Re: [PATCH v3 26/30] net: sk_buff rbnode reorg

Christoph Paasch
2018-10-18T16:01:57Z

Hello,

On Thu, Sep 13, 2018 at 8:00 AM Stephen Hemminger
<stephen@networkplumber.org> wrote:
>
> From: Eric Dumazet <edumazet@google.com>
>
> commit bffa72cf7f9df842f0016ba03586039296b4caaf upstream
>
> skb->rbnode shares space with skb->next, skb->prev and skb->tstamp
>
> Current uses (TCP receive ofo queue and netem) need to save/restore
> tstamp, while skb->dev is either NULL (TCP) or a constant for a given
> queue (netem).
>
> Since we plan using an RB tree for TCP retransmit queue to speedup SACK
> processing with large BDP, this patch exchanges skb->dev and
> skb->tstamp.
>
> This saves some overhead in both TCP and netem.
>
> v2: removes the swtstamp field from struct tcp_skb_cb
>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Cc: Soheil Hassas Yeganeh <soheil@google.com>
> Cc: Wei Wang <weiwan@google.com>
> Cc: Willem de Bruijn <willemb@google.com>
> Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
> Signed-off-by: David S. Miller <davem@davemloft.net>
> ---
> include/linux/skbuff.h | 24 ++--
> include/net/inet_frag.h | 3 +-
> net/ipv4/inet_fragment.c | 16 ++-
> net/ipv4/ip_fragment.c | 182 +++++++++++++-----------
> net/ipv6/netfilter/nf_conntrack_reasm.c | 1 +
> net/ipv6/reassembly.c | 1 +
> 6 files changed, 129 insertions(+), 98 deletions(-)

this change is missing the pieces in sch_netem.c that Eric did as well.

Thus, there is a panic that can be hit à la:
https://github.com/multipath-tcp/mptcp/issues/285#issuecomment-431001559

The following diff fixes it (I can submit a proper patch later today).
Storing of the tstamp can be changed as well (as Eric did in the
original patch), but that's not causing any issues. I see a backport
to v4.9 has been submitted as well...
---
diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 2a2ab6bfe5d8..3d325b840802 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -624,6 +624,10 @@ static struct sk_buff *netem_dequeue(struct Qdisc *sch)
skb->next = NULL;
skb->prev = NULL;
skb->tstamp = netem_skb_cb(skb)->tstamp_save;
+ /* skb->dev shares skb->rbnode area,
+ * we need to restore its value.
+ */
+ skb->dev = qdisc_dev(sch);

#ifdef CONFIG_NET_CLS_ACT
/*

>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 2837e55df03e..f64e88444082 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -663,23 +663,27 @@ struct sk_buff {
> struct sk_buff *prev;
>
> union {
> - ktime_t tstamp;
> - u64 skb_mstamp;
> + struct net_device *dev;
> + /* Some protocols might use this space to store information,
> + * while device pointer would be NULL.
> + * UDP receive path is one user.
> + */
> + unsigned long dev_scratch;
> };
> };
> - struct rb_node rbnode; /* used in netem & tcp stack */
> + struct rb_node rbnode; /* used in netem, ip4 defrag, and tcp stack */
> + struct list_head list;
> };
> - struct sock *sk;
>
> union {
> - struct net_device *dev;
> - /* Some protocols might use this space to store information,
> - * while device pointer would be NULL.
> - * UDP receive path is one user.
