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[Xen-devel] [PATCH net-next v4 6/9] xen-netback: Handle guests with too many frags



Xen network protocol had implicit dependency on MAX_SKB_FRAGS. Netback has to
handle guests sending up to XEN_NETBK_LEGACY_SLOTS_MAX slots. To achieve that:
- create a new skb
- map the leftover slots to its frags (no linear buffer here!)
- chain it to the previous through skb_shinfo(skb)->frag_list
- map them
- copy the whole stuff into a brand new skb and send it to the stack
- unmap the 2 old skb's pages

v3:
- adding extra check for frag number
- consolidate alloc_skb's into xenvif_alloc_skb()
- BUG_ON(frag_overflow > MAX_SKB_FRAGS)

v4:
- handle error of skb_copy_expand()

Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>

---
 drivers/net/xen-netback/netback.c |  125 ++++++++++++++++++++++++++++++++++---
 1 file changed, 115 insertions(+), 10 deletions(-)

diff --git a/drivers/net/xen-netback/netback.c 
b/drivers/net/xen-netback/netback.c
index c2b2597..345c6a2 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -802,6 +802,20 @@ static inline void xenvif_tx_create_gop(struct xenvif *vif,
 
 }
 
+static inline struct sk_buff *xenvif_alloc_skb(unsigned int size)
+{
+       struct sk_buff *skb =
+               alloc_skb(size + NET_SKB_PAD + NET_IP_ALIGN,
+                         GFP_ATOMIC | __GFP_NOWARN);
+       if (unlikely(skb == NULL))
+               return NULL;
+
+       /* Packets passed to netif_rx() must have some headroom. */
+       skb_reserve(skb, NET_SKB_PAD + NET_IP_ALIGN);
+
+       return skb;
+}
+
 static struct gnttab_map_grant_ref *xenvif_get_requests(struct xenvif *vif,
                                                        struct sk_buff *skb,
                                                        struct 
xen_netif_tx_request *txp,
@@ -812,11 +826,16 @@ static struct gnttab_map_grant_ref 
*xenvif_get_requests(struct xenvif *vif,
        u16 pending_idx = *((u16 *)skb->data);
        int start;
        pending_ring_idx_t index;
-       unsigned int nr_slots;
+       unsigned int nr_slots, frag_overflow = 0;
 
        /* At this point shinfo->nr_frags is in fact the number of
         * slots, which can be as large as XEN_NETBK_LEGACY_SLOTS_MAX.
         */
+       if (shinfo->nr_frags > MAX_SKB_FRAGS) {
+               frag_overflow = shinfo->nr_frags - MAX_SKB_FRAGS;
+               BUG_ON(frag_overflow > MAX_SKB_FRAGS);
+               shinfo->nr_frags = MAX_SKB_FRAGS;
+       }
        nr_slots = shinfo->nr_frags;
 
        /* Skip first skb fragment if it is on same page as header fragment. */
@@ -832,6 +851,29 @@ static struct gnttab_map_grant_ref 
*xenvif_get_requests(struct xenvif *vif,
 
        BUG_ON(shinfo->nr_frags > MAX_SKB_FRAGS);
 
+       if (frag_overflow) {
+               struct sk_buff *nskb = xenvif_alloc_skb(0);
+               if (unlikely(nskb == NULL)) {
+                       netdev_err(vif->dev,
+                                  "Can't allocate the frag_list skb.\n");
+                       return NULL;
+               }
+
+               shinfo = skb_shinfo(nskb);
+               frags = shinfo->frags;
+
+               for (shinfo->nr_frags = 0; shinfo->nr_frags < frag_overflow;
+                    shinfo->nr_frags++, txp++, gop++) {
+                       index = pending_index(vif->pending_cons++);
+                       pending_idx = vif->pending_ring[index];
+                       xenvif_tx_create_gop(vif, pending_idx, txp, gop);
+                       frag_set_pending_idx(&frags[shinfo->nr_frags],
+                                            pending_idx);
+               }
+
+               skb_shinfo(skb)->frag_list = nskb;
+       }
+
        return gop;
 }
 
@@ -845,6 +887,7 @@ static int xenvif_tx_check_gop(struct xenvif *vif,
        struct pending_tx_info *tx_info;
        int nr_frags = shinfo->nr_frags;
        int i, err, start;
+       struct sk_buff *first_skb = NULL;
 
        /* Check status of header. */
        err = gop->status;
@@ -867,6 +910,7 @@ static int xenvif_tx_check_gop(struct xenvif *vif,
        /* Skip first skb fragment if it is on same page as header fragment. */
        start = (frag_get_pending_idx(&shinfo->frags[0]) == pending_idx);
 
+check_frags:
        for (i = start; i < nr_frags; i++) {
                int j, newerr;
 
