datapath: Simplify make_writable().

The current implementation of make_writable() is both overly complex
and unnecessarily aggressive about copying data.  We can improve
performance by only making a copy of the data if someone else holds
a reference to the portion of the data that we want to modify.  This
means that if a clone is held by the TCP stack for retransmission then
we do not need to make a copy if we are changing the IP header because
it will get regenerated on retransmit anyways.  Even when it is necessary
to copy we avoid duplicating struct sk_buff.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>

diff --git a/datapath/actions.c b/datapath/actions.c
index 319c783..a8884a3 100644 (file)
--- a/datapath/actions.c
+++ b/datapath/actions.c
@@ -31,44 +31,31 @@
 static int do_execute_actions(struct datapath *, struct sk_buff *,
                              struct sw_flow_actions *acts);
 
-static struct sk_buff *make_writable(struct sk_buff *skb, unsigned min_headroom)
+static int make_writable(struct sk_buff *skb, int write_len)
 {
-       if (skb_cloned(skb)) {
-               struct sk_buff *nskb;
-               unsigned headroom = max(min_headroom, skb_headroom(skb));
+       if (!skb_cloned(skb) || skb_clone_writable(skb, write_len))
+               return 0;
 
-               nskb = skb_copy_expand(skb, headroom, skb_tailroom(skb), GFP_ATOMIC);
-               if (nskb) {
-                       set_skb_csum_bits(skb, nskb);
-                       kfree_skb(skb);
-                       return nskb;
-               }
-       } else {
-               unsigned int hdr_len = (skb_transport_offset(skb)
-                                       + sizeof(struct tcphdr));
-               if (pskb_may_pull(skb, min(hdr_len, skb->len)))
-                       return skb;
-       }
-       kfree_skb(skb);
-       return NULL;
+       return pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
 }
 
-static struct sk_buff *strip_vlan(struct sk_buff *skb)
+static int strip_vlan(struct sk_buff *skb)
 {
        struct ethhdr *eh;
+       int err;
 
        if (vlan_tx_tag_present(skb)) {
                vlan_set_tci(skb, 0);
-               return skb;
+               return 0;
        }
 
        if (unlikely(skb->protocol != htons(ETH_P_8021Q) ||
            skb->len < VLAN_ETH_HLEN))
-               return skb;
+               return 0;
 
-       skb = make_writable(skb, 0);
-       if (unlikely(!skb))
-               return NULL;
+       err = make_writable(skb, VLAN_ETH_HLEN);
+       if (unlikely(err))
+               return err;
 
        if (get_ip_summed(skb) == OVS_CSUM_COMPLETE)
                skb->csum = csum_sub(skb->csum, csum_partial(skb->data
@@ -81,21 +68,25 @@ static struct sk_buff *strip_vlan(struct sk_buff *skb)
        skb->protocol = eh->h_proto;
        skb->mac_header += VLAN_HLEN;
 
-       return skb;
+       return 0;
 }
 
-static struct sk_buff *modify_vlan_tci(struct sk_buff *skb, __be16 tci)
+static int modify_vlan_tci(struct sk_buff *skb, __be16 tci)
 {
        if (!vlan_tx_tag_present(skb) && skb->protocol == htons(ETH_P_8021Q)) {
+               int err;
+
                if (unlikely(skb->len < VLAN_ETH_HLEN))
-                       return skb;
+                       return 0;
 
-               skb = strip_vlan(skb);
-               if (unlikely(!skb))
-                       return NULL;
+               err = strip_vlan(skb);
+               if (unlikely(err))
+                       return err;
        }
 
-       return __vlan_hwaccel_put_tag(skb, ntohs(tci));
+       __vlan_hwaccel_put_tag(skb, ntohs(tci));
+
+       return 0;
 }
 
 static bool is_ip(struct sk_buff *skb)
@@ -118,19 +109,21 @@ static __sum16 *get_l4_checksum(struct sk_buff *skb)
        return NULL;
 }
 
-static struct sk_buff *set_nw_addr(struct sk_buff *skb, const struct nlattr *a)
+static int set_nw_addr(struct sk_buff *skb, const struct nlattr *a)
 {
        __be32 new_nwaddr = nla_get_be32(a);
        struct iphdr *nh;
        __sum16 *check;
        __be32 *nwaddr;
+       int err;
 
        if (unlikely(!is_ip(skb)))
-               return skb;
+               return 0;
 
-       skb = make_writable(skb, 0);
-       if (unlikely(!skb))
-               return NULL;
+       err = make_writable(skb, skb_network_offset(skb) +
+                                sizeof(struct iphdr));
+       if (unlikely(err))
+               return err;
 
        nh = ip_hdr(skb);
        nwaddr = nla_type(a) == ODP_ACTION_ATTR_SET_NW_SRC ? &nh->saddr : &nh->daddr;
@@ -144,47 +137,52 @@ static struct sk_buff *set_nw_addr(struct sk_buff *skb, const struct nlattr *a)
 
        *nwaddr = new_nwaddr;
 
