[PATCH 3/7] Documentation/mm: Don't kmap*() pages which can't come from HIGHMEM

[Fabio M. De Francesco]

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 allocated, for instance, with
alloc_page(GFP_NOFS) and other similar allocations.

Therefore, add a paragraph to highmem.rst, to explain better that a
plain page_address() should be used for getting the address of pages
which cannot come from ZONE_HIGHMEM.

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..f266354c82ab 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 should instead call a plain
+  page_address() for getting the address of memory pages which, depending
+  on the GFP_* flags, cannot come from ZONE_HIGHMEM.
+
   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