highmem: document kunmap_local()

Some users of kmap() add an offset to the kmap() address to be used
during the mapping.

When converting to kmap_local_page() the base address does not need to
be stored because any address within the page can be used in
kunmap_local().  However, this was not clear from the documentation and
cause some questions.[1]

Document that any address in the page can be used in kunmap_local() to
clarify this for future users.

[1] https://lore.kernel.org/lkml/20211213154543.GM3538886@iweiny-DESK2.sc.intel.com/

[ira.weiny@intel.com: updates per Christoph]
Link: https://lkml.kernel.org/r/20220124182138.816693-1-ira.weiny@intel.com
Link: https://lkml.kernel.org/r/20220124013045.806718-1-ira.weiny@intel.com
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/highmem-internal.h b/include/linux/highmem-internal.h
index 0a0b2b0..a77be56 100644 (file)
--- a/include/linux/highmem-internal.h
+++ b/include/linux/highmem-internal.h
@@ -246,6 +246,16 @@ do {                                                               \
        __kunmap_atomic(__addr);                                \
 } while (0)
 
+/**
+ * kunmap_local - Unmap a page mapped via kmap_local_page().
+ * @__addr: An address within the page mapped
+ *
+ * @__addr can be any address within the mapped page.  Commonly it is the
+ * address return from kmap_local_page(), but it can also include offsets.
+ *
+ * Unmapping should be done in the reverse order of the mapping.  See
+ * kmap_local_page() for details.
+ */
 #define kunmap_local(__addr)                                   \
 do {                                                           \
        BUILD_BUG_ON(__same_type((__addr), struct page *));     \