-       return skb;
+       return 0;
 }
 
-static struct sk_buff *set_nw_tos(struct sk_buff *skb, u8 nw_tos)
+static int set_nw_tos(struct sk_buff *skb, u8 nw_tos)
 {
+       struct iphdr *nh = ip_hdr(skb);
+       u8 old, new;
+       int err;
+
        if (unlikely(!is_ip(skb)))
-               return skb;
-
-       skb = make_writable(skb, 0);
-       if (skb) {
-               struct iphdr *nh = ip_hdr(skb);
-               u8 *f = &nh->tos;
-               u8 old = *f;
-               u8 new;
-
-               /* Set the DSCP bits and preserve the ECN bits. */
-               new = nw_tos | (nh->tos & INET_ECN_MASK);
-               csum_replace4(&nh->check, (__force __be32)old,
-                                         (__force __be32)new);
-               *f = new;
-       }
-       return skb;
+               return 0;
+
+       err = make_writable(skb, skb_network_offset(skb) +
+                                sizeof(struct iphdr));
+       if (unlikely(err))
+               return err;
+
+       /* Set the DSCP bits and preserve the ECN bits. */
+       old = nh->tos;
+       new = nw_tos | (nh->tos & INET_ECN_MASK);
+       csum_replace4(&nh->check, (__force __be32)old,
+                                 (__force __be32)new);
+       nh->tos = new;
+
+       return 0;
 }
 
-static struct sk_buff *set_tp_port(struct sk_buff *skb, const struct nlattr *a)
+static int set_tp_port(struct sk_buff *skb, const struct nlattr *a)
 {
        struct udphdr *th;
        __sum16 *check;
        __be16 *port;
+       int err;
 
        if (unlikely(!is_ip(skb)))
-               return skb;
+               return 0;
 
-       skb = make_writable(skb, 0);
-       if (unlikely(!skb))
-               return NULL;
+       err = make_writable(skb, skb_transport_offset(skb) +
+                                sizeof(struct tcphdr));
+       if (unlikely(err))
+               return err;
 
        /* Must follow make_writable() since that can move the skb data. */
        check = get_l4_checksum(skb);
        if (unlikely(!check))
-               return skb;
+               return 0;
 
        /*
         * Update port and checksum.
@@ -199,7 +197,7 @@ static struct sk_buff *set_tp_port(struct sk_buff *skb, const struct nlattr *a)
        *port = nla_get_be16(a);
        skb_clear_rxhash(skb);
 
-       return skb;
+       return 0;
 }
 
 static void do_output(struct datapath *dp, struct sk_buff *skb, int out_port)
@@ -248,10 +246,12 @@ static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
        int prev_port = -1;
        u32 priority = skb->priority;
        const struct nlattr *a;
-       int rem, err;
+       int rem;
 
        for (a = acts->actions, rem = acts->actions_len; rem > 0;
             a = nla_next(a, &rem)) {
+               int err = 0;
+
                if (prev_port != -1) {
                        do_output(dp, skb_clone(skb, GFP_ATOMIC), prev_port);
                        prev_port = -1;
@@ -264,10 +264,6 @@ static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
 
                case ODP_ACTION_ATTR_CONTROLLER:
                        err = output_control(dp, skb, nla_get_u64(a));
-                       if (err) {
-                               kfree_skb(skb);
-                               return err;
-                       }
                        break;
 
                case ODP_ACTION_ATTR_SET_TUNNEL:
@@ -275,39 +271,37 @@ static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
                        break;
 
                case ODP_ACTION_ATTR_SET_DL_TCI:
-                       skb = modify_vlan_tci(skb, nla_get_be16(a));
+                       err = modify_vlan_tci(skb, nla_get_be16(a));
                        break;
 
                case ODP_ACTION_ATTR_STRIP_VLAN:
-                       skb = strip_vlan(skb);
+                       err = strip_vlan(skb);
                        break;
 
                case ODP_ACTION_ATTR_SET_DL_SRC:
-                       skb = make_writable(skb, 0);
-                       if (!skb)
-                               return -ENOMEM;
-                       memcpy(eth_hdr(skb)->h_source, nla_data(a), ETH_ALEN);
+                       err = make_writable(skb, ETH_HLEN);
+                       if (likely(!err))
+                               memcpy(eth_hdr(skb)->h_source, nla_data(a), ETH_ALEN);
                        break;
 
                case ODP_ACTION_ATTR_SET_DL_DST:
-                       skb = make_writable(skb, 0);
-                       if (!skb)
-                               return -ENOMEM;
-                       memcpy(eth_hdr(skb)->h_dest, nla_data(a), ETH_ALEN);
+                       err = make_writable(skb, ETH_HLEN);
+                       if (likely(!err))
+                               memcpy(eth_hdr(skb)->h_dest, nla_data(a), ETH_ALEN);
                        break;
 
                case ODP_ACTION_ATTR_SET_NW_SRC:
                case ODP_ACTION_ATTR_SET_NW_DST:
-                       skb = set_nw_addr(skb, a);
+                       err = set_nw_addr(skb, a);
                        break;
 
                case ODP_ACTION_ATTR_SET_NW_TOS:
-                       skb = set_nw_tos(skb, nla_get_u8(a));
+                       err = set_nw_tos(skb, nla_get_u8(a));
                        break;
 
                case ODP_ACTION_ATTR_SET_TP_SRC:
                case ODP_ACTION_ATTR_SET_TP_DST:
-                       skb = set_tp_port(skb, a);
+                       err = set_tp_port(skb, a);
                        break;
 
                case ODP_ACTION_ATTR_SET_PRIORITY:
@@ -318,14 +312,18 @@ static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
                        skb->priority = priority;
                        break;
                }
-               if (!skb)
-                       return -ENOMEM;
+
+               if (unlikely(err)) {
+                       kfree_skb(skb);
+                       return err;
+               }
        }
 
        if (prev_port != -1)
                do_output(dp, skb, prev_port);
        else
                kfree_skb(skb);
+
        return 0;
 }