm/highmem.rst | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/Documentation/vm/highmem.rst b/Documentation/vm/highmem.rst
index c9887f241c6c..34d7097d3ce8 100644
--- a/Documentation/vm/highmem.rst
+++ b/Documentation/vm/highmem.rst
@@ -71,6 +71,12 @@ list shows them in order of preference of use.
kmap_local_page() always returns a valid virtual address and it is assumed
that kunmap_local() will never fail.
+ On CONFIG_HIGHMEM=n kernels and for low memory pages this returns the
+ virtual address of the direct mapping. Only real highmem pages are
+ temporarily mapped. Therefore, users may call a plain page_address()
+ for pages which are known to not come from ZONE_HIGHMEM. However, it is
+ always safe to use kmap_local_page() / kunmap_local().
+
Nesting kmap_local_page() and kmap_atomic() mappings is allowed to a certain
extent (up to KMAP_TYPE_NR) but their invocations have to be strictly ordered
because the map implementation is stack based. See kmap_local_page() kdocs
--
2.37.1
