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Documentation/mm: don't kmap*() pages which can't come from HIGHMEM

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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>
This commit is contained in:
Fabio M. De Francesco 2022-07-28 17:48:40 +02:00 committed by Andrew Morton
parent 383bbef283
commit 516ea046ec
1 changed files with 6 additions and 0 deletions
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Documentation/mm/highmem.rst
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@ -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 kmap_local_page() always returns a valid virtual address and it is assumed
that kunmap_local() will never fail. 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 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 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 because the map implementation is stack based. See kmap_local_page() kdocs