netdev - Re: [PATCH net-next 3/3] udp: only use paged allocation with scatter-gather

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Date:   Mon, 14 May 2018 19:30:04 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next 3/3] udp: only use paged allocation with scatter-gather

On Mon, May 14, 2018 at 7:12 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
> On 05/14/2018 04:07 PM, Willem de Bruijn wrote:
>> From: Willem de Bruijn <willemb@...gle.com>
>>
>> Paged allocation stores most payload in skb frags. This helps udp gso
>> by avoiding copying from the gso skb to segment skb in skb_segment.
>>
>> But without scatter-gather, data must be linear, so do not use paged
>> mode unless NETIF_F_SG.
>>
>> Fixes: 15e36f5b8e98 ("udp: paged allocation with gso")
>> Reported-by: Sean Tranchetti <stranche@...eaurora.org>
>> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
>> ---
>>  net/ipv4/ip_output.c  | 2 +-
>>  net/ipv6/ip6_output.c | 2 +-
>>  2 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
>> index b5e21eb198d8..b38731d8a44f 100644
>> --- a/net/ipv4/ip_output.c
>> +++ b/net/ipv4/ip_output.c
>> @@ -884,7 +884,7 @@ static int __ip_append_data(struct sock *sk,
>>
>>       exthdrlen = !skb ? rt->dst.header_len : 0;
>>       mtu = cork->gso_size ? IP_MAX_MTU : cork->fragsize;
>> -     paged = !!cork->gso_size;
>> +     paged = cork->gso_size && (rt->dst.dev->features & NETIF_F_SG);
>>
>>       if (cork->tx_flags & SKBTX_ANY_SW_TSTAMP &&
>>           sk->sk_tsflags & SOF_TIMESTAMPING_OPT_ID)
>> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
>> index 7f4493080df6..35a940b9f208 100644
>> --- a/net/ipv6/ip6_output.c
>> +++ b/net/ipv6/ip6_output.c
>> @@ -1262,7 +1262,7 @@ static int __ip6_append_data(struct sock *sk,
>>               dst_exthdrlen = rt->dst.header_len - rt->rt6i_nfheader_len;
>>       }
>>
>> -     paged = !!cork->gso_size;
>> +     paged = cork->gso_size && (rt->dst.dev->features & NETIF_F_SG);
>>       mtu = cork->gso_size ? IP6_MAX_MTU : cork->fragsize;
>>       orig_mtu = mtu;
>>
>>
>
> As I said, this wont help for stacked device
>
> bonding might advertise NETIF_F_SG, but one slave might not.

I don't quite follow. The reported crash happens in the protocol layer,
because of this check. With pagedlen we have not allocated
sufficient space for the skb_put.

                if (!(rt->dst.dev->features&NETIF_F_SG)) {
                        unsigned int off;

                        off = skb->len;
                        if (getfrag(from, skb_put(skb, copy),
                                        offset, copy, off, skb) < 0) {
                                __skb_trim(skb, off);
                                err = -EFAULT;
                                goto error;
                        }
                } else {
                        int i = skb_shinfo(skb)->nr_frags;

Are you referring to a separate potential issue in the gso layer?
If a bonding device advertises SG, but a slave does not, then
skb_segment on the slave should build linear segs? I have not
tested that.