From: John Hubbard <jhubbard@xxxxxxxxxx> Subject: mm/gup: documentation corrections for gup/pup Patch series "A few gup refactorings and documentation updates", v3. While reviewing some of the other things going on around gup.c, I noticed that the documentation was wrong for a few of the routines that I wrote. And then I noticed that there was some significant code duplication too. So this fixes those issues. This is not entirely risk-free, but after looking closely at this, I think it's actually a useful improvement, getting rid of the code duplication here. This patch (of 3): The documentation for try_grab_compound_head() and try_grab_page() has fallen a little out of date. Update and clarify a few points. Also make it kerneldoc-correct, by adding @args documentation. Link: https://lkml.kernel.org/r/20210813044133.1536842-1-jhubbard@xxxxxxxxxx Link: https://lkml.kernel.org/r/20210813044133.1536842-2-jhubbard@xxxxxxxxxx Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: Christoph Hellwig <hch@xxxxxx> Cc: Heiko Carstens <hca@xxxxxxxxxxxxx> Cc: Vasily Gorbik <gor@xxxxxxxxxxxxx> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/gup.c | 27 ++++++++++++++++++++------- 1 file changed, 20 insertions(+), 7 deletions(-) --- a/mm/gup.c~mm-gup-documentation-corrections-for-gup-pup +++ a/mm/gup.c @@ -92,10 +92,17 @@ static inline struct page *try_get_compo return head; } -/* +/** * try_grab_compound_head() - attempt to elevate a page's refcount, by a * flags-dependent amount. * + * Even though the name includes "compound_head", this function is still + * appropriate for callers that have a non-compound @page to get. + * + * @page: pointer to page to be grabbed + * @refs: the value to (effectively) add to the page's refcount + * @flags: gup flags: these are the FOLL_* flag values. + * * "grab" names in this file mean, "look at flags to decide whether to use * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount. * @@ -103,8 +110,14 @@ static inline struct page *try_get_compo * 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 @@ -141,6 +154,8 @@ __maybe_unused struct page *try_grab_com * * 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); @@ -184,10 +199,8 @@ static void put_compound_head(struct pag * @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 _
