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| -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 |