[PATCH v2 4/7] Documentation/mm: Avoid invalid use of addresses from kmap_local_page()

[Fabio M. De Francesco]

Users of kmap_local_page() must be absolutely sure to not hand kernel
virtual address obtained calling kmap_local_page() on highmem pages to
other contexts because those pointers are thread local, therefore, they
are no longer valid across different contexts.

Extend the documentation of kmap_local_page() to warn users about the
above-mentioned potential invalid use of pointers returned by
kmap_local_page().

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 | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/vm/highmem.rst b/Documentation/vm/highmem.rst
index 34d7097d3ce8..71dc09563ff8 100644
--- a/Documentation/vm/highmem.rst
+++ b/Documentation/vm/highmem.rst
@@ -77,6 +77,13 @@ list shows them in order of preference of use.
   for pages which are known to not come from ZONE_HIGHMEM. However, it is
   always safe to use kmap_local_page() / kunmap_local().
 
+  While it is significantly faster than kmap(), for the higmem case it
+  comes with restrictions about the pointers validity. Contrary to kmap()
+  mappings, the local mappings are only valid in the context of the caller
+  and cannot be handed to other contexts. This implies that users must
+  be absolutely sure to keep the use of the return address local to the
+  thread which mapped it.
+
   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