From: Eric Dumazet edumazet@google.com
ip_defrag uses skb->cb[] to store the fragment offset, and unfortunately this integer is currently in a different cache line than skb->next, meaning that we use two cache lines per skb when finding the insertion point.
By aliasing skb->ip_defrag_offset and skb->dev, we pack all the fields in a single cache line and save precious memory bandwidth.
Note that after the fast path added by Changli Gao in commit d6bebca92c66 ("fragment: add fast path for in-order fragments") this change wont help the fast path, since we still need to access prev->len (2nd cache line), but will show great benefits when slow path is entered, since we perform a linear scan of a potentially long list.
Also, note that this potential long list is an attack vector, we might consider also using an rb-tree there eventually.
Signed-off-by: Eric Dumazet edumazet@google.com Signed-off-by: David S. Miller davem@davemloft.net (cherry picked from commit bf66337140c64c27fa37222b7abca7e49d63fb57) --- include/linux/skbuff.h | 1 + net/ipv4/ip_fragment.c | 35 ++++++++++++++--------------------- 2 files changed, 15 insertions(+), 21 deletions(-)
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index 6dd77767fd5b..f4749678b7ee 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -678,6 +678,7 @@ struct sk_buff { * UDP receive path is one user. */ unsigned long dev_scratch; + int ip_defrag_offset; }; /* * This is the control buffer. It is free to use for every diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c index 88fa8ffc5558..5331a0d68374 100644 --- a/net/ipv4/ip_fragment.c +++ b/net/ipv4/ip_fragment.c @@ -57,14 +57,6 @@ */ static const char ip_frag_cache_name[] = "ip4-frags";
-struct ipfrag_skb_cb -{ - struct inet_skb_parm h; - int offset; -}; - -#define FRAG_CB(skb) ((struct ipfrag_skb_cb *)((skb)->cb)) - /* Describe an entry in the "incomplete datagrams" queue. */ struct ipq { struct inet_frag_queue q; @@ -353,13 +345,13 @@ static int ip_frag_queue(struct ipq *qp, struct sk_buff *skb) * this fragment, right? */ prev = qp->q.fragments_tail; - if (!prev || FRAG_CB(prev)->offset < offset) { + if (!prev || prev->ip_defrag_offset < offset) { next = NULL; goto found; } prev = NULL; for (next = qp->q.fragments; next != NULL; next = next->next) { - if (FRAG_CB(next)->offset >= offset) + if (next->ip_defrag_offset >= offset) break; /* bingo! */ prev = next; } @@ -370,7 +362,7 @@ static int ip_frag_queue(struct ipq *qp, struct sk_buff *skb) * any overlaps are eliminated. */ if (prev) { - int i = (FRAG_CB(prev)->offset + prev->len) - offset; + int i = (prev->ip_defrag_offset + prev->len) - offset;
if (i > 0) { offset += i; @@ -387,8 +379,8 @@ static int ip_frag_queue(struct ipq *qp, struct sk_buff *skb)
err = -ENOMEM;
- while (next && FRAG_CB(next)->offset < end) { - int i = end - FRAG_CB(next)->offset; /* overlap is 'i' bytes */ + while (next && next->ip_defrag_offset < end) { + int i = end - next->ip_defrag_offset; /* overlap is 'i' bytes */
if (i < next->len) { int delta = -next->truesize; @@ -401,7 +393,7 @@ static int ip_frag_queue(struct ipq *qp, struct sk_buff *skb) delta += next->truesize; if (delta) add_frag_mem_limit(qp->q.net, delta); - FRAG_CB(next)->offset += i; + next->ip_defrag_offset += i; qp->q.meat -= i; if (next->ip_summed != CHECKSUM_UNNECESSARY) next->ip_summed = CHECKSUM_NONE; @@ -425,7 +417,13 @@ static int ip_frag_queue(struct ipq *qp, struct sk_buff *skb) } }
- FRAG_CB(skb)->offset = offset; + /* Note : skb->ip_defrag_offset and skb->dev share the same location */ + dev = skb->dev; + if (dev) + qp->iif = dev->ifindex; + /* Makes sure compiler wont do silly aliasing games */ + barrier(); + skb->ip_defrag_offset = offset;
/* Insert this fragment in the chain of fragments. */ skb->next = next; @@ -436,11 +434,6 @@ static int ip_frag_queue(struct ipq *qp, struct sk_buff *skb) else qp->q.fragments = skb;
- dev = skb->dev; - if (dev) { - qp->iif = dev->ifindex; - skb->dev = NULL; - } qp->q.stamp = skb->tstamp; qp->q.meat += skb->len; qp->ecn |= ecn; @@ -516,7 +509,7 @@ static int ip_frag_reasm(struct ipq *qp, struct sk_buff *prev, }
WARN_ON(!head); - WARN_ON(FRAG_CB(head)->offset != 0); + WARN_ON(head->ip_defrag_offset != 0);
/* Allocate a new buffer for the datagram. */ ihlen = ip_hdrlen(head);