[PATCH] highmem: Document kunmap_local()

[PATCH] highmem: Document kunmap_local()

[ira.weiny]

From: Ira Weiny <ira.weiny@intel.com>

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
caused 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/

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
---
 include/linux/highmem-internal.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/linux/highmem-internal.h b/include/linux/highmem-internal.h
index 0a0b2b09b1b8..fb2d3e033c01 100644
--- a/include/linux/highmem-internal.h
+++ b/include/linux/highmem-internal.h
@@ -246,6 +246,17 @@ do {								\
 	__kunmap_atomic(__addr);				\
 } while (0)
 
+/**
+ * kunmap_local - Unmap a page mapped via kmap_local_page().
+ * @__addr: An address within the page mapped
+ *
+ * __addr is often an address returned from kmap_local_page().  However,
+ * this address can be any address within the mapped page.  It does not need to
+ * be the exact address returned from kmap_local_page()
+ *
+ * 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 *));	\
-- 
2.31.1

Re: [PATCH] highmem: Document kunmap_local()

[Christoph Hellwig]

On Sun, Jan 23, 2022 at 05:30:44PM -0800, ira.weiny@intel.com wrote:
> + * __addr is often an address returned from kmap_local_page().  However,
> + * this address can be any address within the mapped page.  It does not need to
> + * be the exact address returned from kmap_local_page()
> + */

I'd word this a little different:

> + * __addr is often an address returned from kmap_local_page().  However,
> + * this address can be any address within the mapped page.  It does not need to
> + * be the exact address returned from kmap_local_page()
> + */

 * @__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.

But documenting this definitively is a good idea!

[PATCH V2] highmem: Document kunmap_local()

[ira.weiny]

From: Ira Weiny <ira.weiny@intel.com>

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/

Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>

---
Changes for V2
	Use suggested verbiage from Christoph Hellwig
---
 include/linux/highmem-internal.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/include/linux/highmem-internal.h b/include/linux/highmem-internal.h
index 0a0b2b09b1b8..a77be5630209 100644
--- 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 *));	\
-- 
2.31.1