Subject: Re: [PATCH 3/3] vhost: fix get_user_pages_fast error handling

From: Juan Quintela <[email-masked]>
Date: Tue, 23 Feb 2010 20:56:56 +0100

"Michael S. Tsirkin" <[email-masked]> wrote:
> get_user_pages_fast returns number of pages on success, negative value
> on failure, but never 0. Fix vhost code to match this logic.

It can return 0 if you ask for 0 pages :)
>>From the comment:

 * Returns number of pages pinned. This may be fewer than the number
 * requested. If nr_pages is 0 or negative, returns 0. If no pages
 * were pinned, returns -errno.
 */

I agree that code was wrong, but the BUG_ON() is not neccessary
IMHO. The important bit is the change in the comparison.

Reviewed-by: Juan Quintela <[email-masked]>

> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> ---
>  drivers/vhost/vhost.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index d4f8fdf..d003504 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -646,8 +646,9 @@ static int set_bit_to_user(int nr, void __user *addr)
>  	int bit = nr + (log % PAGE_SIZE) * 8;
>  	int r;
>  	r = get_user_pages_fast(log, 1, 1, &page);
> -	if (r)
> +	if (r < 0)
>  		return r;
> +	BUG_ON(r != 1);
>  	base = kmap_atomic(page, KM_USER0);
>  	set_bit(bit, base);
>  	kunmap_atomic(base, KM_USER0);