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authorYOSHIFUJI Hideaki / 吉藤英明 <yoshfuji@linux-ipv6.org>2013-01-04 03:58:04 +0000
committerDavid S. Miller <davem@davemloft.net>2013-01-04 15:16:44 -0800
commitb7dc8c3959fd43bfa0dbcf65375628c86665cb94 (patch)
treee1d73a23c35cd8de6bac74902d6d7b15be732535 /drivers/net/ethernet
parentff43da86c69d76a726ffe7d1666148960dc1d108 (diff)
downloadvexpress-lsk-b7dc8c3959fd43bfa0dbcf65375628c86665cb94.tar.gz
ndisc: Remove unused space at tail of skb for ndisc messages. (TAKE 3)
Currently, the size of skb allocated for NDISC is MAX_HEADER + LL_RESERVED_SPACE(dev) + packet length + dev->needed_tailroom, but only LL_RESERVED_SPACE(dev) bytes is "reserved" for headers. As a result, the skb looks like this (after construction of the message): head data tail end +--------------------------------------------------------------+ + | | | | +--------------------------------------------------------------+ |<-hlen---->|<---ipv6 packet------>|<--tlen-->|<--MAX_HEADER-->| =LL_ = dev RESERVED_ ->needed_ SPACE(dev) tailroom As the name implies, "MAX_HEADER" is used for headers, and should be "reserved" in prior to packet construction. Or, if some space is really required at the tail of ther skb, it should be explicitly documented. We have several option after construction of NDISC message: Option 1: head data tail end +---------------------------------------------+ + | | | +---------------------------------------------+ |<-hlen---->|<---ipv6 packet------>|<--tlen-->| =LL_ = dev RESERVED_ ->needed_ SPACE(dev) tailroom Option 2: head data tail end +--------------------------------------------------+ + | | | +--------------------------------------------------+ |<--MAX_HEADER-->|<---ipv6 packet------>|<--tlen-->| = dev ->needed_ tailroom Option 3: head data tail end +--------------------------------------------------------------+ + | | | | +--------------------------------------------------------------+ |<--MAX_HEADER-->|<-hlen---->|<---ipv6 packet------>|<--tlen-->| =LL_ = dev RESERVED_ ->needed_ SPACE(dev) tailroom Our tunnel drivers try expanding headroom and the space for tunnel encapsulation was not a mandatory space -- so we are not seeing bugs here --, but just for optimization for performance critial situations. Since NDISC messages are not performance critical unlike TCP, and as we know outgoing device, LL_RESERVED_SPACE(dev) should be just enough for the device in most (if not all) cases: LL_RESERVED_SPACE(dev) <= LL_MAX_HEADER <= MAX_HEADER Note that LL_RESERVED_SPACE(dev) is also enough for NDISC over SIT (e.g., ISATAP). So, I think Option 1 is just fine here. Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org> Acked-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'drivers/net/ethernet')
0 files changed, 0 insertions, 0 deletions