On 25.02.22 09:50, John Hubbard wrote:
> pin_user_page() is an externally-usable version of try_grab_page(), but
> with semantics that match get_page(), so that it can act as a drop-in
> replacement for get_page(). Specifically, pin_user_page() has a void
> return type.
>
> pin_user_page() elevates a page's refcount is using FOLL_PIN rules. This
> means that the caller must release the page via unpin_user_page().
>
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> ---
> include/linux/mm.h | 1 +
> mm/gup.c | 34 ++++++++++++++++++++++++++++++++++
> 2 files changed, 35 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 929488a47181..bb51f5487aef 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1914,6 +1914,7 @@ long pin_user_pages_remote(struct mm_struct *mm,
> long get_user_pages(unsigned long start, unsigned long nr_pages,
> unsigned int gup_flags, struct page **pages,
> struct vm_area_struct **vmas);
> +void pin_user_page(struct page *page);
> long pin_user_pages(unsigned long start, unsigned long nr_pages,
> unsigned int gup_flags, struct page **pages,
> struct vm_area_struct **vmas);
> diff --git a/mm/gup.c b/mm/gup.c
> index 5c3f6ede17eb..44446241c3a9 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -3034,6 +3034,40 @@ long pin_user_pages(unsigned long start, unsigned long nr_pages,
> }
> EXPORT_SYMBOL(pin_user_pages);
>
> +/**
> + * pin_user_page() - apply a FOLL_PIN reference to a page ()
> + *
> + * @page: the page to be pinned.
> + *
> + * Similar to get_user_pages(), in that the page's refcount is elevated using
> + * FOLL_PIN rules.
> + *
> + * IMPORTANT: That means that the caller must release the page via
> + * unpin_user_page().
> + *
> + */
> +void pin_user_page(struct page *page)
> +{
> + struct folio *folio = page_folio(page);
> +
> + WARN_ON_ONCE(folio_ref_count(folio) <= 0);
> +
> + /*
> + * Similar to try_grab_page(): be sure to *also*
> + * increment the normal page refcount field at least once,
> + * so that the page really is pinned.
> + */
> + if (folio_test_large(folio)) {
> + folio_ref_add(folio, 1);
> + atomic_add(1, folio_pincount_ptr(folio));
> + } else {
> + folio_ref_add(folio, GUP_PIN_COUNTING_BIAS);
> + }
> +
> + node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, 1);
> +}
> +EXPORT_SYMBOL(pin_user_page);
> +
> /*
> * pin_user_pages_unlocked() is the FOLL_PIN variant of
> * get_user_pages_unlocked(). Behavior is the same, except that this one sets
I assume that function will only get called on a page that has been
obtained by a previous pin_user_pages_fast(), correct?
--
Thanks,
David / dhildenb
