diff options
| author | Fabio M. De Francesco <[email protected]> | 2022-07-28 17:48:41 +0200 |
|---|---|---|
| committer | Andrew Morton <[email protected]> | 2022-08-08 18:06:45 -0700 |
| commit | 6b3afe2eeec27a148b83235484149f42b22ef793 (patch) | |
| tree | d1e02a13185f2d1ab7cac003c33c7a811ccb69d7 | |
| parent | 516ea046ec5558882b4a8e5f6b0c9b4c957db279 (diff) | |
Documentation/mm: avoid invalid use of addresses from kmap_local_page()
Users of kmap_local_page() must be absolutely sure to not hand kernel
virtual address obtained calling kmap_local_page() on highmem pages to
other contexts because those pointers are thread local, therefore, they
are no longer valid across different contexts.
Extend the documentation of kmap_local_page() to warn users about the
above-mentioned potential invalid use of pointers returned by
kmap_local_page().
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 | 7 |
1 files changed, 7 insertions, 0 deletions
diff --git a/Documentation/mm/highmem.rst b/Documentation/mm/highmem.rst index 34d7097d3ce8..71dc09563ff8 100644 --- a/Documentation/mm/highmem.rst +++ b/Documentation/mm/highmem.rst @@ -77,6 +77,13 @@ list shows them in order of preference of use. for pages which are known to not come from ZONE_HIGHMEM. However, it is always safe to use kmap_local_page() / kunmap_local(). + While it is significantly faster than kmap(), for the higmem case it + comes with restrictions about the pointers validity. Contrary to kmap() + mappings, the local mappings are only valid in the context of the caller + and cannot be handed to other contexts. This implies that users must + be absolutely sure to keep the use of the return address local to the + thread which mapped it. + 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 |