diff mbox series

[net-next,v8] net: ipv4: Cache pmtu for all packet paths if multipath enabled

Message ID 20241106133012.289861-1-deliran@verdict.gg (mailing list archive)
State Superseded
Delegated to: Netdev Maintainers
Headers show
Series [net-next,v8] net: ipv4: Cache pmtu for all packet paths if multipath enabled | expand

Checks

Context Check Description
netdev/series_format success Single patches do not need cover letters
netdev/tree_selection success Clearly marked for net-next
netdev/ynl success Generated files up to date; no warnings/errors; no diff in generated;
netdev/fixes_present success Fixes tag not required for -next series
netdev/header_inline success No static functions without inline keyword in header files
netdev/build_32bit success Errors and warnings before: 3 this patch: 3
netdev/build_tools success Errors and warnings before: 2 (+0) this patch: 2 (+0)
netdev/cc_maintainers success CCed 8 of 8 maintainers
netdev/build_clang success Errors and warnings before: 3 this patch: 3
netdev/verify_signedoff success Signed-off-by tag matches author and committer
netdev/deprecated_api success None detected
netdev/check_selftest success net selftest script(s) already in Makefile
netdev/verify_fixes success No Fixes tag
netdev/build_allmodconfig_warn success Errors and warnings before: 4 this patch: 4
netdev/checkpatch warning WARNING: line length of 81 exceeds 80 columns WARNING: line length of 82 exceeds 80 columns WARNING: line length of 84 exceeds 80 columns WARNING: line length of 85 exceeds 80 columns WARNING: line length of 93 exceeds 80 columns WARNING: line length of 98 exceeds 80 columns
netdev/build_clang_rust success No Rust files in patch. Skipping build
netdev/kdoc success Errors and warnings before: 0 this patch: 0
netdev/source_inline success Was 0 now: 0
netdev/contest success net-next-2024-11-06--18-00 (tests: 782)

Commit Message

Vladimir Vdovin Nov. 6, 2024, 1:30 p.m. UTC
Check number of paths by fib_info_num_path(),
and update_or_create_fnhe() for every path.
Problem is that pmtu is cached only for the oif
that has received icmp message "need to frag",
other oifs will still try to use "default" iface mtu.

An example topology showing the problem:

                    |  host1
                +---------+
                |  dummy0 | 10.179.20.18/32  mtu9000
                +---------+
        +-----------+----------------+
    +---------+                     +---------+
    | ens17f0 |  10.179.2.141/31    | ens17f1 |  10.179.2.13/31
    +---------+                     +---------+
        |    (all here have mtu 9000)    |
    +------+                         +------+
    | ro1  |  10.179.2.140/31        | ro2  |  10.179.2.12/31
    +------+                         +------+
        |                                |
---------+------------+-------------------+------
                        |
                    +-----+
                    | ro3 | 10.10.10.10  mtu1500
                    +-----+
                        |
    ========================================
                some networks
    ========================================
                        |
                    +-----+
                    | eth0| 10.10.30.30  mtu9000
                    +-----+
                        |  host2

host1 have enabled multipath and
sysctl net.ipv4.fib_multipath_hash_policy = 1:

default proto static src 10.179.20.18
        nexthop via 10.179.2.12 dev ens17f1 weight 1
        nexthop via 10.179.2.140 dev ens17f0 weight 1

When host1 tries to do pmtud from 10.179.20.18/32 to host2,
host1 receives at ens17f1 iface an icmp packet from ro3 that ro3 mtu=1500.
And host1 caches it in nexthop exceptions cache.

Problem is that it is cached only for the iface that has received icmp,
and there is no way that ro3 will send icmp msg to host1 via another path.

Host1 now have this routes to host2:

ip r g 10.10.30.30 sport 30000 dport 443
10.10.30.30 via 10.179.2.12 dev ens17f1 src 10.179.20.18 uid 0
    cache expires 521sec mtu 1500

ip r g 10.10.30.30 sport 30033 dport 443
10.10.30.30 via 10.179.2.140 dev ens17f0 src 10.179.20.18 uid 0
    cache

So when host1 tries again to reach host2 with mtu>1500,
if packet flow is lucky enough to be hashed with oif=ens17f1 its ok,
if oif=ens17f0 it blackholes and still gets icmp msgs from ro3 to ens17f1,
until lucky day when ro3 will send it through another flow to ens17f0.

