The documentation for try_grab_compound_head() and try_grab_page() has fallen a little out of date.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
