[Xen-devel] [PATCH V2] xen/netfront: handle compound page fragments on transmit

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

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>
Cc: ANNIE LI <annie.li@xxxxxxxxxx>
Cc: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
Cc: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
---
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 |   95 ++++++++++++++++++++++++++++++++++----------
 1 files changed, 74 insertions(+), 21 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index caa0110..d9b16f4 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -452,29 +452,82 @@ 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);
+               unsigned long size = skb_frag_size(frag);
+               unsigned long offset = frag->page_offset;
 
-               tx->flags |= XEN_NETTXF_more_data;
+               /* Data must not cross a page boundary. */
+               BUG_ON(size + offset > PAGE_SIZE<<compound_order(page));
 
-               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);
+               /* Skip unused frames from start of page */
+               page += offset >> PAGE_SHIFT;
+               offset &= ~PAGE_MASK;
 
-               mfn = pfn_to_mfn(page_to_pfn(skb_frag_page(frag)));
-               gnttab_grant_foreign_access_ref(ref, np->xbdev->otherend_id,
-                                               mfn, GNTMAP_readonly);
+               while (size > 0) {
+                       unsigned long bytes;
 
-               tx->gref = np->grant_tx_ref[id] = ref;
-               tx->offset = frag->page_offset;
-               tx->size = skb_frag_size(frag);
-               tx->flags = 0;
+                       BUG_ON(offset >= PAGE_SIZE);
+
+                       bytes = PAGE_SIZE - offset;
+                       if (bytes > size)
+                               bytes = size;
+
+                       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;
+                       size -= bytes;
+
+                       /* Next frame */
+                       if (offset == PAGE_SIZE && size) {
+                               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 +540,23 @@ 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)) {
+               printk(KERN_ALERT "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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