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 16:09:27 -0200

On Mon, Jan 11, 2016 at 12:20:17PM -0500, Vlad Yasevich wrote:
> On 01/11/2016 11:00 AM, [email-masked] wrote:
> > 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.
> 
> I have a hard time excepting this argument.  Just because a given test
> scenario may be unreasonable now, doesn't make so in the future.  If
> there is a way to solve the problem, then it should be done.  Saying
> this isn't really a problem isn't going to make it go away.

Heh, I understand..

There is still the other part of this thread to be worked on (re
->dead), maybe that will justify extra stuff in here but I really
wouldn't like to add extra structures and locks on this just to satisfy
an unreasonable scenario like this. This hash is very busy, the lean it
is, the better.

Maybe we could keep the loop as is for now, as a fail-safe, and add a
pr_warn_once() if it gets hit?

  Marcelo