> - */
> - unsigned long dev_scratch;
> + struct sock *sk;
> int ip_defrag_offset;
> };
> +
> + union {
> + ktime_t tstamp;
> + u64 skb_mstamp;
> + };
> /*
> * This is the control buffer. It is free to use for every
> * layer. Please put your private variables there. If you
> diff --git a/include/net/inet_frag.h b/include/net/inet_frag.h
> index ed07e3786d98..e4c71a7644be 100644
> --- a/include/net/inet_frag.h
> +++ b/include/net/inet_frag.h
> @@ -75,7 +75,8 @@ struct inet_frag_queue {
> struct timer_list timer;
> spinlock_t lock;
> refcount_t refcnt;
> - struct sk_buff *fragments;
> + struct sk_buff *fragments; /* Used in IPv6. */
> + struct rb_root rb_fragments; /* Used in IPv4. */
> struct sk_buff *fragments_tail;
> ktime_t stamp;
> int len;
> diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
> index c9e35b81d093..6904cbb7de1a 100644
> --- a/net/ipv4/inet_fragment.c
> +++ b/net/ipv4/inet_fragment.c
> @@ -136,12 +136,16 @@ void inet_frag_destroy(struct inet_frag_queue *q)
> fp = q->fragments;
> nf = q->net;
> f = nf->f;
> - while (fp) {
> - struct sk_buff *xp = fp->next;
> -
> - sum_truesize += fp->truesize;
> - kfree_skb(fp);
> - fp = xp;
> + if (fp) {
> + do {
> + struct sk_buff *xp = fp->next;
> +
> + sum_truesize += fp->truesize;
> + kfree_skb(fp);
> + fp = xp;
> + } while (fp);
> + } else {
> + sum_truesize = skb_rbtree_purge(&q->rb_fragments);
> }
> sum = sum_truesize + f->qsize;
>
> diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
> index 960bf5eab59f..0e8f8de77e71 100644
> --- a/net/ipv4/ip_fragment.c
> +++ b/net/ipv4/ip_fragment.c
> @@ -136,7 +136,7 @@ static void ip_expire(struct timer_list *t)
> {
> struct inet_frag_queue *frag = from_timer(frag, t, timer);
> const struct iphdr *iph;
> - struct sk_buff *head;
> + struct sk_buff *head = NULL;
> struct net *net;
> struct ipq *qp;
> int err;
> @@ -152,14 +152,31 @@ static void ip_expire(struct timer_list *t)
>
> ipq_kill(qp);
> __IP_INC_STATS(net, IPSTATS_MIB_REASMFAILS);
> -
> - head = qp->q.fragments;
> -
> __IP_INC_STATS(net, IPSTATS_MIB_REASMTIMEOUT);
>
> - if (!(qp->q.flags & INET_FRAG_FIRST_IN) || !head)
> + if (!qp->q.flags & INET_FRAG_FIRST_IN)
> goto out;
>
> + /* sk_buff::dev and sk_buff::rbnode are unionized. So we
> + * pull the head out of the tree in order to be able to
> + * deal with head->dev.
> + */
> + if (qp->q.fragments) {
> + head = qp->q.fragments;
> + qp->q.fragments = head->next;
> + } else {
> + head = skb_rb_first(&qp->q.rb_fragments);
> + if (!head)
> + goto out;
> + rb_erase(&head->rbnode, &qp->q.rb_fragments);
> + memset(&head->rbnode, 0, sizeof(head->rbnode));
> + barrier();
> + }
> + if (head == qp->q.fragments_tail)
> + qp->q.fragments_tail = NULL;
> +
> + sub_frag_mem_limit(qp->q.net, head->truesize);
> +
> head->dev = dev_get_by_index_rcu(net, qp->iif);
> if (!head->dev)
> goto out;
> @@ -179,16 +196,16 @@ static void ip_expire(struct timer_list *t)
