Re: [Xen-devel] [PATCH V3] xen/netfront: handle compound page fragments on transmit

[Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>]

On Wed, Nov 21, 2012 at 12:02:16PM +0000, Ian Campbell wrote:
> An SKB paged fragment can consist of a compound page with order > 0.
> However the netchannel protocol deals only in PAGE_SIZE frames.
> 
> Handle this in xennet_make_frags by iterating over the frames which
> make up the page.
> 
> This is the netfront equivalent to 6a8ed462f16b for netback.
> 
> Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> Cc: netdev@xxxxxxxxxxxxxxx
> Cc: xen-devel@xxxxxxxxxxxxx
> Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> Cc: Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>

Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

David, would you like me to send it to Linus via my tree or are you
OK sending him a git pull with this patch (and hopefully some other
ones?)

Thank you.

> Cc: ANNIE LI <annie.li@xxxxxxxxxx>
> Cc: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
> Cc: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
> ---
> v3: limit to 80-characters. Use net_alert_ratelimited.
> v2: check we have enough room in the ring and that the other end can
>     cope with the number of slots in a single frame
> ---
>  drivers/net/xen-netfront.c |   98 ++++++++++++++++++++++++++++++++++---------
>  1 files changed, 77 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index caa0110..fc24eb9 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -452,29 +452,85 @@ static void xennet_make_frags(struct sk_buff *skb, 
> struct net_device *dev,
>       /* Grant backend access to each skb fragment page. */
>       for (i = 0; i < frags; i++) {
>               skb_frag_t *frag = skb_shinfo(skb)->frags + i;
> +             struct page *page = skb_frag_page(frag);
>  
> -             tx->flags |= XEN_NETTXF_more_data;
> +             len = skb_frag_size(frag);
> +             offset = frag->page_offset;
>  
> -             id = get_id_from_freelist(&np->tx_skb_freelist, np->tx_skbs);
> -             np->tx_skbs[id].skb = skb_get(skb);
> -             tx = RING_GET_REQUEST(&np->tx, prod++);
> -             tx->id = id;
> -             ref = gnttab_claim_grant_reference(&np->gref_tx_head);
> -             BUG_ON((signed short)ref < 0);
> +             /* Data must not cross a page boundary. */
> +             BUG_ON(len + offset > PAGE_SIZE<<compound_order(page));
>  
> -             mfn = pfn_to_mfn(page_to_pfn(skb_frag_page(frag)));
> -             gnttab_grant_foreign_access_ref(ref, np->xbdev->otherend_id,
> -                                             mfn, GNTMAP_readonly);
> +             /* Skip unused frames from start of page */
> +             page += offset >> PAGE_SHIFT;
> +             offset &= ~PAGE_MASK;
>  
> -             tx->gref = np->grant_tx_ref[id] = ref;
> -             tx->offset = frag->page_offset;
> -             tx->size = skb_frag_size(frag);
> -             tx->flags = 0;
> +             while (len > 0) {
> +                     unsigned long bytes;
> +
> +                     BUG_ON(offset >= PAGE_SIZE);
> +
> +                     bytes = PAGE_SIZE - offset;
> +                     if (bytes > len)
> +                             bytes = len;
> +
> +                     tx->flags |= XEN_NETTXF_more_data;
> +
> +                     id = get_id_from_freelist(&np->tx_skb_freelist,
> +                                               np->tx_skbs);
> +                     np->tx_skbs[id].skb = skb_get(skb);
> +                     tx = RING_GET_REQUEST(&np->tx, prod++);
> +                     tx->id = id;
> +                     ref = gnttab_claim_grant_reference(&np->gref_tx_head);
> +                     BUG_ON((signed short)ref < 0);
> +
> +                     mfn = pfn_to_mfn(page_to_pfn(page));
> +                     gnttab_grant_foreign_access_ref(ref,
> +                                                     np->xbdev->otherend_id,
> +                                                     mfn, GNTMAP_readonly);
> +
> +                     tx->gref = np->grant_tx_ref[id] = ref;
> +                     tx->offset = offset;
> +                     tx->size = bytes;
> +                     tx->flags = 0;
> +
> +                     offset += bytes;
> +                     len -= bytes;
> +
> +                     /* Next frame */
> +                     if (offset == PAGE_SIZE && len) {
> +                             BUG_ON(!PageCompound(page));
> +                             page++;
> +                             offset = 0;
> +                     }
> +             }
>       }
>  
>       np->tx.req_prod_pvt = prod;
>  }
>  
> +/*
> + * Count how many ring slots are required to send the frags of this
> + * skb. Each frag might be a compound page.
> + */
> +static int xennet_count_skb_frag_slots(struct sk_buff *skb)
> +{
> +     int i, frags = skb_shinfo(skb)->nr_frags;
> +     int pages = 0;
> +
> +     for (i = 0; i < frags; i++) {
> +             skb_frag_t *frag = skb_shinfo(skb)->frags + i;
> +             unsigned long size = skb_frag_size(frag);
> +             unsigned long offset = frag->page_offset;
> +
> +             /* Skip unused frames from start of page */
> +             offset &= ~PAGE_MASK;
> +
> +             pages += PFN_UP(offset + size);
> +     }
> +
> +     return pages;
> +}
> +
>  static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  {
>       unsigned short id;
> @@ -487,23 +543,23 @@ static int xennet_start_xmit(struct sk_buff *skb, 
> struct net_device *dev)
>       grant_ref_t ref;
>       unsigned long mfn;
>       int notify;
> -     int frags = skb_shinfo(skb)->nr_frags;
> +     int slots;
>       unsigned int offset = offset_in_page(data);
>       unsigned int len = skb_headlen(skb);
>       unsigned long flags;
>  
> -     frags += DIV_ROUND_UP(offset + len, PAGE_SIZE);
> -     if (unlikely(frags > MAX_SKB_FRAGS + 1)) {
> -             printk(KERN_ALERT "xennet: skb rides the rocket: %d frags\n",
> -                    frags);
> -             dump_stack();
> +     slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
> +             xennet_count_skb_frag_slots(skb);
> +     if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
> +             net_alert_ratelimited(
> +                     "xennet: skb rides the rocket: %d slots\n", slots);
>               goto drop;
>       }
>  
>       spin_lock_irqsave(&np->tx_lock, flags);
>  
>       if (unlikely(!netif_carrier_ok(dev) ||
> -                  (frags > 1 && !xennet_can_sg(dev)) ||
> +                  (slots > 1 && !xennet_can_sg(dev)) ||
>                    netif_needs_gso(skb, netif_skb_features(skb)))) {
>               spin_unlock_irqrestore(&np->tx_lock, flags);
>               goto drop;
> -- 
> 1.7.2.5
> 

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