The patch titled
Subject: Documentation/mm: don't kmap*() pages which can't come from HIGHMEM
has been added to the -mm mm-unstable branch. Its filename is
documentation-mm-dont-kmap-pages-which-cant-come-from-highmem.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/documentation-mm-dont-kmap-pages-which-cant-come-from-highmem.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@xxxxxxxxx>
Subject: Documentation/mm: don't kmap*() pages which can't come from HIGHMEM
Date: Thu, 28 Jul 2022 17:48:40 +0200
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.
Link: https://lkml.kernel.org/r/20220728154844.10874-4-fmdefrancesco@xxxxxxxxx
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>
Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Peter Collingbourne <pcc@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/mm/highmem.rst | 6 ++++++
1 file changed, 6 insertions(+)
--- a/Documentation/mm/highmem.rst~documentation-mm-dont-kmap-pages-which-cant-come-from-highmem
+++ a/Documentation/mm/highmem.rst
@@ -71,6 +71,12 @@ list shows them in order of preference o
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
_
Patches currently in -mm which might be from fmdefrancesco@xxxxxxxxx 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