> (skb_rtable(head)->rt_type != RTN_LOCAL))
> goto out;
>
> - skb_get(head);
> spin_unlock(&qp->q.lock);
> icmp_send(head, ICMP_TIME_EXCEEDED, ICMP_EXC_FRAGTIME, 0);
> - kfree_skb(head);
> goto out_rcu_unlock;
>
> out:
> spin_unlock(&qp->q.lock);
> out_rcu_unlock:
> rcu_read_unlock();
> + if (head)
> + kfree_skb(head);
> ipq_put(qp);
> }
>
> @@ -231,7 +248,7 @@ static int ip_frag_too_far(struct ipq *qp)
> end = atomic_inc_return(&peer->rid);
> qp->rid = end;
>
> - rc = qp->q.fragments && (end - start) > max;
> + rc = qp->q.fragments_tail && (end - start) > max;
>
> if (rc) {
> struct net *net;
> @@ -245,7 +262,6 @@ static int ip_frag_too_far(struct ipq *qp)
>
> static int ip_frag_reinit(struct ipq *qp)
> {
> - struct sk_buff *fp;
> unsigned int sum_truesize = 0;
>
> if (!mod_timer(&qp->q.timer, jiffies + qp->q.net->timeout)) {
> @@ -253,20 +269,14 @@ static int ip_frag_reinit(struct ipq *qp)
> return -ETIMEDOUT;
> }
>
> - fp = qp->q.fragments;
> - do {
> - struct sk_buff *xp = fp->next;
> -
> - sum_truesize += fp->truesize;
> - kfree_skb(fp);
> - fp = xp;
> - } while (fp);
> + sum_truesize = skb_rbtree_purge(&qp->q.rb_fragments);
> sub_frag_mem_limit(qp->q.net, sum_truesize);
>
> qp->q.flags = 0;
> qp->q.len = 0;
> qp->q.meat = 0;
> qp->q.fragments = NULL;
> + qp->q.rb_fragments = RB_ROOT;
> qp->q.fragments_tail = NULL;
> qp->iif = 0;
> qp->ecn = 0;
> @@ -278,7 +288,8 @@ static int ip_frag_reinit(struct ipq *qp)
> static int ip_frag_queue(struct ipq *qp, struct sk_buff *skb)
> {
> struct net *net = container_of(qp->q.net, struct net, ipv4.frags);
> - struct sk_buff *prev, *next;
> + struct rb_node **rbn, *parent;
> + struct sk_buff *skb1;
> struct net_device *dev;
> unsigned int fragsize;
> int flags, offset;
> @@ -341,58 +352,58 @@ static int ip_frag_queue(struct ipq *qp, struct sk_buff *skb)
> if (err)
> goto err;
>
> - /* Find out which fragments are in front and at the back of us
> - * in the chain of fragments so far. We must know where to put
> - * this fragment, right?
> - */
> - prev = qp->q.fragments_tail;
> - if (!prev || prev->ip_defrag_offset < offset) {
> - next = NULL;
> - goto found;
> - }
> - prev = NULL;
> - for (next = qp->q.fragments; next != NULL; next = next->next) {
> - if (next->ip_defrag_offset >= offset)
> - break; /* bingo! */
> - prev = next;
> - }
> + /* Note : skb->rbnode and skb->dev share the same location. */
> + dev = skb->dev;
> + /* Makes sure compiler wont do silly aliasing games */
> + barrier();
>
> -found:
> /* RFC5722, Section 4, amended by Errata ID : 3089
> * When reassembling an IPv6 datagram, if
> * one or more its constituent fragments is determined to be an
> * overlapping fragment, the entire datagram (and any constituent
> * fragments) MUST be silently discarded.
> *
> - * We do the same here for IPv4.
> + * We do the same here for IPv4 (and increment an snmp counter).
