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

[Jan Kara]

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@xxxxxxxxxx>

Looks good. You can add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								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@xxxxxxxx>
SUSE Labs, CR