From 6b3afe2eeec27a148b83235484149f42b22ef793 Mon Sep 17 00:00:00 2001 From: "Fabio M. De Francesco" Date: Thu, 28 Jul 2022 17:48:41 +0200 Subject: Documentation/mm: avoid invalid use of addresses from kmap_local_page() 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kmap_local_page() on highmem pages to other contexts because those pointers are thread local, therefore, they are no longer valid across different contexts.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(). Link: https://lkml.kernel.org/r/20220728154844.10874-5-fmdefrancesco@gmail.com Signed-off-by: Fabio M. De Francesco Suggested-by: Ira Weiny Cc: Matthew Wilcox (Oracle) Cc: Mike Rapoport Cc: Sebastian Andrzej Siewior Cc: Thomas Gleixner Cc: Catalin Marinas Cc: Jonathan Corbet Cc: Peter Collingbourne Cc: Vlastimil Babka Cc: Will Deacon Signed-off-by: Andrew Morton --- Documentation/mm/highmem.rst | 7 +++++++ 1 file changed, 7 insertions(+) (limited to 'Documentation/mm') diff --git a/Documentation/mm/highmem.rst b/Documentation/mm/highmem.rst index 34d7097d3ce8..71dc09563ff8 100644 --- a/Documentation/mm/highmem.rst +++ b/Documentation/mm/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 -- cgit v1.2.3