Signed-off-by: Vladimir Vdovin <deliran@verdict.gg>
---
V8:
  selftests in pmtu.sh:
    - Change var names from "dummy" to "host"
    - Fix errors caused by incorrect iface arguments pass
    - Add src addr to setup_multipath_new
    - Change multipath* func order
    - Change route_get_dst_exception() && route_get_dst_pmtu_from_exception()
      and arguments pass where they are used
      as Ido suggested in https://lore.kernel.org/all/ZykH_fdcMBdFgXix@shredder/

V7:
  selftest in pmtu.sh:
    - add setup_multipath() with old and new nh tests
    - add global "dummy_v4" addr variables
    - add documentation
    - remove dummy netdev usage in mp nh test
    - remove useless sysctl opts in mp nh test

V6:
  - make commit message cleaner

V5:
  - make self test cleaner

V4:
  - fix selftest, do route lookup before checking cached exceptions

V3:
  - add selftest
  - fix compile error

V2:
  - fix fib_info_num_path parameter pass
---
 net/ipv4/route.c                    |  13 +++
 tools/testing/selftests/net/pmtu.sh | 119 ++++++++++++++++++++++++----
 2 files changed, 115 insertions(+), 17 deletions(-)


base-commit: 66600fac7a984dea4ae095411f644770b2561ede

Comments

David Ahern Nov. 6, 2024, 3:44 p.m. UTC | #1
On 11/6/24 6:30 AM, Vladimir Vdovin wrote:
> Check number of paths by fib_info_num_path(),
> and update_or_create_fnhe() for every path.
> Problem is that pmtu is cached only for the oif
> that has received icmp message "need to frag",
> other oifs will still try to use "default" iface mtu.
> 
> An example topology showing the problem:
> 
>                     |  host1
>                 +---------+
>                 |  dummy0 | 10.179.20.18/32  mtu9000
>                 +---------+
>         +-----------+----------------+
>     +---------+                     +---------+
>     | ens17f0 |  10.179.2.141/31    | ens17f1 |  10.179.2.13/31
>     +---------+                     +---------+
>         |    (all here have mtu 9000)    |
>     +------+                         +------+
>     | ro1  |  10.179.2.140/31        | ro2  |  10.179.2.12/31
>     +------+                         +------+
>         |                                |
> ---------+------------+-------------------+------
>                         |
>                     +-----+
>                     | ro3 | 10.10.10.10  mtu1500
>                     +-----+
>                         |
>     ========================================
>                 some networks
>     ========================================
>                         |
>                     +-----+
>                     | eth0| 10.10.30.30  mtu9000
>                     +-----+
>                         |  host2
> 
> host1 have enabled multipath and
> sysctl net.ipv4.fib_multipath_hash_policy = 1:
> 
> default proto static src 10.179.20.18
>         nexthop via 10.179.2.12 dev ens17f1 weight 1
>         nexthop via 10.179.2.140 dev ens17f0 weight 1
> 
> When host1 tries to do pmtud from 10.179.20.18/32 to host2,
> host1 receives at ens17f1 iface an icmp packet from ro3 that ro3 mtu=1500.
> And host1 caches it in nexthop exceptions cache.
> 
> Problem is that it is cached only for the iface that has received icmp,
> and there is no way that ro3 will send icmp msg to host1 via another path.
> 
> Host1 now have this routes to host2:
> 
> ip r g 10.10.30.30 sport 30000 dport 443
> 10.10.30.30 via 10.179.2.12 dev ens17f1 src 10.179.20.18 uid 0
>     cache expires 521sec mtu 1500
> 
> ip r g 10.10.30.30 sport 30033 dport 443
> 10.10.30.30 via 10.179.2.140 dev ens17f0 src 10.179.20.18 uid 0
>     cache
> 
> So when host1 tries again to reach host2 with mtu>1500,
> if packet flow is lucky enough to be hashed with oif=ens17f1 its ok,
> if oif=ens17f0 it blackholes and still gets icmp msgs from ro3 to ens17f1,
> until lucky day when ro3 will send it through another flow to ens17f0.
> 
> Signed-off-by: Vladimir Vdovin <deliran@verdict.gg>
> ---
> V8:
>   selftests in pmtu.sh:
>     - Change var names from "dummy" to "host"
>     - Fix errors caused by incorrect iface arguments pass
>     - Add src addr to setup_multipath_new
>     - Change multipath* func order
>     - Change route_get_dst_exception() && route_get_dst_pmtu_from_exception()
>       and arguments pass where they are used
>       as Ido suggested in https://lore.kernel.org/all/ZykH_fdcMBdFgXix@shredder/
> 
> V7:
>   selftest in pmtu.sh:
>     - add setup_multipath() with old and new nh tests
>     - add global "dummy_v4" addr variables
>     - add documentation
>     - remove dummy netdev usage in mp nh test
>     - remove useless sysctl opts in mp nh test
> 
> V6:
>   - make commit message cleaner
> 
> V5:
>   - make self test cleaner
> 
> V4:
>   - fix selftest, do route lookup before checking cached exceptions
> 
> V3:
>   - add selftest
>   - fix compile error
> 
> V2:
>   - fix fib_info_num_path parameter pass
> ---
>  net/ipv4/route.c                    |  13 +++
>  tools/testing/selftests/net/pmtu.sh | 119 ++++++++++++++++++++++++----
>  2 files changed, 115 insertions(+), 17 deletions(-)
> 

