Merge branch 'ipv6-no-rtnl-for-ipv6-routing-table'

Kuniyuki Iwashima says:

====================
ipv6: No RTNL for IPv6 routing table.

IPv6 routing tables are protected by each table's lock and work in
the interrupt context, which means we basically don't need RTNL to
modify an IPv6 routing table itself.

Currently, the control paths require RTNL because we may need to
perform device and nexthop lookups; we must prevent dev/nexthop from
going away from the netns.

This, however, can be achieved by RCU as well.

If we are in the RCU critical section while adding an IPv6 route,
synchronize_net() in __dev_change_net_namespace() and
unregister_netdevice_many_notify() guarantee that the dev will not be
moved to another netns or removed.

Also, nexthop is guaranteed not to be freed during the RCU grace period.

If we care about a race between nexthop removal and IPv6 route addition,
we can get rid of RTNL from the control paths.

Patch 1 moves a validation for RTA_MULTIPATH earlier.
Patch 2 removes RTNL for SIOCDELRT and RTM_DELROUTE.
Patch 3 ~ 11 moves validation and memory allocation earlier.
Patch 12 prevents a race between two requests for the same table.
Patch 13 & 14 prevents the nexthop race mentioned above.
Patch 15 removes RTNL for SIOCADDRT and RTM_NEWROUTE.

Test:

The script [0] lets each CPU-X create 100000 routes on table-X in a
batch.

On c7a.metal-48xl EC2 instance with 192 CPUs,

without this series:

$ sudo ./route_test.sh
start adding routes
added 19200000 routes (100000 routes * 192 tables).
total routes: 19200006
Time elapsed: 191577 milliseconds.

with this series:

$ sudo ./route_test.sh
start adding routes
added 19200000 routes (100000 routes * 192 tables).
total routes: 19200006
Time elapsed: 62854 milliseconds.

I changed the number of routes (1000 ~ 100000 per CPU/table) and
consistently saw it finish 3x faster with this series.

[0]

mkdir tmp

NS="test"
ip netns add $NS
ip -n $NS link add veth0 type veth peer veth1
ip -n $NS link set veth0 up
ip -n $NS link set veth1 up

TABLES=()
for i in $(seq $(nproc)); do
TABLES+=("$i")
done

ROUTES=()
for i in {1..100}; do
for j in {1..1000}; do
ROUTES+=("2001:$i:$j::/64")
done
done

for TABLE in "${TABLES[@]}"; do
(
FILE="./tmp/batch-table-$TABLE.txt"
> $FILE
for ROUTE in "${ROUTES[@]}"; do
echo "route add $ROUTE dev veth0 table $TABLE" >> $FILE
done
) &
done

wait

echo "start adding routes"

START_TIME=$(date +%s%3N)
for TABLE in "${TABLES[@]}"; do
ip -n $NS -6 -batch "./tmp/batch-table-$TABLE.txt" &
done

wait
END_TIME=$(date +%s%3N)
ELAPSED_TIME=$((END_TIME - START_TIME))

echo "added $((${#ROUTES[@]} * ${#TABLES[@]})) routes (${#ROUTES[@]} routes * ${#TABLES[@]} tables)."
echo "total routes: $(ip -n $NS -6 route show table all | wc -l)" # Just for debug
echo "Time elapsed: ${ELAPSED_TIME} milliseconds."

ip netns del $NS
rm -fr ./tmp/

v2: https://lore.kernel.org/netdev/20250409011243.26195-1-kuniyu@amazon.com/
v1: https://lore.kernel.org/netdev/20250321040131.21057-1-kuniyu@amazon.com/
====================

Link: https://patch.msgid.link/20250418000443.43734-1-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>