Re: [PATCH iproute2] lib/libnetlink: ensure a minimum of 32KB for the buffer used in rtnl_recvmsg()

[Hangbin Liu <liuhangbin@gmail.com>]

Hi David,
On Tue, Feb 12, 2019 at 06:08:13PM -0800, Eric Dumazet wrote:
> >> diff --git a/lib/libnetlink.c b/lib/libnetlink.c
> >> index 1892a02ab5d0d73776c9882ffc77edcd2c663d01..0d48a3d43cf03065dacbd419578ab10af56431a4 100644
> >> --- a/lib/libnetlink.c
> >> +++ b/lib/libnetlink.c
> >> @@ -718,6 +718,8 @@ static int rtnl_recvmsg(int fd, struct msghdr *msg, char **answer)
> >>  	if (len < 0)
> >>  		return len;
> >>  
> >> +	if (len < 32768)
> >> +		len = 32768;
> >>  	buf = malloc(len);
> >>  	if (!buf) {
> >>  		fprintf(stderr, "malloc error: not enough buffer\n");
> >>
> > 
> > I believe that negates the whole point of 2d34851cd341 - which I have no
> > problem with. 2 recvmsg calls per message is overkill.

It should not affects ip cmd too much. But for ss, as Eric pointed, it
cause performance issue.

> > 
> 
> It does not negates the point at all.
> 
> The main point was to eventually be able to allocate more than 32KB.
> 
> We need to have a minimum size of 32KB so that the kernel can cook reasonably sized skbs
> 
> Because trying to allocate 4KB only in 2019 is kind of stupid...
> 
> ( Especially considering ss currently buffers the whole thing before calling render() !!! )

This makes sense to me.
 
> > Do we know of any single message sizes > 32k? 2d34851cd341 cites
> > increasing VF's but at some point there is a limit. If not, the whole
> > PEEK thing should go away and we just malloc 32k (or 64k) buffers for
> > each recvmsg.

Apart from the 200 VFs example, I think there will be more and more virtual
interfaces be used in cloud environment, like openstack/OVS, so useing 32K
or 64K is still not safe.

What do you think.

Thanks
Hangbin