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Documentation/mm: rrefer kmap_local_page() and avoid kmap() 

The reasoning for converting kmap() to kmap_local_page() was questioned
recently.[1]

There are two main problems with kmap(): (1) It comes with an overhead as
mapping space is restricted and protected by a global lock for
synchronization and (2) kmap() also requires global TLB invalidation when
its pool wraps and it might block when the mapping space is fully utilized
until a slot becomes available.

Warn users to avoid the use of kmap() and instead use kmap_local_page(),
by designing their code to map pages in the same context the mapping will
be used.

[1] https://lore.kernel.org/lkml/1891319.taCxCBeP46@opensuse/

Link: https://lkml.kernel.org/r/20220728154844.10874-6-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 17:48:42 +02:00 committed by Andrew Morton
parent 6b3afe2eee
commit 84b86f6054
1 changed files with 5 additions and 0 deletions
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Documentation/mm/highmem.rst
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@ -84,6 +84,11 @@ list shows them in order of preference of use.
be absolutely sure to keep the use of the return address local to the be absolutely sure to keep the use of the return address local to the
thread which mapped it. thread which mapped it.
Most code can be designed to use thread local mappings. User should
therefore try to design their code to avoid the use of kmap() by mapping
pages in the same thread the address will be used and prefer
kmap_local_page().
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