Re: [PATCH v3 06/12] mm/gup: require FOLL_GET for get_user_pages_fast()

Jan Kara
2020-02-03T14:18:17Z

On Fri 31-01-20 19:40:23, John Hubbard wrote:
> Internal to mm/gup.c, require that get_user_pages_fast()
> and __get_user_pages_fast() identify themselves, by setting
> FOLL_GET. This is required in order to be able to make decisions
> based on "FOLL_PIN, or FOLL_GET, or both or neither are set", in
> upcoming patches.
>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>

Looks good. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

> ---
> mm/gup.c | 19 +++++++++++++++++--
> 1 file changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 83473c2165f4..e899d2e6398c 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2390,6 +2390,14 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
> unsigned long len, end;
> unsigned long flags;
> int nr = 0;
> + /*
> + * Internally (within mm/gup.c), gup fast variants must set FOLL_GET,
> + * because gup fast is always a "pin with a +1 page refcount" request.
> + */
> + unsigned int gup_flags = FOLL_GET;
> +
> + if (write)
> + gup_flags |= FOLL_WRITE;
>
> start = untagged_addr(start) & PAGE_MASK;
> len = (unsigned long) nr_pages << PAGE_SHIFT;
> @@ -2415,7 +2423,7 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
> if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
> gup_fast_permitted(start, end)) {
> local_irq_save(flags);
> - gup_pgd_range(start, end, write ? FOLL_WRITE : 0, pages, &nr);
> + gup_pgd_range(start, end, gup_flags, pages, &nr);
> local_irq_restore(flags);
> }
>
> @@ -2454,7 +2462,7 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
> int nr = 0, ret = 0;
>
> if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
> - FOLL_FORCE | FOLL_PIN)))
> + FOLL_FORCE | FOLL_PIN | FOLL_GET)))
> return -EINVAL;
>
> start = untagged_addr(start) & PAGE_MASK;
> @@ -2521,6 +2529,13 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
> if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> return -EINVAL;
>
> + /*
> + * The caller may or may not have explicitly set FOLL_GET; either way is
> + * OK. However, internally (within mm/gup.c), gup fast variants must set
> + * FOLL_GET, because gup fast is always a "pin with a +1 page refcount"
> + * request.
> + */
> + gup_flags |= FOLL_GET;
> return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
> }
> EXPORT_SYMBOL_GPL(get_user_pages_fast);
> --
> 2.25.0
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR