diff options
| author | Fabio M. De Francesco <[email protected]> | 2022-07-28 17:48:42 +0200 |
|---|---|---|
| committer | Andrew Morton <[email protected]> | 2022-08-08 18:06:45 -0700 |
| commit | 84b86f6054c42776c0d6482ab9a9071b247159fa (patch) | |
| tree | b41c9b5f3572b0b64cc06a27267d72bbab0a4a9f | |
| parent | 6b3afe2eeec27a148b83235484149f42b22ef793 (diff) | |
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/[email protected]
Signed-off-by: Fabio M. De Francesco <[email protected]>
Suggested-by: Ira Weiny <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Peter Collingbourne <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
| -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 |