[PATCH 5/7] Documentation/mm: Prefer kmap_local_page() and avoid kmap()

Fabio M. De Francesco
2022-07-21T21:02:46Z

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@linux-foundation.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Suggested-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
---
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