From 84b86f6054c42776c0d6482ab9a9071b247159fa Mon Sep 17 00:00:00 2001 From: "Fabio M. De Francesco" Date: Thu, 28 Jul 2022 17:48:42 +0200 Subject: Documentation/mm: rrefer kmap_local_page() and avoid kmap() The reasoning for converting kmap() to kmap_local_page() was questioned recently.[1] There are two main problems with kmap(): (1) It comes with an overhead as mapping space is restricted and protected by a global lock for synchronization and (2) kmap() also requires global TLB invalidation when its pool wraps and it might block when the mapping space is fully utilized until a slot becomes available. Warn users to avoid the use of kmap() and instead use kmap_local_page(), by designing their code to map pages in the same context the mapping will be used. [1] https://lore.kernel.org/lkml/1891319.taCxCBeP46@opensuse/ Link: https://lkml.kernel.org/r/20220728154844.10874-6-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 | 5 +++++ 1 file changed, 5 insertions(+) (limited to 'Documentation/mm') diff --git a/Documentation/mm/highmem.rst b/Documentation/mm/highmem.rst index 71dc09563ff8..e045a4b7b3da 100644 --- a/Documentation/mm/highmem.rst +++ b/Documentation/mm/highmem.rst @@ -84,6 +84,11 @@ list shows them in order of preference of use. be absolutely sure to keep the use of the return address local to the thread which mapped it. + Most code can be designed to use thread local mappings. User should + therefore try to design their code to avoid the use of kmap() by mapping + pages in the same thread the address will be used and prefer + kmap_local_page(). + 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