diff options
| author | Fabio M. De Francesco <[email protected]> | 2022-07-28 17:48:40 +0200 |
|---|---|---|
| committer | Andrew Morton <[email protected]> | 2022-08-08 18:06:45 -0700 |
| commit | 516ea046ec5558882b4a8e5f6b0c9b4c957db279 (patch) | |
| tree | a7ce490c6e2eb1771a982cfe75511fbcdf78b3af | |
| parent | 383bbef283920411379c5c93829102ff7859fea5 (diff) | |
Documentation/mm: don't kmap*() pages which can't come from HIGHMEM
There is no need to kmap*() pages which are guaranteed to come from
ZONE_NORMAL (or lower). Linux has currently several call sites of
kmap{,_atomic,_local_page}() on pages which are clearly known which can't
come from ZONE_HIGHMEM.
Therefore, add a paragraph to highmem.rst, to explain better that a plain
page_address() may be used for getting the address of pages which cannot
come from ZONE_HIGHMEM, although it is always safe to use
kmap_local_page() / kunmap_local() also on those pages.
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 | 6 |
1 files changed, 6 insertions, 0 deletions
diff --git a/Documentation/mm/highmem.rst b/Documentation/mm/highmem.rst index c9887f241c6c..34d7097d3ce8 100644 --- a/Documentation/mm/highmem.rst +++ b/Documentation/mm/highmem.rst @@ -71,6 +71,12 @@ list shows them in order of preference of use. kmap_local_page() always returns a valid virtual address and it is assumed that kunmap_local() will never fail. + On CONFIG_HIGHMEM=n kernels and for low memory pages this returns the + virtual address of the direct mapping. Only real highmem pages are + temporarily mapped. Therefore, users may call a plain page_address() + for pages which are known to not come from ZONE_HIGHMEM. However, it is + always safe to use kmap_local_page() / kunmap_local(). + 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 |