Reviewed-by: David Ahern <dsahern@kernel.org>
Ido Schimmel Nov. 7, 2024, 9:14 a.m. UTC | #2
On Wed, Nov 06, 2024 at 01:30:07PM +0000, Vladimir Vdovin wrote:
> +test_pmtu_ipv4_mp_exceptions() {
> +	setup namespaces routing multipath || return $ksft_skip
> +
> +	trace "${ns_a}"  veth_A-R1    "${ns_r1}" veth_R1-A \
> +	      "${ns_r1}" veth_R1-B    "${ns_b}"  veth_B-R1 \
> +	      "${ns_a}"  veth_A-R2    "${ns_r2}" veth_R2-A \
> +	      "${ns_r2}" veth_R2-B    "${ns_b}"  veth_B-R2
> +
> +	# Set up initial MTU values
> +	mtu "${ns_a}"  veth_A-R1 2000
> +	mtu "${ns_r1}" veth_R1-A 2000
> +	mtu "${ns_r1}" veth_R1-B 1500
> +	mtu "${ns_b}"  veth_B-R1 1500
> +
> +	mtu "${ns_a}"  veth_A-R2 2000
> +	mtu "${ns_r2}" veth_R2-A 2000
> +	mtu "${ns_r2}" veth_R2-B 1500
> +	mtu "${ns_b}"  veth_B-R2 1500
> +
> +	fail=0
> +
> +	# Ping and expect two nexthop exceptions for two routes in nh group
> +	run_cmd ${ns_a} ping -q -M want -i 0.1 -c 1 -s 1800 "${host4_b_addr}"
> +
> +	# Do route lookup before checking cached exceptions.
> +	# The following commands are needed for dst entries to be cached
> +	# in both paths exceptions and therefore dumped to user space
> +	# Check that exceptions have been created with the correct PMTU
> +	pmtu="$(route_get_dst_pmtu_from_exception "${ns_a}" "${host4_b_addr}" oif veth_A-R1)"
> +	pmtu="$(route_get_dst_pmtu_from_exception "${ns_a}" "${host4_b_addr}" oif veth_A-R2)"

This overrides the first PMTU

> +
> +	check_pmtu_value "1500" "${pmtu}" "exceeding MTU (veth_A-R2)" || return 1
> +	check_pmtu_value "1500" "${pmtu}" "exceeding MTU (veth_A-R1)" || return 1
> +
> +	return ${fail}

And "fail" does not seem to be used anymore.

