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| author | Fabio M. De Francesco <fmdefrancesco@gmail.com> | 2022-07-28 17:48:42 +0200 |
|---|---|---|
| committer | Andrew Morton <akpm@linux-foundation.org> | 2022-08-08 18:06:45 -0700 |
| commit | 84b86f6054c42776c0d6482ab9a9071b247159fa (patch) | |
| tree | b41c9b5f3572b0b64cc06a27267d72bbab0a4a9f /Documentation/mm | |
| parent | 6b3afe2eeec27a148b83235484149f42b22ef793 (diff) | |
| download | lwn-84b86f6054c42776c0d6482ab9a9071b247159fa.tar.gz lwn-84b86f6054c42776c0d6482ab9a9071b247159fa.zip | |
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>
Diffstat (limited to 'Documentation/mm')
| -rw-r--r-- | Documentation/mm/highmem.rst | 5 |
1 files changed, 5 insertions, 0 deletions
diff --git a/Documentation/mm/highmem.rst b/Documentation/mm/highmem.rst index 71dc09563ff8..e045a4b7b3da 100644 --- a/Documentation/mm/highmem.rst +++ b/Documentation/mm/highmem.rst @@ -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 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 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 |