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

Juan Quintela
2010-02-23T19:56:56Z

"Michael S. Tsirkin" <mst@redhat.com> 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 <quintela@redhat.com>

> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
> 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);