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| author | Fabio M. De Francesco <fmdefrancesco@gmail.com> | 2022-07-28 17:48:40 +0200 |
|---|---|---|
| committer | Andrew Morton <akpm@linux-foundation.org> | 2022-08-08 18:06:45 -0700 |
| commit | 516ea046ec5558882b4a8e5f6b0c9b4c957db279 (patch) | |
| tree | a7ce490c6e2eb1771a982cfe75511fbcdf78b3af /Documentation/mm | |
| parent | 383bbef283920411379c5c93829102ff7859fea5 (diff) | |
| download | lwn-516ea046ec5558882b4a8e5f6b0c9b4c957db279.tar.gz lwn-516ea046ec5558882b4a8e5f6b0c9b4c957db279.zip | |
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')
| -rw-r--r-- | Documentation/mm/highmem.rst | 6 |
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 |