linux-kernel - [PATCH v2 5/7] Documentation/mm: Prefer kmap_local

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Message-Id: <20220728154844.10874-6-fmdefrancesco@gmail.com>
Date:   Thu, 28 Jul 2022 17:48:42 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Mike Rapoport <rppt@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ira Weiny <ira.weiny@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Will Deacon <will@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Subject: [PATCH v2 5/7] Documentation/mm: Prefer 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/

Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
Cc: Mike Rapoport <rppt@...ux.ibm.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Thomas Gleixner <tglx@...utronix.de>
Suggested-by: Ira Weiny <ira.weiny@...el.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
---
 Documentation/vm/highmem.rst | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Documentation/vm/highmem.rst b/Documentation/vm/highmem.rst
index 71dc09563ff8..e045a4b7b3da 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