Return-Path: Message-ID: <543B90FA.2060603@xsilon.com> Date: Mon, 13 Oct 2014 09:44:42 +0100 From: Martin Townsend MIME-Version: 1.0 To: Alexander Aring , Martin Townsend CC: linux-bluetooth@vger.kernel.org, linux-wpan@vger.kernel.org, marcel@holtmann.org, jukka.rissanen@linux.intel.com Subject: Re: [PATCH v5 bluetooth-next] 6lowpan: Use pskb_expand_head in IPHC decompression. References: <1412840794-17283-1-git-send-email-martin.townsend@xsilon.com> <1412840794-17283-2-git-send-email-martin.townsend@xsilon.com> <20141011073620.GA28500@omega> In-Reply-To: <20141011073620.GA28500@omega> Content-Type: text/plain; charset=utf-8 List-ID: Hi Alex, On 11/10/14 08:36, Alexander Aring wrote: > Hi Martin, > > This is only a summarize what we should now to try to do here for next > version. > > I know... this patch ends in a global debate on principles how we deal > with the replacement of 6LoWPAN -> IPv6 header. Current behaviour is > more a hack. > > On Thu, Oct 09, 2014 at 08:46:34AM +0100, Martin Townsend wrote: >> Currently there are potentially 2 skb_copy_expand calls in IPHC >> decompression. This patch replaces this with one call to >> pskb_expand_head. It also checks to see if there is enough headroom >> first to ensure it's only done if necessary. >> As pskb_expand_head must only have one reference the calling code >> now ensures this. >> >> Signed-off-by: Martin Townsend >> --- >> net/6lowpan/iphc.c | 51 ++++++++++++++++++++++++------------------------- >> net/bluetooth/6lowpan.c | 7 +++++++ >> 2 files changed, 32 insertions(+), 26 deletions(-) >> >> diff --git a/net/6lowpan/iphc.c b/net/6lowpan/iphc.c >> index 142eef5..853b4b8 100644 >> --- a/net/6lowpan/iphc.c >> +++ b/net/6lowpan/iphc.c >> @@ -174,30 +174,22 @@ static int uncompress_context_based_src_addr(struct sk_buff *skb, >> static int skb_deliver(struct sk_buff *skb, struct ipv6hdr *hdr, >> struct net_device *dev, skb_delivery_cb deliver_skb) >> { >> - struct sk_buff *new; >> int stat; >> >> - new = skb_copy_expand(skb, sizeof(struct ipv6hdr), skb_tailroom(skb), >> - GFP_ATOMIC); >> - kfree_skb(skb); >> - >> - if (!new) >> - return -ENOMEM; >> - >> - skb_push(new, sizeof(struct ipv6hdr)); >> - skb_reset_network_header(new); >> - skb_copy_to_linear_data(new, hdr, sizeof(struct ipv6hdr)); >> + skb_push(skb, sizeof(struct ipv6hdr)); >> + skb_reset_network_header(skb); >> + skb_copy_to_linear_data(skb, hdr, sizeof(struct ipv6hdr)); >> >> - new->protocol = htons(ETH_P_IPV6); >> - new->pkt_type = PACKET_HOST; >> - new->dev = dev; >> + skb->protocol = htons(ETH_P_IPV6); >> + skb->pkt_type = PACKET_HOST; >> + skb->dev = dev; >> >> raw_dump_table(__func__, "raw skb data dump before receiving", >> - new->data, new->len); >> + skb->data, skb->len); >> >> - stat = deliver_skb(new, dev); >> + stat = deliver_skb(skb, dev); >> >> - kfree_skb(new); >> + consume_skb(skb); >> >> return stat; >> } >> @@ -460,7 +452,7 @@ int lowpan_process_data(struct sk_buff *skb, struct net_device *dev, >> /* UDP data uncompression */ >> if (iphc0 & LOWPAN_IPHC_NH_C) { >> struct udphdr uh; >> - struct sk_buff *new; >> + const int needed = sizeof(struct udphdr) + sizeof(hdr); >> >> if (uncompress_udp_header(skb, &uh)) >> goto drop; >> @@ -468,14 +460,13 @@ int lowpan_process_data(struct sk_buff *skb, struct net_device *dev, >> /* replace the compressed UDP head by the uncompressed UDP >> * header >> */ >> - new = skb_copy_expand(skb, sizeof(struct udphdr), >> - skb_tailroom(skb), GFP_ATOMIC); >> - kfree_skb(skb); >> - >> - if (!new) >> - return -ENOMEM; >> - >> - skb = new; >> + if (skb_headroom(skb) < needed) { >> + err = pskb_expand_head(skb, needed, 0, GFP_ATOMIC); >> + if (unlikely(err)) { >> + kfree_skb(skb); >> + return err; >> + } >> + } > use skb_cow_head here. I don't know much about skb_cow_head but from the description it says that only the head is writable which I assume is the newly requested headroom? I imagine this is for the usual case where you are adding headers and you go through the protocol layers. For 6lowpan aren't we removing one header and replacing with a decompressed header which would suggest that we are going to modify the data (which would contain the 6lowpan header). skb_cow seems a good fit to me though. > >> >> skb_push(skb, sizeof(struct udphdr)); >> skb_reset_transport_header(skb); >> @@ -485,6 +476,14 @@ int lowpan_process_data(struct sk_buff *skb, struct net_device *dev, >> (u8 *)&uh, sizeof(uh)); >> >> hdr.nexthdr = UIP_PROTO_UDP; >> + } else { >> + if (skb_headroom(skb) < sizeof(hdr)) { >> + err = pskb_expand_head(skb, sizeof(hdr), 0, GFP_ATOMIC); >> + if (unlikely(err)) { >> + kfree_skb(skb); >> + return err; >> + } >> + } > same here. > >> } >> >> hdr.payload_len = htons(skb->len); >> diff --git a/net/bluetooth/6lowpan.c b/net/bluetooth/6lowpan.c >> index c2e0d14..6643a7c 100644 >> --- a/net/bluetooth/6lowpan.c >> +++ b/net/bluetooth/6lowpan.c >> @@ -343,6 +343,13 @@ static int recv_pkt(struct sk_buff *skb, struct net_device *dev, >> kfree_skb(local_skb); >> kfree_skb(skb); >> } else { >> + /* Decompression may use pskb_expand_head so no shared skb's */ >> + skb = skb_share_check(skb, GFP_ATOMIC); >> + if (!skb) { >> + dev->stats.rx_dropped++; >> + return NET_RX_DROP; >> + } >> + > I see now that case LOWPAN_DISPATCH_IPHC: runs a "local_skb = > skb_clone(skb, GFP_ATOMIC);" and send the local_skb to process_data. So > is the share_check really needed? In my opinion there are several things > wrong. > > What bluetooth should do here is: > > Always run skb = skb_share_check(skb, GFP_ATOMIC); at first of this function. > > In if (skb->data[0] == LOWPAN_DISPATCH_IPV6): > > - only run a skb_pull (doesn't change the data buffer, only skb attribute) > for the one byte dispatch. > - run other things like skb_reset_network_header, etc... > > -> Currently this works because we run a complete skb_copy_expand here. > But this is not needed, we need a clone because we don't modify the > data buffer. > > -> To Jukka: I don't see a skb_pull for the one byte dispatch value? > What's happend here? Also IPv6 dispatch is part of rfc4944 and btle > 6LoWPAN is only realted to rfc6282 (IPHC). I think you don't need handling > for this here. > > > > In "case LOWPAN_DISPATCH_IPHC:" we should remove the skb_clone, because > this is already handled by "skb_share_check" then. Not necessarily, if the skb isn't shared, ie no-one has called skb_get on it then the clone will not happen. I tried removing the skb_clone as in theory psk_expand_head should handle cloned skb's but I think it caused the oops that Jukka was seeing. Because of this I only want to make changes absolutely necessary to get pskb_expand_head or skb_cow working as I can't test the bluetooth code and have to rely on Jukka. If it's ok with everyone I'll change pskb_expand_head to skb_cow or skb_cow_head and leave the bluetooth code as it is with this v5 patch. > > - Alex > -- > To unsubscribe from this list: send the line "unsubscribe linux-wpan" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html - Martin