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Date:   Fri, 17 May 2019 13:32:33 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        netdev@...r.kernel.org
Cc:     Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next RFC] ipv6: elide flowlabel check if no exclusive
 leases exist



On 5/17/19 8:56 AM, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@...gle.com>
> 
> Processes can request ipv6 flowlabels with cmsg IPV6_FLOWINFO.
> If not set, by default an autogenerated flowlabel is selected.
> 
> Explicit flowlabels require a control operation per label plus a
> datapath check on every connection (every datagram if unconnected).
> 
> This is particularly expensive on unconnected sockets with many
> connections, such as QUIC.
> 
> In the common case, where no lease is exclusive, the check can be
> safely elided, as both lease request and check trivially succeed.
> Indeed, autoflowlabel does the same (even with exclusive leases).
> 
> Elide the check if no process has requested an exclusive lease.
> 
> This is an optimization. Robust applications still have to revert to
> requesting leases if the fast path fails due to an exclusive lease.
> 
> This is decidedly an RFC patch:
> - need to update all fl6_sock_lookup callers, not just udp
> - behavior should be per-netns isolated
> 
> Other approaches considered:
> - a single "get all flowlabels, non-exclusive" flowlabel get request
>   if set, elide fl6_sock_lookup and fail exclusive lease requests
> 
> - sysctls (only useful if on by default, with static_branch)
>   A) "non-exclusive mode", failing all exclusive lease requests:
>      processes already have to be robust against lease failure
>   B) just bypass check in fl6_sock_lookup, like autoflowlabel
> 
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> ---
>  include/net/ipv6.h       | 11 +++++++++++
>  net/ipv6/ip6_flowlabel.c |  6 ++++++
>  net/ipv6/udp.c           |  8 ++++----
>  3 files changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/include/net/ipv6.h b/include/net/ipv6.h
> index daf80863d3a50..8881cee572410 100644
> --- a/include/net/ipv6.h
> +++ b/include/net/ipv6.h
> @@ -17,6 +17,7 @@
>  #include <linux/hardirq.h>
>  #include <linux/jhash.h>
>  #include <linux/refcount.h>
> +#include <linux/jump_label.h>
>  #include <net/if_inet6.h>
>  #include <net/ndisc.h>
>  #include <net/flow.h>
> @@ -343,7 +344,17 @@ static inline void txopt_put(struct ipv6_txoptions *opt)
>  		kfree_rcu(opt, rcu);
>  }
>  
> +extern struct static_key_false ipv6_flowlabel_exclusive;
>  struct ip6_flowlabel *fl6_sock_lookup(struct sock *sk, __be32 label);
> +static inline struct ip6_flowlabel *fl6_sock_verify(struct sock *sk,
> +						    __be32 label)
> +{
> +	if (static_branch_unlikely(&ipv6_flowlabel_exclusive))
> +		return fl6_sock_lookup(sk, label) ? : ERR_PTR(-ENOENT);
> +
> +	return NULL;
> +}
> +
>  struct ipv6_txoptions *fl6_merge_options(struct ipv6_txoptions *opt_space,
>  					 struct ip6_flowlabel *fl,
>  					 struct ipv6_txoptions *fopt);
> diff --git a/net/ipv6/ip6_flowlabel.c b/net/ipv6/ip6_flowlabel.c
> index be5f3d7ceb966..d5f4233b04e0c 100644
> --- a/net/ipv6/ip6_flowlabel.c
> +++ b/net/ipv6/ip6_flowlabel.c
> @@ -57,6 +57,8 @@ static DEFINE_SPINLOCK(ip6_fl_lock);
>  
>  static DEFINE_SPINLOCK(ip6_sk_fl_lock);
>  
> +DEFINE_STATIC_KEY_FALSE(ipv6_flowlabel_exclusive);
> +
>  #define for_each_fl_rcu(hash, fl)				\
>  	for (fl = rcu_dereference_bh(fl_ht[(hash)]);		\
>  	     fl != NULL;					\
> @@ -98,6 +100,8 @@ static void fl_free_rcu(struct rcu_head *head)
>  {
>  	struct ip6_flowlabel *fl = container_of(head, struct ip6_flowlabel, rcu);
>  
> +	if (fl->share != IPV6_FL_S_NONE && fl->share != IPV6_FL_S_ANY)
> +		static_branch_dec(&ipv6_flowlabel_exclusive);

static_branch_dec() can not be invoked from a rcu call back.

>  	if (fl->share == IPV6_FL_S_PROCESS)
>  		put_pid(fl->owner.pid);
>  	kfree(fl->opt);
> @@ -423,6 +427,8 @@ fl_create(struct net *net, struct sock *sk, struct in6_flowlabel_req *freq,
>  	}
>  	fl->dst = freq->flr_dst;
>  	atomic_set(&fl->users, 1);
> +	if (fl->share != IPV6_FL_S_ANY)
> +		static_branch_inc(&ipv6_flowlabel_exclusive);


Can this be used by unpriv users ?

If yes, then you want to use static_key_false_deferred instead


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