Commit 52900d22 authored by Willem de Bruijn's avatar Willem de Bruijn Committed by David S. Miller
Browse files

udp: elide zerocopy operation in hot path 



With MSG_ZEROCOPY, each skb holds a reference to a struct ubuf_info.
Release of its last reference triggers a completion notification.

The TCP stack in tcp_sendmsg_locked holds an extra ref independent of
the skbs, because it can build, send and free skbs within its loop,
possibly reaching refcount zero and freeing the ubuf_info too soon.

The UDP stack currently also takes this extra ref, but does not need
it as all skbs are sent after return from __ip(6)_append_data.

Avoid the extra refcount_inc and refcount_dec_and_test, and generally
the sock_zerocopy_put in the common path, by passing the initial
reference to the first skb.

This approach is taken instead of initializing the refcount to 0, as
that would generate error "refcount_t: increment on 0" on the
next skb_zcopy_set.

Changes
  v3 -> v4
    - Move skb_zcopy_set below the only kfree_skb that might cause
      a premature uarg destroy before skb_zerocopy_put_abort
      - Move the entire skb_shinfo assignment block, to keep that
        cacheline access in one place

Signed-off-by: default avatarWillem de Bruijn <willemb@google.com>
Acked-by: default avatarPaolo Abeni <pabeni@redhat.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent b5947e5d

include/linux/skbuff.h

+8 −4
Original line number Diff line number Diff line
@@ -481,7 +481,7 @@ static inline void sock_zerocopy_get(struct ubuf_info *uarg) 
}



void sock_zerocopy_put(struct ubuf_info *uarg);

void sock_zerocopy_put_abort(struct ubuf_info *uarg);

void sock_zerocopy_put_abort(struct ubuf_info *uarg, bool have_uref);



void sock_zerocopy_callback(struct ubuf_info *uarg, bool success);
@@ -1326,9 +1326,13 @@ static inline struct ubuf_info *skb_zcopy(struct sk_buff *skb) 
	return is_zcopy ? skb_uarg(skb) : NULL;

}



static inline void skb_zcopy_set(struct sk_buff *skb, struct ubuf_info *uarg)

static inline void skb_zcopy_set(struct sk_buff *skb, struct ubuf_info *uarg,

				 bool *have_ref)

{

	if (skb && uarg && !skb_zcopy(skb)) {

		if (unlikely(have_ref && *have_ref))

			*have_ref = false;

		else

			sock_zerocopy_get(uarg);

		skb_shinfo(skb)->destructor_arg = uarg;

		skb_shinfo(skb)->tx_flags |= SKBTX_ZEROCOPY_FRAG;
@@ -1374,7 +1378,7 @@ static inline void skb_zcopy_abort(struct sk_buff *skb) 
	struct ubuf_info *uarg = skb_zcopy(skb);



	if (uarg) {

		sock_zerocopy_put_abort(uarg);

		sock_zerocopy_put_abort(uarg, false);

		skb_shinfo(skb)->tx_flags &= ~SKBTX_ZEROCOPY_FRAG;

	}

}

net/core/skbuff.c

+5 −4
Original line number Diff line number Diff line
@@ -1089,7 +1089,7 @@ void sock_zerocopy_put(struct ubuf_info *uarg) 
}

EXPORT_SYMBOL_GPL(sock_zerocopy_put);



void sock_zerocopy_put_abort(struct ubuf_info *uarg)

void sock_zerocopy_put_abort(struct ubuf_info *uarg, bool have_uref)

{

	if (uarg) {

		struct sock *sk = skb_from_uarg(uarg)->sk;
@@ -1097,6 +1097,7 @@ void sock_zerocopy_put_abort(struct ubuf_info *uarg) 
		atomic_dec(&sk->sk_zckey);

		uarg->len--;



		if (have_uref)

			sock_zerocopy_put(uarg);

	}

}
@@ -1137,7 +1138,7 @@ int skb_zerocopy_iter_stream(struct sock *sk, struct sk_buff *skb, 
		return err;

	}



	skb_zcopy_set(skb, uarg);

	skb_zcopy_set(skb, uarg, NULL);

	return skb->len - orig_len;

}

EXPORT_SYMBOL_GPL(skb_zerocopy_iter_stream);
@@ -1157,7 +1158,7 @@ static int skb_zerocopy_clone(struct sk_buff *nskb, struct sk_buff *orig, 
			if (skb_copy_ubufs(nskb, GFP_ATOMIC))

				return -EIO;

		}

		skb_zcopy_set(nskb, skb_uarg(orig));

		skb_zcopy_set(nskb, skb_uarg(orig), NULL);

	}

	return 0;

}

net/ipv4/ip_output.c

+11 −11
Original line number Diff line number Diff line
@@ -881,8 +881,8 @@ static int __ip_append_data(struct sock *sk, 
	int csummode = CHECKSUM_NONE;

	struct rtable *rt = (struct rtable *)cork->dst;

	unsigned int wmem_alloc_delta = 0;

	bool paged, extra_uref;

	u32 tskey = 0;

	bool paged;



	skb = skb_peek_tail(queue);
@@ -921,12 +921,13 @@ static int __ip_append_data(struct sock *sk, 
		uarg = sock_zerocopy_realloc(sk, length, skb_zcopy(skb));

		if (!uarg)

			return -ENOBUFS;

		extra_uref = true;

		if (rt->dst.dev->features & NETIF_F_SG &&

		    csummode == CHECKSUM_PARTIAL) {

			paged = true;

