[PATCH 1/3] mm/gup: documentation corrections for gup/pup
From: John Hubbard
Date: Sun Aug 08 2021 - 19:50:38 EST
The documentation for try_grab_compound_head() and try_grab_page() has
fallen a little out of date. Update and clarify a few points.
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
---
mm/gup.c | 18 ++++++++++++------
1 file changed, 12 insertions(+), 6 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c
index 77150624f77a..5cb18b62921c 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -103,8 +103,14 @@ static inline struct page *try_get_compound_head(struct page *page, int refs)
* same time. (That's true throughout the get_user_pages*() and
* pin_user_pages*() APIs.) Cases:
*
- * FOLL_GET: page's refcount will be incremented by 1.
- * FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
+ * FOLL_GET: page's refcount will be incremented by refs.
+ *
+ * FOLL_PIN on compound pages that are > two pages long: page's refcount will
+ * be incremented by refs, and page[2].hpage_pinned_refcount will be
+ * incremented by refs * GUP_PIN_COUNTING_BIAS.
+ *
+ * FOLL_PIN on normal pages, or compound pages that are two pages long:
+ * page's refcount will be incremented by refs * GUP_PIN_COUNTING_BIAS.
*
* Return: head page (with refcount appropriately incremented) for success, or
* NULL upon failure. If neither FOLL_GET nor FOLL_PIN was set, that's
@@ -143,6 +149,8 @@ __maybe_unused struct page *try_grab_compound_head(struct page *page,
*
* However, be sure to *also* increment the normal page refcount
* field at least once, so that the page really is pinned.
+ * That's why the refcount from the earlier
+ * try_get_compound_head() is left intact.
*/
if (hpage_pincount_available(page))
hpage_pincount_add(page, refs);
@@ -186,10 +194,8 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags)
* @flags: gup flags: these are the FOLL_* flag values.
*
* Either FOLL_PIN or FOLL_GET (or neither) may be set, but not both at the same
- * time. Cases:
- *
- * FOLL_GET: page's refcount will be incremented by 1.
- * FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
+ * time. Cases: please see the try_grab_compound_head() documentation, with
+ * "refs=1".
*
* Return: true for success, or if no action was required (if neither FOLL_PIN
* nor FOLL_GET was set, nothing is done). False for failure: FOLL_GET or
--
2.32.0