@@ -903,11 +947,20 @@ static int xenvif_tx_check_gop(struct xenvif *vif,
                /* Not the first error? Preceding frags already invalidated. */
                if (err)
                        continue;
-
                /* First error: invalidate header and preceding fragments. */
-               pending_idx = *((u16 *)skb->data);
-               xenvif_idx_unmap(vif, pending_idx);
-               xenvif_idx_release(vif, pending_idx, XEN_NETIF_RSP_OKAY);
+               if (!first_skb) {
+                       pending_idx = *((u16 *)skb->data);
+                       xenvif_idx_unmap(vif, pending_idx);
+                       xenvif_idx_release(vif,
+                                          pending_idx,
+                                          XEN_NETIF_RSP_OKAY);
+               } else {
+                       pending_idx = *((u16 *)first_skb->data);
+                       xenvif_idx_unmap(vif, pending_idx);
+                       xenvif_idx_release(vif,
+                                          pending_idx,
+                                          XEN_NETIF_RSP_OKAY);
+               }
                for (j = start; j < i; j++) {
                        pending_idx = frag_get_pending_idx(&shinfo->frags[j]);
                        xenvif_idx_unmap(vif, pending_idx);
@@ -919,6 +972,32 @@ static int xenvif_tx_check_gop(struct xenvif *vif,
                err = newerr;
        }
 
+       if (shinfo->frag_list) {
+               first_skb = skb;
+               skb = shinfo->frag_list;
+               shinfo = skb_shinfo(skb);
+               nr_frags = shinfo->nr_frags;
+               start = 0;
+
+               goto check_frags;
+       }
+
+       /* There was a mapping error in the frag_list skb. We have to unmap
+        * the first skb's frags
+        */
+       if (first_skb && err) {
+               int j;
+               shinfo = skb_shinfo(first_skb);
+               pending_idx = *((u16 *)first_skb->data);
+               start = (frag_get_pending_idx(&shinfo->frags[0]) == 
pending_idx);
+               for (j = start; j < shinfo->nr_frags; j++) {
+                       pending_idx = frag_get_pending_idx(&shinfo->frags[j]);
+                       xenvif_idx_unmap(vif, pending_idx);
+                       xenvif_idx_release(vif, pending_idx,
+                                          XEN_NETIF_RSP_OKAY);
+               }
+       }
+
        *gopp = gop + 1;
        return err;
 }
@@ -1422,8 +1501,7 @@ static unsigned xenvif_tx_build_gops(struct xenvif *vif, 
int budget)
                            ret < XEN_NETBK_LEGACY_SLOTS_MAX) ?
                        PKT_PROT_LEN : txreq.size;
 
-               skb = alloc_skb(data_len + NET_SKB_PAD + NET_IP_ALIGN,
-                               GFP_ATOMIC | __GFP_NOWARN);
+               skb = xenvif_alloc_skb(data_len);
                if (unlikely(skb == NULL)) {
                        netdev_dbg(vif->dev,
                                   "Can't allocate a skb in start_xmit.\n");
@@ -1431,9 +1509,6 @@ static unsigned xenvif_tx_build_gops(struct xenvif *vif, 
int budget)
                        break;
                }
 
-               /* Packets passed to netif_rx() must have some headroom. */
-               skb_reserve(skb, NET_SKB_PAD + NET_IP_ALIGN);
-
                if (extras[XEN_NETIF_EXTRA_TYPE_GSO - 1].type) {
                        struct xen_netif_extra_info *gso;
                        gso = &extras[XEN_NETIF_EXTRA_TYPE_GSO - 1];
@@ -1495,6 +1570,7 @@ static int xenvif_tx_submit(struct xenvif *vif)
                struct xen_netif_tx_request *txp;
                u16 pending_idx;
                unsigned data_len;
+               struct sk_buff *nskb = NULL;
 
                pending_idx = *((u16 *)skb->data);
                txp = &vif->pending_tx_info[pending_idx].req;
@@ -1537,6 +1613,32 @@ static int xenvif_tx_submit(struct xenvif *vif)
                                  pending_idx :
                                  INVALID_PENDING_IDX);
 
+               if (skb_shinfo(skb)->frag_list) {
+                       nskb = skb_shinfo(skb)->frag_list;
+                       xenvif_fill_frags(vif, nskb, INVALID_PENDING_IDX);
+                       skb->len += nskb->len;
+                       skb->data_len += nskb->len;
+                       skb->truesize += nskb->truesize;
+                       skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
+                       skb_shinfo(nskb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
+                       vif->tx_zerocopy_sent += 2;
+                       nskb = skb;
+
+                       skb = skb_copy_expand(skb,
+                                             0,
+                                             0,
+                                             GFP_ATOMIC | __GFP_NOWARN);
+                       if (!skb) {
+                               netdev_dbg(vif->dev,
+                                          "Can't consolidate skb with too many 
fragments\n");
+                               if (skb_shinfo(nskb)->destructor_arg)
+                                       skb_shinfo(nskb)->tx_flags |=
+                                               SKBTX_DEV_ZEROCOPY;
+                               kfree_skb(nskb);
+                               continue;
+                       }
+                       skb_shinfo(skb)->destructor_arg = NULL;
+               }
                if (skb_is_nonlinear(skb) && skb_headlen(skb) < PKT_PROT_LEN) {
                        int target = min_t(int, skb->len, PKT_PROT_LEN);
                        __pskb_pull_tail(skb, target - skb_headlen(skb));
@@ -1590,6 +1692,9 @@ static int xenvif_tx_submit(struct xenvif *vif)
                }
 
                netif_receive_skb(skb);
+
+               if (nskb)
+                       kfree_skb(nskb);
        }
 
        return work_done;

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