> */
>
> - /* Is there an overlap with the previous fragment? */
> - if (prev &&
> - (prev->ip_defrag_offset + prev->len) > offset)
> - goto discard_qp;
> -
> - /* Is there an overlap with the next fragment? */
> - if (next && next->ip_defrag_offset < end)
> - goto discard_qp;
> + /* Find out where to put this fragment. */
> + skb1 = qp->q.fragments_tail;
> + if (!skb1) {
> + /* This is the first fragment we've received. */
> + rb_link_node(&skb->rbnode, NULL, &qp->q.rb_fragments.rb_node);
> + qp->q.fragments_tail = skb;
> + } else if ((skb1->ip_defrag_offset + skb1->len) < end) {
> + /* This is the common/special case: skb goes to the end. */
> + /* Detect and discard overlaps. */
> + if (offset < (skb1->ip_defrag_offset + skb1->len))
> + goto discard_qp;
> + /* Insert after skb1. */
> + rb_link_node(&skb->rbnode, &skb1->rbnode, &skb1->rbnode.rb_right);
> + qp->q.fragments_tail = skb;
> + } else {
> + /* Binary search. Note that skb can become the first fragment, but
> + * not the last (covered above). */
> + rbn = &qp->q.rb_fragments.rb_node;
> + do {
> + parent = *rbn;
> + skb1 = rb_to_skb(parent);
> + if (end <= skb1->ip_defrag_offset)
> + rbn = &parent->rb_left;
> + else if (offset >= skb1->ip_defrag_offset + skb1->len)
> + rbn = &parent->rb_right;
> + else /* Found an overlap with skb1. */
> + goto discard_qp;
> + } while (*rbn);
> + /* Here we have parent properly set, and rbn pointing to
> + * one of its NULL left/right children. Insert skb. */
> + rb_link_node(&skb->rbnode, parent, rbn);
> + }
> + rb_insert_color(&skb->rbnode, &qp->q.rb_fragments);
>
> - /* 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;
> - if (!next)
> - qp->q.fragments_tail = skb;
> - if (prev)
> - prev->next = skb;
> - else
> - qp->q.fragments = skb;
> -
> qp->q.stamp = skb->tstamp;
> qp->q.meat += skb->len;
> qp->ecn |= ecn;
> @@ -414,7 +425,7 @@ static int ip_frag_queue(struct ipq *qp, struct sk_buff *skb)
> unsigned long orefdst = skb->_skb_refdst;
>
> skb->_skb_refdst = 0UL;
> - err = ip_frag_reasm(qp, prev, dev);
> + err = ip_frag_reasm(qp, skb, dev);
> skb->_skb_refdst = orefdst;
> return err;
> }
> @@ -431,15 +442,15 @@ static int ip_frag_queue(struct ipq *qp, struct sk_buff *skb)
> return err;
> }
>
> -
> /* Build a new IP datagram from all its fragments. */
> -
> -static int ip_frag_reasm(struct ipq *qp, struct sk_buff *prev,
> +static int ip_frag_reasm(struct ipq *qp, struct sk_buff *skb,
> struct net_device *dev)
> {
> struct net *net = container_of(qp->q.net, struct net, ipv4.frags);
> struct iphdr *iph;
> - struct sk_buff *fp, *head = qp->q.fragments;
> + struct sk_buff *fp, *head = skb_rb_first(&qp->q.rb_fragments);
> + struct sk_buff **nextp; /* To build frag_list. */
> + struct rb_node *rbn;
> int len;
> int ihlen;
> int err;
> @@ -453,25 +464,20 @@ static int ip_frag_reasm(struct ipq *qp, struct sk_buff *prev,
> goto out_fail;
> }
> /* Make the one we just received the head. */
> - if (prev) {
> - head = prev->next;
> - fp = skb_clone(head, GFP_ATOMIC);
> + if (head != skb) {
> + fp = skb_clone(skb, GFP_ATOMIC);
> if (!fp)
> goto out_nomem;
> -
> - fp->next = head->next;
> - if (!fp->next)
> + rb_replace_node(&skb->rbnode, &fp->rbnode, &qp->q.rb_fragments);
> + if (qp->q.fragments_tail == skb)
> qp->q.fragments_tail = fp;
> - prev->next = fp;
> -
