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/ 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 | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/Documentation/vm/highmem.rst b/Documentation/vm/highmem.rst index 50d2b1fac24a..564017b447b1 100644 --- a/Documentation/vm/highmem.rst +++ b/Documentation/vm/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 -- 2.37.1
