Messages in this thread Patch in this message | | | From | "Fabio M. De Francesco" <> | Subject | [PATCH v2 3/7] Documentation/mm: Don't kmap*() pages which can't come from HIGHMEM | Date | Thu, 28 Jul 2022 17:48:40 +0200 |
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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.
Cc: Andrew Morton <akpm@linux-foundation.org> 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> Suggested-by: Ira Weiny <ira.weiny@intel.com> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.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
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