Re: [PATCH v2 net] udp: gso: do not drop small packets when PMTU reduces

[Willem de Bruijn]

Yan Zhai wrote:
> Commit 4094871db1d6 ("udp: only do GSO if # of segs > 1") avoided GSO
> for small packets. But the kernel currently dismisses GSO requests only
> after checking MTU/PMTU on gso_size. This means any packets, regardless
> of their payload sizes, could be dropped when PMTU becomes smaller than
> requested gso_size. We encountered this issue in production and it
> caused a reliability problem that new QUIC connection cannot be
> established before PMTU cache expired, while non GSO sockets still
> worked fine at the same time.
> 
> Ideally, do not check any GSO related constraints when payload size is
> smaller than requested gso_size, and return EMSGSIZE instead of EINVAL
> on MTU/PMTU check failure to be more specific on the error cause.
> 
> Fixes: 4094871db1d6 ("udp: only do GSO if # of segs > 1")
> Signed-off-by: Yan Zhai <yan@xxxxxxxxxxxxxx>
> --
> v1->v2: add a missing MTU check when fall back to no GSO mode suggested
> by Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx>; Fixed up commit
> message to be more precise.
> 
> v1: https://lore.kernel.org/all/Z5cgWh%2F6bRQm9vVU@debian.debian/
> ---
>  net/ipv4/udp.c                       | 28 +++++++++++++++++++---------
>  net/ipv6/udp.c                       | 28 +++++++++++++++++++---------
>  tools/testing/selftests/net/udpgso.c | 14 ++++++++++++++
>  3 files changed, 52 insertions(+), 18 deletions(-)
> 
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index c472c9a57cf6..0b5010238d05 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -1141,9 +1141,20 @@ static int udp_send_skb(struct sk_buff *skb, struct flowi4 *fl4,
>  		const int hlen = skb_network_header_len(skb) +
>  				 sizeof(struct udphdr);
>  
> +		if (datalen <= cork->gso_size) {
> +			/*
> +			 * check MTU again: it's skipped previously when
> +			 * gso_size != 0
> +			 */
> +			if (hlen + datalen > cork->fragsize) {
> +				kfree_skb(skb);
> +				return -EMSGSIZE;
> +			}
> +			goto no_gso;

This is almost the same as the test below.

How about just

    if (hlen + min(cork->gso_size, datalen) > cork->fragsize)

And don't bypass the subsequent checks with a goto, or modify the
rest of the code.

> +		}
>  		if (hlen + cork->gso_size > cork->fragsize) {
>  			kfree_skb(skb);
> -			return -EINVAL;
> +			return -EMSGSIZE;
>  		}
>  		if (datalen > cork->gso_size * UDP_MAX_SEGMENTS) {
>  			kfree_skb(skb);
> @@ -1158,17 +1169,16 @@ static int udp_send_skb(struct sk_buff *skb, struct flowi4 *fl4,
>  			return -EIO;
>  		}
>  
> -		if (datalen > cork->gso_size) {
> -			skb_shinfo(skb)->gso_size = cork->gso_size;
> -			skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
> -			skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
> -								 cork->gso_size);
> +		skb_shinfo(skb)->gso_size = cork->gso_size;
> +		skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
> +		skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
> +							 cork->gso_size);
>  
> -			/* Don't checksum the payload, skb will get segmented */
> -			goto csum_partial;
> -		}
> +		/* Don't checksum the payload, skb will get segmented */
> +		goto csum_partial;
>  	}
>  
> +no_gso:
>  	if (is_udplite)  				 /*     UDP-Lite      */
>  		csum = udplite_csum(skb);
>  
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> index 6671daa67f4f..d97befa7f80d 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -1389,9 +1389,20 @@ static int udp_v6_send_skb(struct sk_buff *skb, struct flowi6 *fl6,
>  		const int hlen = skb_network_header_len(skb) +
>  				 sizeof(struct udphdr);
>  
> +		if (datalen <= cork->gso_size) {
> +			/*
> +			 * check MTU again: it's skipped previously when
> +			 * gso_size != 0
> +			 */
> +			if (hlen + datalen > cork->fragsize) {
> +				kfree_skb(skb);
> +				return -EMSGSIZE;
> +			}
> +			goto no_gso;
> +		}
>  		if (hlen + cork->gso_size > cork->fragsize) {
>  			kfree_skb(skb);
> -			return -EINVAL;
> +			return -EMSGSIZE;
>  		}
>  		if (datalen > cork->gso_size * UDP_MAX_SEGMENTS) {
>  			kfree_skb(skb);
> @@ -1406,17 +1417,16 @@ static int udp_v6_send_skb(struct sk_buff *skb, struct flowi6 *fl6,
>  			return -EIO;
>  		}
>  
> -		if (datalen > cork->gso_size) {
> -			skb_shinfo(skb)->gso_size = cork->gso_size;
> -			skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
> -			skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
> -								 cork->gso_size);
> +		skb_shinfo(skb)->gso_size = cork->gso_size;
> +		skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
> +		skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
> +							 cork->gso_size);
>  
> -			/* Don't checksum the payload, skb will get segmented */
> -			goto csum_partial;
> -		}
> +		/* Don't checksum the payload, skb will get segmented */
> +		goto csum_partial;
>  	}
>  
> +no_gso:
>  	if (is_udplite)
>  		csum = udplite_csum(skb);
>  	else if (udp_get_no_check6_tx(sk)) {   /* UDP csum disabled */
> diff --git a/tools/testing/selftests/net/udpgso.c b/tools/testing/selftests/net/udpgso.c
> index 3f2fca02fec5..fb73f1c331fb 100644
> --- a/tools/testing/selftests/net/udpgso.c
> +++ b/tools/testing/selftests/net/udpgso.c
> @@ -102,6 +102,13 @@ struct testcase testcases_v4[] = {
>  		.gso_len = CONST_MSS_V4,
>  		.r_num_mss = 1,
>  	},
> +	{
> +		/* datalen <= MSS < gso_len: will fall back to no GSO */
> +		.tlen = CONST_MSS_V4,
> +		.gso_len = CONST_MSS_V4 + 1,
> +		.r_num_mss = 0,
> +		.r_len_last = CONST_MSS_V4,
> +	},

Please also add a test where datalen > MSS < gso_len (with .tfail = true).

>  	{
>  		/* send a single MSS + 1B */
>  		.tlen = CONST_MSS_V4 + 1,
> @@ -205,6 +212,13 @@ struct testcase testcases_v6[] = {
>  		.gso_len = CONST_MSS_V6,
>  		.r_num_mss = 1,
>  	},
> +	{
> +		/* datalen <= MSS < gso_len: will fall back to no GSO */
> +		.tlen = CONST_MSS_V6,
> +		.gso_len = CONST_MSS_V6 + 1,
> +		.r_num_mss = 0,
> +		.r_len_last = CONST_MSS_V6,
> +	},
>  	{
>  		/* send a single MSS + 1B */
>  		.tlen = CONST_MSS_V6 + 1,
> -- 
> 2.30.2
> 
>