Re: [PATCH v2] mm: vmscan: add VM_BUG_ON on illegal return values from scan_objects

[Pekka Enberg <penberg@kernel.org>]

On Tue, May 21, 2013 at 10:13 AM, Oskar Andero
<oskar.andero@sonymobile.com> wrote:
> Add a VM_BUG_ON to catch any illegal value from the shrinkers. It's a
> potential bug if scan_objects returns a negative other than -1 and
> would lead to undefined behaviour.
>
> Cc: Glauber Costa <glommer@openvz.org>
> Cc: Dave Chinner <dchinner@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Oskar Andero <oskar.andero@sonymobile.com>
> ---
>  mm/vmscan.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 6bac41e..63fec86 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -293,6 +293,7 @@ shrink_slab_one(struct shrinker *shrinker, struct shrink_control *shrinkctl,
>                 ret = shrinker->scan_objects(shrinker, shrinkctl);
>                 if (ret == -1)
>                         break;
> +               VM_BUG_ON(ret < -1);

It seems to me relaxing the shrinker API restrictions and changing the
"ret == -1" to "ret < 0" would be a much more robust approach...

>                 freed += ret;
>
>                 count_vm_events(SLABS_SCANNED, nr_to_scan);
> --
> 1.8.1.5
>
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