linux-kernel - [PATCH v2 3/7] Documentation/mm: Don't kmap*() pages which can't come from HIGHMEM

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Date:   Thu, 28 Jul 2022 17:48:40 +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 3/7] Documentation/mm: Don't kmap*() pages which can't come from HIGHMEM

There is no need to kmap*() pages which are guaranteed to come from
ZONE_NORMAL (or lower). Linux has currently several call sites of
kmap{,_atomic,_local_page}() on pages which are clearly known which
can't come from ZONE_HIGHMEM.

Therefore, add a paragraph to highmem.rst, to explain better that a
plain page_address() may be used for getting the address of pages
which cannot come from ZONE_HIGHMEM, although it is always safe to use
kmap_local_page() / kunmap_local() also on those pages.

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 | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Documentation/vm/highmem.rst b/Documentation/vm/highmem.rst
index c9887f241c6c..34d7097d3ce8 100644
--- a/Documentation/vm/highmem.rst
+++ b/Documentation/vm/highmem.rst
@@ -71,6 +71,12 @@ list shows them in order of preference of use.
   kmap_local_page() always returns a valid virtual address and it is assumed
   that kunmap_local() will never fail.
 
+  On CONFIG_HIGHMEM=n kernels and for low memory pages this returns the
+  virtual address of the direct mapping. Only real highmem pages are
+  temporarily mapped. Therefore, users may call a plain page_address()
+  for pages which are known to not come from ZONE_HIGHMEM. However, it is
+  always safe to use kmap_local_page() / kunmap_local().
+
   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