Subject: Re: [PATCH net-next v11 08/12] virtio_net: Use existing classifier if possible

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 18 Nov 2025 16:55:55 -0500

On Tue, Nov 18, 2025 at 08:38:58AM -0600, Daniel Jurgens wrote:
> Classifiers can be used by more than one rule. If there is an existing
> classifier, use it instead of creating a new one.
> 
> Signed-off-by: Daniel Jurgens <[email-masked]>
> Reviewed-by: Parav Pandit <[email-masked]>
> Reviewed-by: Shahar Shitrit <[email-masked]>
> Reviewed-by: Xuan Zhuo <[email-masked]>
> ---
> v4:
>     - Fixed typo in commit message
>     - for (int -> for (
> 
> v8:
>     - Removed unused num_classifiers. Jason Wang
> ---
>  drivers/net/virtio_net.c | 40 +++++++++++++++++++++++++++-------------
>  1 file changed, 27 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index de1a23c71449..f392ea30f2c7 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -299,7 +299,6 @@ struct virtnet_ff {
>  	struct virtio_net_ff_cap_mask_data *ff_mask;
>  	struct virtio_net_ff_actions *ff_actions;
>  	struct xarray classifiers;
> -	int num_classifiers;
>  	struct virtnet_ethtool_ff ethtool;
>  };
>  
> @@ -6827,6 +6826,7 @@ struct virtnet_ethtool_rule {
>  /* The classifier struct must be the last field in this struct */
>  struct virtnet_classifier {
>  	size_t size;
> +	refcount_t refcount;
>  	u32 id;
>  	struct virtio_net_resource_obj_ff_classifier classifier;
>  };
> @@ -6920,11 +6920,24 @@ static bool validate_mask(const struct virtnet_ff *ff,
>  	return false;
>  }
>  
> -static int setup_classifier(struct virtnet_ff *ff, struct virtnet_classifier *c)
> +static int setup_classifier(struct virtnet_ff *ff,
> +			    struct virtnet_classifier **c)
>  {
> +	struct virtnet_classifier *tmp;
> +	unsigned long i;
>  	int err;
>  
> -	err = xa_alloc(&ff->classifiers, &c->id, c,
> +	xa_for_each(&ff->classifiers, i, tmp) {
> +		if ((*c)->size == tmp->size &&
> +		    !memcmp(&tmp->classifier, &(*c)->classifier, tmp->size)) {

note that classifier has padding bytes.
comparing these with memcmp is not safe, is it?

> +			refcount_inc(&tmp->refcount);
> +			kfree(*c);
> +			*c = tmp;
> +			goto out;
> +		}
> +	}
> +
> +	err = xa_alloc(&ff->classifiers, &(*c)->id, *c,
>  		       XA_LIMIT(0, le32_to_cpu(ff->ff_caps->classifiers_limit) - 1),
>  		       GFP_KERNEL);
>  	if (err)

what kind of locking prevents two threads racing in this code?

> @@ -6932,29 +6945,30 @@ static int setup_classifier(struct virtnet_ff *ff, struct virtnet_classifier *c)
>  
>  	err = virtio_admin_obj_create(ff->vdev,
>  				      VIRTIO_NET_RESOURCE_OBJ_FF_CLASSIFIER,
> -				      c->id,
> +				      (*c)->id,
>  				      VIRTIO_ADMIN_GROUP_TYPE_SELF,
>  				      0,
> -				      &c->classifier,
> -				      c->size);
> +				      &(*c)->classifier,
> +				      (*c)->size);
>  	if (err)
>  		goto err_xarray;
>  
> +	refcount_set(&(*c)->refcount, 1);

so you insert uninitialized refcount? can't another thread find it
meanwhile?

> +out:
>  	return 0;
>  
>  err_xarray:
> -	xa_erase(&ff->classifiers, c->id);
> +	xa_erase(&ff->classifiers, (*c)->id);
>  
>  	return err;
>  }
>  
> -static void destroy_classifier(struct virtnet_ff *ff,
> -			       u32 classifier_id)
> +static void try_destroy_classifier(struct virtnet_ff *ff, u32 classifier_id)
>  {
>  	struct virtnet_classifier *c;
>  
>  	c = xa_load(&ff->classifiers, classifier_id);
> -	if (c) {
> +	if (c && refcount_dec_and_test(&c->refcount)) {
>  		virtio_admin_obj_destroy(ff->vdev,
>  					 VIRTIO_NET_RESOURCE_OBJ_FF_CLASSIFIER,
>  					 c->id,

same questions about locking.

> @@ -6978,7 +6992,7 @@ static void destroy_ethtool_rule(struct virtnet_ff *ff,
>  				 0);
>  
>  	xa_erase(&ff->ethtool.rules, eth_rule->flow_spec.location);
> -	destroy_classifier(ff, eth_rule->classifier_id);
> +	try_destroy_classifier(ff, eth_rule->classifier_id);
>  	kfree(eth_rule);
>  }
>  
> @@ -7159,14 +7173,14 @@ static int build_and_insert(struct virtnet_ff *ff,
>  	if (err)
>  		goto err_key;
>  
> -	err = setup_classifier(ff, c);
> +	err = setup_classifier(ff, &c);
>  	if (err)
>  		goto err_classifier;
>  
>  	err = insert_rule(ff, eth_rule, c->id, key, key_size);
>  	if (err) {
>  		/* destroy_classifier will free the classifier */

will free is no longer correct, is it?

> -		destroy_classifier(ff, c->id);
> +		try_destroy_classifier(ff, c->id);
>  		goto err_key;
>  	}
>  
> -- 
> 2.50.1