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Message-ID: <1273554387.10889.23.camel@edumazet-laptop> Date: Tue, 11 May 2010 07:06:27 +0200 From: Eric Dumazet <eric.dumazet@...il.com> To: Sharat Masetty <sharat_masetty@...oo.com> Cc: netdev@...r.kernel.org, linux-kernel <linux-kernel@...r.kernel.org> Subject: Re: Question about more headroom in skb Le lundi 10 mai 2010 à 13:09 -0700, Sharat Masetty a écrit : > Hello All, Please dont use too long lines > > For my project I need 3 words of headroom in the skb in the network > driver level, to add a custom header to the ethernet packet. I > looked into the tcp code and figured out tcp uses sk->sk_prot->max_header > for header allocation size. But I was not able to confirm that all other > transport protocol use the same mechanism(?) For example in UDP/ICMP I was > not able to figure out from the code where the allocation and header > reservation happens(Any light here would be really helpful.) > > I have also looked at an API in skbuff skb_pad() which does what I want > (add either headroom or tailroom), but I want to avoid that for performance > reasons(skb_pad does kmalloc and memcpy). I want to figure out a good way > (may be tune some parameters) to allocate extra 3 words for any skbuff > independant of the transport protocol being used. > Any light here would be very much appreciated. LL_RESERVED_SPACE() is the magic you need. #define LL_RESERVED_SPACE(dev) \ ((((dev)->hard_header_len+(dev)->needed_headroom)&~(HH_DATA_MOD - 1)) + HH_DATA_MOD) sendmsg() -> ip_append_data() ... hh_len = LL_RESERVED_SPACE(rt->u.dst.dev); fragheaderlen = sizeof(struct iphdr) + (opt ? opt->optlen : 0); ... if (transhdrlen) { skb = sock_alloc_send_skb(sk, alloclen + hh_len + 15, (flags & MSG_DONTWAIT), &err); } else { skb = NULL; if (atomic_read(&sk->sk_wmem_alloc) <= 2 * sk->sk_sndbuf) skb = sock_wmalloc(sk, alloclen + hh_len + 15, 1, sk->sk_allocation); -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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