Can you squash the following diff to v9?

diff --git a/tools/testing/selftests/net/pmtu.sh b/tools/testing/selftests/net/pmtu.sh
index 9e34b54d9c15..1b4d4c598d66 100755
--- a/tools/testing/selftests/net/pmtu.sh
+++ b/tools/testing/selftests/net/pmtu.sh
@@ -2396,22 +2396,14 @@ test_pmtu_ipv4_mp_exceptions() {
 	mtu "${ns_r2}" veth_R2-B 1500
 	mtu "${ns_b}"  veth_B-R2 1500
 
-	fail=0
-
 	# Ping and expect two nexthop exceptions for two routes in nh group
 	run_cmd ${ns_a} ping -q -M want -i 0.1 -c 1 -s 1800 "${host4_b_addr}"
 
-	# Do route lookup before checking cached exceptions.
-	# The following commands are needed for dst entries to be cached
-	# in both paths exceptions and therefore dumped to user space
 	# Check that exceptions have been created with the correct PMTU
 	pmtu="$(route_get_dst_pmtu_from_exception "${ns_a}" "${host4_b_addr}" oif veth_A-R1)"
+	check_pmtu_value "1500" "${pmtu}" "exceeding MTU (veth_A-R1)" || return 1
 	pmtu="$(route_get_dst_pmtu_from_exception "${ns_a}" "${host4_b_addr}" oif veth_A-R2)"
-
 	check_pmtu_value "1500" "${pmtu}" "exceeding MTU (veth_A-R2)" || return 1
-	check_pmtu_value "1500" "${pmtu}" "exceeding MTU (veth_A-R1)" || return 1
-
-	return ${fail}
 }
 
 usage() {
Vladimir Vdovin Nov. 7, 2024, 9:51 a.m. UTC | #3
On Thu Nov 7, 2024 at 12:14 PM MSK, Ido Schimmel wrote:
> On Wed, Nov 06, 2024 at 01:30:07PM +0000, Vladimir Vdovin wrote:
> > +test_pmtu_ipv4_mp_exceptions() {
> > +	setup namespaces routing multipath || return $ksft_skip
> > +
> > +	trace "${ns_a}"  veth_A-R1    "${ns_r1}" veth_R1-A \
> > +	      "${ns_r1}" veth_R1-B    "${ns_b}"  veth_B-R1 \
> > +	      "${ns_a}"  veth_A-R2    "${ns_r2}" veth_R2-A \
> > +	      "${ns_r2}" veth_R2-B    "${ns_b}"  veth_B-R2
> > +
> > +	# Set up initial MTU values
> > +	mtu "${ns_a}"  veth_A-R1 2000
> > +	mtu "${ns_r1}" veth_R1-A 2000
> > +	mtu "${ns_r1}" veth_R1-B 1500
> > +	mtu "${ns_b}"  veth_B-R1 1500
> > +
> > +	mtu "${ns_a}"  veth_A-R2 2000
> > +	mtu "${ns_r2}" veth_R2-A 2000
> > +	mtu "${ns_r2}" veth_R2-B 1500
> > +	mtu "${ns_b}"  veth_B-R2 1500
> > +
> > +	fail=0
> > +
> > +	# Ping and expect two nexthop exceptions for two routes in nh group
> > +	run_cmd ${ns_a} ping -q -M want -i 0.1 -c 1 -s 1800 "${host4_b_addr}"
> > +
> > +	# Do route lookup before checking cached exceptions.
> > +	# The following commands are needed for dst entries to be cached
> > +	# in both paths exceptions and therefore dumped to user space
> > +	# Check that exceptions have been created with the correct PMTU
> > +	pmtu="$(route_get_dst_pmtu_from_exception "${ns_a}" "${host4_b_addr}" oif veth_A-R1)"
> > +	pmtu="$(route_get_dst_pmtu_from_exception "${ns_a}" "${host4_b_addr}" oif veth_A-R2)"
>
> This overrides the first PMTU
>
> > +
> > +	check_pmtu_value "1500" "${pmtu}" "exceeding MTU (veth_A-R2)" || return 1
> > +	check_pmtu_value "1500" "${pmtu}" "exceeding MTU (veth_A-R1)" || return 1
> > +
> > +	return ${fail}
>
> And "fail" does not seem to be used anymore.
>
> Can you squash the following diff to v9?
Thanks again! Sent v9
And seems that v10 will come next day, cause I did not apply your diff and forgot to
remove fail=0 :/

>
> diff --git a/tools/testing/selftests/net/pmtu.sh b/tools/testing/selftests/net/pmtu.sh
> index 9e34b54d9c15..1b4d4c598d66 100755
> --- a/tools/testing/selftests/net/pmtu.sh
> +++ b/tools/testing/selftests/net/pmtu.sh
> @@ -2396,22 +2396,14 @@ test_pmtu_ipv4_mp_exceptions() {
>  	mtu "${ns_r2}" veth_R2-B 1500
>  	mtu "${ns_b}"  veth_B-R2 1500
>  
> -	fail=0
> -
>  	# Ping and expect two nexthop exceptions for two routes in nh group
>  	run_cmd ${ns_a} ping -q -M want -i 0.1 -c 1 -s 1800 "${host4_b_addr}"
>  
> -	# Do route lookup before checking cached exceptions.
> -	# The following commands are needed for dst entries to be cached
> -	# in both paths exceptions and therefore dumped to user space
>  	# Check that exceptions have been created with the correct PMTU
>  	pmtu="$(route_get_dst_pmtu_from_exception "${ns_a}" "${host4_b_addr}" oif veth_A-R1)"
> +	check_pmtu_value "1500" "${pmtu}" "exceeding MTU (veth_A-R1)" || return 1
>  	pmtu="$(route_get_dst_pmtu_from_exception "${ns_a}" "${host4_b_addr}" oif veth_A-R2)"
> -
>  	check_pmtu_value "1500" "${pmtu}" "exceeding MTU (veth_A-R2)" || return 1
> -	check_pmtu_value "1500" "${pmtu}" "exceeding MTU (veth_A-R1)" || return 1
> -
> -	return ${fail}
>  }
>  
>  usage() {
diff mbox series

Patch

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 723ac9181558..652f603d29fe 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1027,6 +1027,19 @@  static void __ip_rt_update_pmtu(struct rtable *rt, struct flowi4 *fl4, u32 mtu)
 		struct fib_nh_common *nhc;
 
 		fib_select_path(net, &res, fl4, NULL);
+#ifdef CONFIG_IP_ROUTE_MULTIPATH
+		if (fib_info_num_path(res.fi) > 1) {
+			int nhsel;
+
+			for (nhsel = 0; nhsel < fib_info_num_path(res.fi); nhsel++) {
+				nhc = fib_info_nhc(res.fi, nhsel);
+				update_or_create_fnhe(nhc, fl4->daddr, 0, mtu, lock,
+						      jiffies + net->ipv4.ip_rt_mtu_expires);
+			}
+			rcu_read_unlock();
+			return;
+		}
+#endif /* CONFIG_IP_ROUTE_MULTIPATH */
 		nhc = FIB_RES_NHC(res);
 		update_or_create_fnhe(nhc, fl4->daddr, 0, mtu, lock,
 				      jiffies + net->ipv4.ip_rt_mtu_expires);
diff --git a/tools/testing/selftests/net/pmtu.sh b/tools/testing/selftests/net/pmtu.sh
index 569bce8b6383..2b79a16d3369 100755
--- a/tools/testing/selftests/net/pmtu.sh
+++ b/tools/testing/selftests/net/pmtu.sh
@@ -197,6 +197,12 @@ 
 #
 # - pmtu_ipv6_route_change
 #	Same as above but with IPv6
+#
+# - pmtu_ipv4_mp_exceptions
+#	Use the same topology as in pmtu_ipv4, but add routeable addresses
+#	on host A and B on lo reachable via both routers. Host A and B
+#	addresses have multipath routes to each other, b_r1 mtu = 1500.
+#	Check that PMTU exceptions are created for both paths.
 
 source lib.sh
 source net_helper.sh
@@ -266,7 +272,8 @@  tests="
 	list_flush_ipv4_exception	ipv4: list and flush cached exceptions	1
 	list_flush_ipv6_exception	ipv6: list and flush cached exceptions	1
 	pmtu_ipv4_route_change		ipv4: PMTU exception w/route replace	1
-	pmtu_ipv6_route_change		ipv6: PMTU exception w/route replace	1"
+	pmtu_ipv6_route_change		ipv6: PMTU exception w/route replace	1
+	pmtu_ipv4_mp_exceptions		ipv4: PMTU multipath nh exceptions	1"
 
 # Addressing and routing for tests with routers: four network segments, with
 # index SEGMENT between 1 and 4, a common prefix (PREFIX4 or PREFIX6) and an
@@ -343,6 +350,9 @@  tunnel6_a_addr="fd00:2::a"
 tunnel6_b_addr="fd00:2::b"
 tunnel6_mask="64"
 