> - skb_morph(head, qp->q.fragments);
> - head->next = qp->q.fragments->next;
> -
> - consume_skb(qp->q.fragments);
> - qp->q.fragments = head;
> + skb_morph(skb, head);
> + rb_replace_node(&head->rbnode, &skb->rbnode,
> + &qp->q.rb_fragments);
> + consume_skb(head);
> + head = skb;
> }
>
> - WARN_ON(!head);
> WARN_ON(head->ip_defrag_offset != 0);
>
> /* Allocate a new buffer for the datagram. */
> @@ -496,24 +502,35 @@ static int ip_frag_reasm(struct ipq *qp, struct sk_buff *prev,
> clone = alloc_skb(0, GFP_ATOMIC);
> if (!clone)
> goto out_nomem;
> - clone->next = head->next;
> - head->next = clone;
> skb_shinfo(clone)->frag_list = skb_shinfo(head)->frag_list;
> skb_frag_list_init(head);
> for (i = 0; i < skb_shinfo(head)->nr_frags; i++)
> plen += skb_frag_size(&skb_shinfo(head)->frags[i]);
> clone->len = clone->data_len = head->data_len - plen;
> - head->data_len -= clone->len;
> - head->len -= clone->len;
> + skb->truesize += clone->truesize;
> clone->csum = 0;
> clone->ip_summed = head->ip_summed;
> add_frag_mem_limit(qp->q.net, clone->truesize);
> + skb_shinfo(head)->frag_list = clone;
> + nextp = &clone->next;
> + } else {
> + nextp = &skb_shinfo(head)->frag_list;
> }
>
> - skb_shinfo(head)->frag_list = head->next;
> skb_push(head, head->data - skb_network_header(head));
>
> - for (fp=head->next; fp; fp = fp->next) {
> + /* Traverse the tree in order, to build frag_list. */
> + rbn = rb_next(&head->rbnode);
> + rb_erase(&head->rbnode, &qp->q.rb_fragments);
> + while (rbn) {
> + struct rb_node *rbnext = rb_next(rbn);
> + fp = rb_to_skb(rbn);
> + rb_erase(rbn, &qp->q.rb_fragments);
> + rbn = rbnext;
> + *nextp = fp;
> + nextp = &fp->next;
> + fp->prev = NULL;
> + memset(&fp->rbnode, 0, sizeof(fp->rbnode));
> head->data_len += fp->len;
> head->len += fp->len;
> if (head->ip_summed != fp->ip_summed)
> @@ -524,7 +541,9 @@ static int ip_frag_reasm(struct ipq *qp, struct sk_buff *prev,
> }
> sub_frag_mem_limit(qp->q.net, head->truesize);
>
> + *nextp = NULL;
> head->next = NULL;
> + head->prev = NULL;
> head->dev = dev;
> head->tstamp = qp->q.stamp;
> IPCB(head)->frag_max_size = max(qp->max_df_size, qp->q.max_size);
> @@ -552,6 +571,7 @@ static int ip_frag_reasm(struct ipq *qp, struct sk_buff *prev,
>
> __IP_INC_STATS(net, IPSTATS_MIB_REASMOKS);
> qp->q.fragments = NULL;
> + qp->q.rb_fragments = RB_ROOT;
> qp->q.fragments_tail = NULL;
> return 0;
>
> diff --git a/net/ipv6/netfilter/nf_conntrack_reasm.c b/net/ipv6/netfilter/nf_conntrack_reasm.c
> index 1d2f07cde01a..82ce0d0f54bf 100644
> --- a/net/ipv6/netfilter/nf_conntrack_reasm.c
> +++ b/net/ipv6/netfilter/nf_conntrack_reasm.c
> @@ -471,6 +471,7 @@ nf_ct_frag6_reasm(struct frag_queue *fq, struct sk_buff *prev, struct net_devic
> head->csum);
>
> fq->q.fragments = NULL;
> + fq->q.rb_fragments = RB_ROOT;
> fq->q.fragments_tail = NULL;
>
> return true;
> diff --git a/net/ipv6/reassembly.c b/net/ipv6/reassembly.c
> index afaad60dc2ac..ede0061b6f5d 100644
> --- a/net/ipv6/reassembly.c
> +++ b/net/ipv6/reassembly.c
> @@ -472,6 +472,7 @@ static int ip6_frag_reasm(struct frag_queue *fq, struct sk_buff *prev,
> __IP6_INC_STATS(net, __in6_dev_get(dev), IPSTATS_MIB_REASMOKS);
> rcu_read_unlock();
> fq->q.fragments = NULL;
> + fq->q.rb_fragments = RB_ROOT;
> fq->q.fragments_tail = NULL;
> return 1;
>
> --
> 2.18.0
>