[LTP] [PATCH 1/1] if-mtu-change.sh: Add max packet size detection for IPv4

[Alexey Kodanev]

On 15.12.2020 22:05, Petr Vorel wrote:
> Although theoretical maximum IP packet size is 65507, this does not work
> on s390x on low MTU. But according to Wenjia Zhang the same problem is
> on more platforms, e.g. x86:
> 
> A big IP packet is fragmented into small packets according to the MTU
> size.  Each small packet consumes an amount of system memory, which are
> recorded in ‘skb->truesize’. And the sum of the space all of the small
> packets need is limited by ‘2 * sk->sk_sndbuf’ (which you can find in
> '__ip_append_data' function in net/ipv4/ip_output.c), that is inherited
> from the system default set during socket initialization. The problem on
> s390 is that ‘skb->truesize’ is got by aligning the size of the data
> buffer to the L1 cache line (using SKB_DATA_ALIGN), which on s390 and
> only on s390 is 256 bytes, while on other processors less than or equal
> to 128 bytes. So the sum on s390 is much easier to exceed the limit,
> that the big packet can not be sent successfully.
> 
> Because of the big cache line size on s390, the maximum size of the
> packet is limited in between 53816 and 64960, if the MTU is set to
> between 484 and 588.
> However, this problem not only occurs on s390, but also on some other
> processor e.g. x86. If the MTU is set to less than 340, an IP packet
> with the theoretical maximum size (65507 bytes) likewise can not be sent
> successfully.
> 
> NOTE: the detection isn't precise. I didn't consider it important enough
> to implement bisection method. Detected only for IPv4, because IPv6 uses
> high enough MTU (and function is not IPv6 ready).
> 
> Link: https://urldefense.com/v3/__https://lore.kernel.org/lkml/ab8289e5-547c-5375-5d0f-e9a363005db1@linux.ibm.com/__;!!GqivPVa7Brio!I-DjoaVAyYBR7vfzBbvQC5DTwXdU7kKn2lkcphosAI5PK2DLKjbAuaaBl6XG6hh1Ydlh$ 
> 
> Signed-off-by: Petr Vorel <pvorel@suse.cz>
> ---
> Hi Alexey,
> 
> This simple change would be enough:
> [ "$(uname -m)" = "s390x" ] && MAX_PACKET_SIZE="61244"
> 
> But I wanted to fix this for all platforms at once.
> 

Hi Petr,

Another option would be to limit max size according to the size of
the set mtu, so that we test packets with and without fragmentation,
i.e. tst_ping -s "1 $((mtu / 2)) $mtu $((5 * mtu))", where "$mtu"
size - for minor fragmentation.

Anyway the patch looks good, minor comments below.

> Kind regards,
> Petr
> 
>  .../network/stress/interface/if-mtu-change.sh | 61 ++++++++++++++-----
>  1 file changed, 45 insertions(+), 16 deletions(-)
> 
> diff --git a/testcases/network/stress/interface/if-mtu-change.sh b/testcases/network/stress/interface/if-mtu-change.sh
> index f7ec35619..bc05ee187 100755
> --- a/testcases/network/stress/interface/if-mtu-change.sh
> +++ b/testcases/network/stress/interface/if-mtu-change.sh
> @@ -21,11 +21,53 @@ CHANGE_VALUES="784 1142 552 1500 552 1500 552 748 552 1142 1500"
>  CHANGE6_VALUES="1280 1445 1335 1390 1500 1280 1500 1280 1335 1500"
>  saved_mtu=
>  
> +MAX_PACKET_SIZE=65507
> +
> +set_mtu()
> +{
> +	local mtu="$1"
> +	local cmd="$2"
> +	local ret=0
> +	local iface=$(tst_iface)
> +	local iface_rmt=$(tst_iface rhost)
> +
> +	case $cmd in
> +		ifconfig) ifconfig $iface mtu $mtu || ret=1
> +			tst_rhost_run -c "ifconfig $iface_rmt mtu $mtu" || ret=1
> +			;;
> +		ip) ip link set $iface mtu $mtu || ret=1
> +			tst_rhost_run -c "ip link set $iface_rmt mtu $mtu" || ret=1
> +			;;
> +		*) tst_brk TBROK "unknown cmd '$cmd'"
> +			;;
> +	esac
> +
> +	return $ret
> +}
> +
> +find_ipv4_max_packet_size()
> +{
> +	local min_mtu=552
> +	local size=$MAX_PACKET_SIZE
> +
> +	set_mtu $min_mtu $CMD || tst_brk TBROK "failed to set MTU to $mtu"
> +	tst_res TINFO "checking max MTU"
> +	while [ $size -gt 0 ]; do
> +		if ping -I $(tst_iface) -c1 -w1 -s $size $(tst_ipaddr rhost) >/dev/null; then
> +			tst_res TINFO "use max MTU $size"
> +			MAX_PACKET_SIZE=$size
> +			return
> +		fi
> +		size=$((size - 500))
> +	done

Should we TBROK here?

> +}
> +
>  do_setup()
>  {
>  	[ "$TST_IPV6" ] && CHANGE_VALUES=$CHANGE6_VALUES
>  	if_setup
>  	saved_mtu="$(cat /sys/class/net/$(tst_iface)/mtu)"
> +	[ "$TST_IPV6" ] || find_ipv4_max_packet_size
>  }
>  
>  do_cleanup()
> @@ -41,9 +83,6 @@ test_body()
>  {
>  	local cmd="$CMD"
>  
> -	local iface=$(tst_iface)
> -	local iface_rmt=$(tst_iface rhost)
> -
>  	tst_res TINFO "'$cmd' changes MTU $MTU_CHANGE_TIMES times" \
>  	               "every $CHANGE_INTERVAL seconds"
>  
> @@ -59,24 +98,14 @@ test_body()
>  		make_background_tcp_traffic
>  
>  		tst_res TINFO "set MTU to $mtu $cnt/$MTU_CHANGE_TIMES"
> -		local ret=0
> -		case $cmd in
> -		ifconfig) ifconfig $iface mtu $mtu || ret=1
> -			tst_rhost_run -c "ifconfig $iface_rmt mtu $mtu"
> -		;;
> -		ip) ip link set $iface mtu $mtu || ret=1
> -			tst_rhost_run -c "ip link set $iface_rmt mtu $mtu"
> -		;;
> -		esac
> -
> -		if [ $? -ne 0 -o $ret -ne 0 ]; then
> -			tst_res TFAIL "Failed to change the mtu at $cnt time"
> +		if ! set_mtu $mtu $cmd; then
> +			tst_res TFAIL "failed to change MTU to $mtu at $cnt time"
>  			return
>  		fi
>  
>  		tst_sleep $CHANGE_INTERVAL
>  
> -		tst_ping -s "1 1000 65507"
> +		tst_ping -s "1 1000 $MAX_PACKET_SIZE"

tst_ping -s "1 $((mtu / 2)) $mtu $MAX_PACKET_SIZE"?