+host4_a_addr="192.168.99.99"
+host4_b_addr="192.168.88.88"
+
 dummy6_0_prefix="fc00:1000::"
 dummy6_1_prefix="fc00:1001::"
 dummy6_mask="64"
@@ -984,6 +994,52 @@  setup_ovs_bridge() {
 	run_cmd ip route add ${prefix6}:${b_r1}::1 via ${prefix6}:${a_r1}::2
 }
 
+setup_multipath_new() {
+	# Set up host A with multipath routes to host B host4_b_addr
+	run_cmd ${ns_a} ip addr add ${host4_a_addr} dev lo
+	run_cmd ${ns_a} ip nexthop add id 401 via ${prefix4}.${a_r1}.2 dev veth_A-R1
+	run_cmd ${ns_a} ip nexthop add id 402 via ${prefix4}.${a_r2}.2 dev veth_A-R2
+	run_cmd ${ns_a} ip nexthop add id 403 group 401/402
+	run_cmd ${ns_a} ip route add ${host4_b_addr} src ${host4_a_addr} nhid 403
+
+	# Set up host B with multipath routes to host A host4_a_addr
+	run_cmd ${ns_b} ip addr add ${host4_b_addr} dev lo
+	run_cmd ${ns_b} ip nexthop add id 401 via ${prefix4}.${b_r1}.2 dev veth_B-R1
+	run_cmd ${ns_b} ip nexthop add id 402 via ${prefix4}.${b_r2}.2 dev veth_B-R2
+	run_cmd ${ns_b} ip nexthop add id 403 group 401/402
+	run_cmd ${ns_b} ip route add ${host4_a_addr} src ${host4_b_addr} nhid 403
+}
+
+setup_multipath_old() {
+	# Set up host A with multipath routes to host B host4_b_addr
+	run_cmd ${ns_a} ip addr add ${host4_a_addr} dev lo
+	run_cmd ${ns_a} ip route add ${host4_b_addr} \
+			src ${host4_a_addr} \
+			nexthop via ${prefix4}.${a_r1}.2 weight 1 \
+			nexthop via ${prefix4}.${a_r2}.2 weight 1
+
+	# Set up host B with multipath routes to host A host4_a_addr
+	run_cmd ${ns_b} ip addr add ${host4_b_addr} dev lo
+	run_cmd ${ns_b} ip route add ${host4_a_addr} \
+			src ${host4_b_addr} \
+			nexthop via ${prefix4}.${b_r1}.2 weight 1 \
+			nexthop via ${prefix4}.${b_r2}.2 weight 1
+}
+
+setup_multipath() {
+	if [ "$USE_NH" = "yes" ]; then
+		setup_multipath_new
+	else
+		setup_multipath_old
+	fi
+
+	# Set up routers with routes to dummies
+	run_cmd ${ns_r1} ip route add ${host4_a_addr} via ${prefix4}.${a_r1}.1
+	run_cmd ${ns_r2} ip route add ${host4_a_addr} via ${prefix4}.${a_r2}.1
+	run_cmd ${ns_r1} ip route add ${host4_b_addr} via ${prefix4}.${b_r1}.1
+	run_cmd ${ns_r2} ip route add ${host4_b_addr} via ${prefix4}.${b_r2}.1
+}
+
 setup() {
 	[ "$(id -u)" -ne 0 ] && echo "  need to run as root" && return $ksft_skip
 
@@ -1076,23 +1132,15 @@  link_get_mtu() {
 }
 
 route_get_dst_exception() {
-	ns_cmd="${1}"
-	dst="${2}"
-	dsfield="${3}"
-
-	if [ -z "${dsfield}" ]; then
-		dsfield=0
-	fi
+	ns_cmd="${1}"; shift
 
-	${ns_cmd} ip route get "${dst}" dsfield "${dsfield}"
+	${ns_cmd} ip route get "$@"
 }
 
 route_get_dst_pmtu_from_exception() {
-	ns_cmd="${1}"
-	dst="${2}"
-	dsfield="${3}"
+	ns_cmd="${1}"; shift
 
-	mtu_parse "$(route_get_dst_exception "${ns_cmd}" "${dst}" "${dsfield}")"
+	mtu_parse "$(route_get_dst_exception "${ns_cmd}" "$@")"
 }
 
 check_pmtu_value() {
@@ -1235,10 +1283,10 @@  test_pmtu_ipv4_dscp_icmp_exception() {
 	run_cmd "${ns_a}" ping -q -M want -Q "${dsfield}" -c 1 -w 1 -s "${len}" "${dst2}"
 
