From 516ea046ec5558882b4a8e5f6b0c9b4c957db279 Mon Sep 17 00:00:00 2001 From: "Fabio M. De Francesco" Date: Thu, 28 Jul 2022 17:48:40 +0200 Subject: [PATCH] Documentation/mm: don't kmap*() pages which can't come from HIGHMEM 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. Link: https://lkml.kernel.org/r/20220728154844.10874-4-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 | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/Documentation/mm/highmem.rst b/Documentation/mm/highmem.rst index c9887f241c6c..34d7097d3ce8 100644 --- a/Documentation/mm/highmem.rst +++ b/Documentation/mm/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