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

[David Rientjes <rientjes@google.com>]

On Mon, 20 May 2013, Oskar Andero wrote:

> Add a BUG_ON to catch any illegal value from the shrinkers. This fixes a
> potential bug if scan_objects returns a negative other than -1, which
> 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..fbe6742 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;
> +		BUG_ON(ret < -1);
>  		freed += ret;
>  
>  		count_vm_events(SLABS_SCANNED, nr_to_scan);

Nack, this doesn't fix anything.  I can see the intention, and for that it 
might make sense to turn this into VM_BUG_ON() so that anybody debugging 
an issue related to this with CONFIG_DEBUG_VM would get the indication, 
but I don't think we need to enforce the API with BUG_ON().

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