Subject: Re: [PATCH net-next 1/5] sctp: add the rhashtable apis for sctp global transport hashtable

From: [email-masked]
Date: Mon, 11 Jan 2016 14:00:40 -0200

On Mon, Jan 11, 2016 at 05:30:12PM +0800, Herbert Xu wrote:
> Xin Long <[email-masked]> wrote:
> >
> > +static inline int sctp_hash_cmp(struct rhashtable_compare_arg *arg,
> > +                               const void *ptr)
> > +{
> > +       const struct sctp_hash_cmp_arg *x = arg->key;
> > +       const struct sctp_transport *t = ptr;
> > +       struct sctp_association *asoc = t->asoc;
> > +       const struct net *net = x->net;
> > +
> > +       if (x->laddr->v4.sin_port != htons(asoc->base.bind_addr.port))
> > +               return 1;
> > +       if (!sctp_cmp_addr_exact(&t->ipaddr, x->paddr))
> > +               return 1;
> > +       if (!net_eq(sock_net(asoc->base.sk), net))
> > +               return 1;
> > +       if (!sctp_bind_addr_match(&asoc->base.bind_addr,
> > +                                 x->laddr, sctp_sk(asoc->base.sk)))
> > +               return 1;
> > +
> > +       return 0;
> > +}
> > +
> > +static inline u32 sctp_hash_obj(const void *data, u32 len, u32 seed)
> > +{
> > +       const struct sctp_transport *t = data;
> > +       const union sctp_addr *paddr = &t->ipaddr;
> > +       const struct net *net = sock_net(t->asoc->base.sk);
> > +       u16 lport = htons(t->asoc->base.bind_addr.port);
> > +       u32 addr;
> > +
> > +       if (paddr->sa.sa_family == AF_INET6)
> > +               addr = jhash(&paddr->v6.sin6_addr, 16, seed);
> > +       else
> > +               addr = paddr->v4.sin_addr.s_addr;
> > +
> > +       return  jhash_3words(addr, ((__u32)paddr->v4.sin_port) << 16 |
> > +                            (__force __u32)lport, net_hash_mix(net), seed);
> > +}
> > +
> > +static inline u32 sctp_hash_key(const void *data, u32 len, u32 seed)
> > +{
> > +       const struct sctp_hash_cmp_arg *x = data;
> > +       const union sctp_addr *paddr = x->paddr;
> > +       const struct net *net = x->net;
> > +       u16 lport = x->laddr->v4.sin_port;
> > +       u32 addr;
> > +
> > +       if (paddr->sa.sa_family == AF_INET6)
> > +               addr = jhash(&paddr->v6.sin6_addr, 16, seed);
> > +       else
> > +               addr = paddr->v4.sin_addr.s_addr;
> > +
> > +       return  jhash_3words(addr, ((__u32)paddr->v4.sin_port) << 16 |
> > +                            (__force __u32)lport, net_hash_mix(net), seed);
> > +}
> 
> There's your problem.  You are allowing multiple objects to hash
> to the same value.  This is unacceptable with rhashtable because
> we use the hash chain length to determine if we're under attack
> and need to rehash.  This is the reason why you would see EBUSY
> during insertion.

Cool. Then I guess we don't really have an issue here. The case that
fails is an artificial load test which is virtually impossible to be hit
in real world, or at least I really hope so. The test, as in Xin's
attachment, will load more than 1600 IP addresses in one host (2 vCPU
during the test) and attempt to start an assoc from each of those using
the very same (lport, daddr, dport)-tuple.

Doing so is just unreasonable. Note that net is also hashed, so
even if we consider it could be 1600 containers, it is fine.

> So instead of inserting your objects as is, you should instead hash
> a list object that then links to all the objects that has the same
> hash key.

Now that it is clarified, I'm thinking we should just get ride of that
loop on EBUSY. No real reason to have extra code only to support an
artificial test. Agree?

Thanks,
Marcelo