+ documentation-mm-prefer-kmap_local_page-and-avoid-kmap.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: Documentation/mm: rrefer kmap_local_page() and avoid kmap()
has been added to the -mm mm-unstable branch.  Its filename is
     documentation-mm-prefer-kmap_local_page-and-avoid-kmap.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/documentation-mm-prefer-kmap_local_page-and-avoid-kmap.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Subject: Documentation/mm: rrefer kmap_local_page() and avoid kmap()
Date: Thu, 28 Jul 2022 17:48:42 +0200

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 <fmdefrancesco@gmail.com>
Suggested-by: Ira Weiny <ira.weiny@intel.com>
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>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Peter Collingbourne <pcc@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 Documentation/mm/highmem.rst |    5 +++++
 1 file changed, 5 insertions(+)

--- a/Documentation/mm/highmem.rst~documentation-mm-prefer-kmap_local_page-and-avoid-kmap
+++ a/Documentation/mm/highmem.rst
@@ -84,6 +84,11 @@ list shows them in order of preference o
   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
_

Patches currently in -mm which might be from fmdefrancesco@gmail.com are

highmem-remove-unneeded-spaces-in-kmap_local_page-kdocs.patch
highmem-specify-that-kmap_local_page-is-callable-from-interrupts.patch
documentation-mm-dont-kmap-pages-which-cant-come-from-highmem.patch
documentation-mm-avoid-invalid-use-of-addresses-from-kmap_local_page.patch
documentation-mm-prefer-kmap_local_page-and-avoid-kmap.patch
highmem-delete-a-sentence-from-kmap_local_page-kdocs.patch
documentation-mm-add-details-about-kmap_local_page-and-preemption.patch