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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Zoltan Kiss
> Sent: 04 June 2014 19:59
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx; Ian Campbell; Wei Liu; Paul Durrant;
> linux@xxxxxxxxxxxxxx
> Cc: netdev@xxxxxxxxxxxxxxx; David Vrabel; davem@xxxxxxxxxxxxx; 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@xxxxxxxxxx>
> Cc: Paul Durrant <paul.durrant@xxxxxxxxxx>

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

> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
> Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
> ---
> 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) &&

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