Commit bf663371 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

inet: frags: get rid of ipfrag_skb_cb/FRAG_CB 



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
d6bebca9 ("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: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent c2615cf5

include/linux/skbuff.h

+1 −0
Original line number Diff line number Diff line
@@ -672,6 +672,7 @@ struct sk_buff { 
				 * UDP receive path is one user.

				 */

				unsigned long		dev_scratch;

				int			ip_defrag_offset;

			};

		};

		struct rb_node	rbnode; /* used in netem & tcp stack */

net/ipv4/ip_fragment.c

+14 −21
Original line number Diff line number Diff line
@@ -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 @@ found: 
	 * 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 @@ found: 


	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) {

			/* Eat head of the next overlapped fragment
@@ -396,7 +388,7 @@ found: 
			 */

			if (!pskb_pull(next, i))

				goto err;

			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;
@@ -420,7 +412,13 @@ found: 
		}

	}



	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;
@@ -431,11 +429,6 @@ found: 
	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;
@@ -511,7 +504,7 @@ 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);

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