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authorFabio M. De Francesco <fmdefrancesco@gmail.com>2022-07-28 17:48:40 +0200
committerAndrew Morton <akpm@linux-foundation.org>2022-08-08 18:06:45 -0700
commit516ea046ec5558882b4a8e5f6b0c9b4c957db279 (patch)
treea7ce490c6e2eb1771a982cfe75511fbcdf78b3af /Documentation/mm/highmem.rst
parent383bbef283920411379c5c93829102ff7859fea5 (diff)
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. Link: https://lkml.kernel.org/r/20220728154844.10874-4-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>
Diffstat (limited to 'Documentation/mm/highmem.rst')
-rw-r--r--Documentation/mm/highmem.rst6
1 files changed, 6 insertions, 0 deletions
diff --git a/Documentation/mm/highmem.rst b/Documentation/mm/highmem.rst
index c9887f241c6c..34d7097d3ce8 100644
--- a/Documentation/mm/highmem.rst
+++ b/Documentation/mm/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