		} else {

			uarg->zerocopy = 0;

			skb_zcopy_set(skb, uarg);

			skb_zcopy_set(skb, uarg, &extra_uref);

		}

	}
@@ -1015,13 +1016,6 @@ alloc_new_skb: 
			skb->csum = 0;

			skb_reserve(skb, hh_len);



			/* only the initial fragment is time stamped */

			skb_shinfo(skb)->tx_flags = cork->tx_flags;

			cork->tx_flags = 0;

			skb_shinfo(skb)->tskey = tskey;

			tskey = 0;

			skb_zcopy_set(skb, uarg);



			/*

			 *	Find where to start putting bytes.

			 */
@@ -1054,6 +1048,13 @@ alloc_new_skb: 
			exthdrlen = 0;

			csummode = CHECKSUM_NONE;



			/* only the initial fragment is time stamped */

			skb_shinfo(skb)->tx_flags = cork->tx_flags;

			cork->tx_flags = 0;

			skb_shinfo(skb)->tskey = tskey;

			tskey = 0;

			skb_zcopy_set(skb, uarg, &extra_uref);



			if ((flags & MSG_CONFIRM) && !skb_prev)

				skb_set_dst_pending_confirm(skb, 1);
@@ -1124,13 +1125,12 @@ alloc_new_skb: 


	if (wmem_alloc_delta)

		refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc);

	sock_zerocopy_put(uarg);

	return 0;



error_efault:

	err = -EFAULT;

error:

	sock_zerocopy_put_abort(uarg);

	sock_zerocopy_put_abort(uarg, extra_uref);

	cork->length -= length;

	IP_INC_STATS(sock_net(sk), IPSTATS_MIB_OUTDISCARDS);

	refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc);

net/ipv4/tcp.c

+1 −1
Original line number Diff line number Diff line
@@ -1423,7 +1423,7 @@ do_error: 
	if (copied + copied_syn)

		goto out;

out_err:

	sock_zerocopy_put_abort(uarg);

	sock_zerocopy_put_abort(uarg, true);

	err = sk_stream_error(sk, flags, err);

	/* make sure we wake any epoll edge trigger waiter */

	if (unlikely(skb_queue_len(&sk->sk_write_queue) == 0 &&

net/ipv6/ip6_output.c

+11 −11
Original line number Diff line number Diff line
@@ -1258,7 +1258,7 @@ static int __ip6_append_data(struct sock *sk, 
	int csummode = CHECKSUM_NONE;

	unsigned int maxnonfragsize, headersize;

	unsigned int wmem_alloc_delta = 0;

	bool paged;

	bool paged, extra_uref;



	skb = skb_peek_tail(queue);

	if (!skb) {
@@ -1327,12 +1327,13 @@ emsgsize: 
		uarg = sock_zerocopy_realloc(sk, length, skb_zcopy(skb));

		if (!uarg)

			return -ENOBUFS;

		extra_uref = true;

		if (rt->dst.dev->features & NETIF_F_SG &&

		    csummode == CHECKSUM_PARTIAL) {

			paged = true;

		} else {

			uarg->zerocopy = 0;

			skb_zcopy_set(skb, uarg);

			skb_zcopy_set(skb, uarg, &extra_uref);

		}

	}
@@ -1454,13 +1455,6 @@ alloc_new_skb: 
			skb_reserve(skb, hh_len + sizeof(struct frag_hdr) +

				    dst_exthdrlen);



			/* Only the initial fragment is time stamped */

			skb_shinfo(skb)->tx_flags = cork->tx_flags;

			cork->tx_flags = 0;

			skb_shinfo(skb)->tskey = tskey;

			tskey = 0;

			skb_zcopy_set(skb, uarg);



			/*

			 *	Find where to start putting bytes

			 */
@@ -1492,6 +1486,13 @@ alloc_new_skb: 
			exthdrlen = 0;

			dst_exthdrlen = 0;



			/* Only the initial fragment is time stamped */

			skb_shinfo(skb)->tx_flags = cork->tx_flags;

			cork->tx_flags = 0;

			skb_shinfo(skb)->tskey = tskey;

			tskey = 0;

			skb_zcopy_set(skb, uarg, &extra_uref);



			if ((flags & MSG_CONFIRM) && !skb_prev)

				skb_set_dst_pending_confirm(skb, 1);
@@ -1562,13 +1563,12 @@ alloc_new_skb: 


	if (wmem_alloc_delta)

		refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc);

	sock_zerocopy_put(uarg);

	return 0;



error_efault:

	err = -EFAULT;

error:

	sock_zerocopy_put_abort(uarg);

	sock_zerocopy_put_abort(uarg, extra_uref);

	cork->length -= length;

	IP6_INC_STATS(sock_net(sk), rt->rt6i_idev, IPSTATS_MIB_OUTDISCARDS);

	refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc);

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