[PATCH v3 hmm 09/12] mm/hmm: Poison hmm_range during unregister

[Christoph Hellwig]

> -	/* Sanity check this really should not happen. */
> -	if (hmm == NULL || range->end <= range->start)
> -		return;
> -
>  	mutex_lock(&hmm->lock);
>  	list_del_rcu(&range->list);
>  	mutex_unlock(&hmm->lock);
>  
>  	/* Drop reference taken by hmm_range_register() */
> -	range->valid = false;
>  	mmput(hmm->mm);
>  	hmm_put(hmm);
> -	range->hmm = NULL;
> +
> +	/*
> +	 * The range is now invalid and the ref on the hmm is dropped, so
> +         * poison the pointer.  Leave other fields in place, for the caller's
> +         * use.
> +         */
> +	range->valid = false;
> +	memset(&range->hmm, POISON_INUSE, sizeof(range->hmm));

Formatting seems to be messed up.  But again I don't see the value
in the poisoning, just let normal linked list debugging do its work.
The other cleanups looks fine to me.