Re: [PATCH net-next v2 3/3] genetlink: fit NLMSG_DONE into same read() as families

On 19/03/2024 19:40, Jakub Kicinski wrote:

On Tue, 19 Mar 2024 18:17:47 +0100 Eric Dumazet wrote:

quoted

quoted

Hi Stefano! I was worried this may happen :( I think we should revert
offending commits, but I'd like to take it on case by case basis.
I'd imagine majority of netlink is only exercised by iproute2 and
libmnl-based tools. Does passt hang specifically on genetlink family
dump? Your commit also mentions RTM_GETROUTE. This is not the only
commit which removed DONE:

$ git log --since='1 month ago' --grep=NLMSG_DONE --no-merges  --oneline

9cc4cc329d30 ipv6: use xa_array iterator to implement inet6_dump_addr()
87d381973e49 genetlink: fit NLMSG_DONE into same read() as families
4ce5dc9316de inet: switch inet_dump_fib() to RCU protection
6647b338fc5c netlink: fix netlink_diag_dump() return value  

Lets not bring back more RTNL locking please for the handlers that
still require it.

Definitely. My git log copy/paste is pretty inaccurate, these two are
better examples:

5d9b7cb383bb nexthop: Simplify dump error handling
02e24903e5a4 netlink: let core handle error cases in dump operations

I was trying to point out that we merged a handful of DONE "coalescing"
patches, and if we need to revert - let's only do that for the exact
commands needed. The comment was raised on my genetlink patch while
the discussion in the link points to RTM_GETROUTE.

quoted

The core can generate an NLMSG_DONE by itself, if we decide this needs
to be done.

Exactly.

We've encountered a new issue recently which I believe is related to
this discussion.

Following Eric's patch:
9cc4cc329d30 ("ipv6: use xa_array iterator to implement inet6_dump_addr()")

Setting the interface mtu to < 1280 results in 'ip addr show eth2'
returning an error, because the ipv6 dump fails. This is a degradation
from the user's perspective.

# ip addr show eth2
4: eth2: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group
default qlen 1000
    link/ether 24:42:53:21:52:44 brd ff:ff:ff:ff:ff:ff
    altname enp6s0f0np0
# ip link set dev eth2 mtu 1000
# ip addr show eth2
RTNETLINK answers: No such device
Dump terminated