 	# Check that exceptions have been created with the correct PMTU
-	pmtu_1="$(route_get_dst_pmtu_from_exception "${ns_a}" "${dst1}" "${policy_mark}")"
+	pmtu_1="$(route_get_dst_pmtu_from_exception "${ns_a}" "${dst1}" dsfield "${policy_mark}")"
 	check_pmtu_value "1400" "${pmtu_1}" "exceeding MTU" || return 1
 
-	pmtu_2="$(route_get_dst_pmtu_from_exception "${ns_a}" "${dst2}" "${policy_mark}")"
+	pmtu_2="$(route_get_dst_pmtu_from_exception "${ns_a}" "${dst2}" dsfield "${policy_mark}")"
 	check_pmtu_value "1500" "${pmtu_2}" "exceeding MTU" || return 1
 }
 
@@ -1285,9 +1333,9 @@  test_pmtu_ipv4_dscp_udp_exception() {
 		UDP:"${dst2}":50000,tos="${dsfield}"
 
 	# Check that exceptions have been created with the correct PMTU
-	pmtu_1="$(route_get_dst_pmtu_from_exception "${ns_a}" "${dst1}" "${policy_mark}")"
+	pmtu_1="$(route_get_dst_pmtu_from_exception "${ns_a}" "${dst1}" dsfield "${policy_mark}")"
 	check_pmtu_value "1400" "${pmtu_1}" "exceeding MTU" || return 1
-	pmtu_2="$(route_get_dst_pmtu_from_exception "${ns_a}" "${dst2}" "${policy_mark}")"
+	pmtu_2="$(route_get_dst_pmtu_from_exception "${ns_a}" "${dst2}" dsfield "${policy_mark}")"
 	check_pmtu_value "1500" "${pmtu_2}" "exceeding MTU" || return 1
 }
 
@@ -2329,6 +2377,43 @@  test_pmtu_ipv6_route_change() {
 	test_pmtu_ipvX_route_change 6
 }
 
+test_pmtu_ipv4_mp_exceptions() {
+	setup namespaces routing multipath || return $ksft_skip
+
+	trace "${ns_a}"  veth_A-R1    "${ns_r1}" veth_R1-A \
+	      "${ns_r1}" veth_R1-B    "${ns_b}"  veth_B-R1 \
+	      "${ns_a}"  veth_A-R2    "${ns_r2}" veth_R2-A \
+	      "${ns_r2}" veth_R2-B    "${ns_b}"  veth_B-R2
+
+	# Set up initial MTU values
+	mtu "${ns_a}"  veth_A-R1 2000
+	mtu "${ns_r1}" veth_R1-A 2000
+	mtu "${ns_r1}" veth_R1-B 1500
+	mtu "${ns_b}"  veth_B-R1 1500
+
+	mtu "${ns_a}"  veth_A-R2 2000
+	mtu "${ns_r2}" veth_R2-A 2000
+	mtu "${ns_r2}" veth_R2-B 1500
+	mtu "${ns_b}"  veth_B-R2 1500
+
+	fail=0
+
+	# Ping and expect two nexthop exceptions for two routes in nh group
+	run_cmd ${ns_a} ping -q -M want -i 0.1 -c 1 -s 1800 "${host4_b_addr}"
+
+	# Do route lookup before checking cached exceptions.
+	# The following commands are needed for dst entries to be cached
+	# in both paths exceptions and therefore dumped to user space
+	# Check that exceptions have been created with the correct PMTU
+	pmtu="$(route_get_dst_pmtu_from_exception "${ns_a}" "${host4_b_addr}" oif veth_A-R1)"
+	pmtu="$(route_get_dst_pmtu_from_exception "${ns_a}" "${host4_b_addr}" oif veth_A-R2)"
+
+	check_pmtu_value "1500" "${pmtu}" "exceeding MTU (veth_A-R2)" || return 1
+	check_pmtu_value "1500" "${pmtu}" "exceeding MTU (veth_A-R1)" || return 1
+
+	return ${fail}
+}
+
 usage() {
 	echo
 	echo "$0 [OPTIONS] [TEST]..."