Re: [PATCH net v2] xen-netback: Fix handling of skbs requiring too many slots

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Zoltan Kiss
> Sent: 04 June 2014 19:59
> To: xen-devel@lists.xenproject.org; Ian Campbell; Wei Liu; Paul Durrant;
> linux@eikelenboom.it
> Cc: netdev@vger.kernel.org; David Vrabel; davem@davemloft.net; Zoltan
> Kiss
> Subject: [PATCH net v2] xen-netback: Fix handling of skbs requiring too many
> slots
> 
> A recent commit (a02eb4 "xen-netback: worse-case estimate in
> xenvif_rx_action is
> underestimating") capped the slot estimation to MAX_SKB_FRAGS, but that
> triggers
> the next BUG_ON a few lines down, as the packet consumes more slots than
> estimated.
> This patch introduces full_coalesce on the skb callback buffer, which is used
> in
> start_new_rx_buffer() to decide whether netback needs coalescing more
> aggresively. By doing that, no packet should need more than
> (XEN_NETIF_MAX_TX_SIZE + 1) / PAGE_SIZE data slots (excluding the
> optional GSO
> slot, it doesn't carry data, therefore irrelevant in this case), as the provided
> buffers are fully utilized.
> 
> Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
> Cc: Paul Durrant <paul.durrant@citrix.com>

Reviewed-by: Paul Durrant <paul.durrant@gmail.com>

> Cc: Wei Liu <wei.liu2@citrix.com>
> Cc: Ian Campbell <ian.campbell@citrix.com>
> Cc: David Vrabel <david.vrabel@citrix.com>
> ---
> v2:
> - clarify that GSO slot doesn't count here
> - fix style
> 
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-
> netback/netback.c
> index 64ab1d1..6a21588 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -163,7 +163,8 @@ bool xenvif_rx_ring_slots_available(struct xenvif *vif,
> int needed)
>   * adding 'size' bytes to a buffer which currently contains 'offset'
>   * bytes.
>   */
> -static bool start_new_rx_buffer(int offset, unsigned long size, int head)
> +static bool start_new_rx_buffer(int offset, unsigned long size, int head,
> +				bool full_coalesce)
>  {
>  	/* simple case: we have completely filled the current buffer. */
>  	if (offset == MAX_BUFFER_OFFSET)
> @@ -175,6 +176,7 @@ static bool start_new_rx_buffer(int offset, unsigned
> long size, int head)
>  	 *     (i)   this frag would fit completely in the next buffer
>  	 * and (ii)  there is already some data in the current buffer
>  	 * and (iii) this is not the head buffer.
> +	 * and (iv)  there is no need to fully utilize the buffers
>  	 *
>  	 * Where:
>  	 * - (i) stops us splitting a frag into two copies
> @@ -185,6 +187,8 @@ static bool start_new_rx_buffer(int offset, unsigned
> long size, int head)
>  	 *   by (ii) but is explicitly checked because
>  	 *   netfront relies on the first buffer being
>  	 *   non-empty and can crash otherwise.
> +	 * - (iv) is needed for skbs which can use up more than
> MAX_SKB_FRAGS
> +	 *   slot
>  	 *
>  	 * This means we will effectively linearise small
>  	 * frags but do not needlessly split large buffers
> @@ -192,7 +196,8 @@ static bool start_new_rx_buffer(int offset, unsigned
> long size, int head)
>  	 * own buffers as before.
>  	 */
>  	BUG_ON(size > MAX_BUFFER_OFFSET);
> -	if ((offset + size > MAX_BUFFER_OFFSET) && offset && !head)
> +	if ((offset + size > MAX_BUFFER_OFFSET) && offset && !head &&
> +	    !full_coalesce)
>  		return true;
> 
>  	return false;
> @@ -227,6 +232,13 @@ static struct xenvif_rx_meta
> *get_next_rx_buffer(struct xenvif *vif,
>  	return meta;
>  }
> 
> +struct xenvif_rx_cb {
> +	int meta_slots_used;
> +	bool full_coalesce;
> +};
> +
> +#define XENVIF_RX_CB(skb) ((struct xenvif_rx_cb *)(skb)->cb)
> +
>  /*
>   * Set up the grant operations for this fragment. If it's a flipping
>   * interface, we also set up the unmap request from here.
> @@ -261,7 +273,10 @@ static void xenvif_gop_frag_copy(struct xenvif *vif,
> struct sk_buff *skb,
>  		if (bytes > size)
>  			bytes = size;
> 
> -		if (start_new_rx_buffer(npo->copy_off, bytes, *head)) {
> +		if (start_new_rx_buffer(npo->copy_off,
> +					bytes,
> +					*head,
> +					XENVIF_RX_CB(skb)->full_coalesce))
> {
>  			/*
>  			 * Netfront requires there to be some data in the
> head
>  			 * buffer.
> @@ -541,12 +556,6 @@ static void xenvif_add_frag_responses(struct xenvif
> *vif, int status,
>  	}
>  }
> 
> -struct xenvif_rx_cb {
> -	int meta_slots_used;
> -};
> -
> -#define XENVIF_RX_CB(skb) ((struct xenvif_rx_cb *)(skb)->cb)
> -
>  void xenvif_kick_thread(struct xenvif *vif)
>  {
>  	wake_up(&vif->wq);
> @@ -602,10 +611,14 @@ static void xenvif_rx_action(struct xenvif *vif)
> 
>  		/* To avoid the estimate becoming too pessimal for some
>  		 * frontends that limit posted rx requests, cap the estimate
> -		 * at MAX_SKB_FRAGS.
> +		 * at MAX_SKB_FRAGS. In this case netback will fully coalesce
> +		 * the skb into the provided slots.
>  		 */
> -		if (max_slots_needed > MAX_SKB_FRAGS)
> +		if (max_slots_needed > MAX_SKB_FRAGS) {
>  			max_slots_needed = MAX_SKB_FRAGS;
> +			XENVIF_RX_CB(skb)->full_coalesce = true;
> +		} else
> +			XENVIF_RX_CB(skb)->full_coalesce = false;
> 
>  		/* We may need one more slot for GSO metadata */
>  		if (skb_is_gso(skb) &&