Re: [PATCH nf,v3] netfilter: nf_tables: coalesce multiple notifications into one skbuff

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Wed, Sep 02, 2020 at 06:39:34PM +0200, Phil Sutter wrote:
> On Wed, Sep 02, 2020 at 06:37:43PM +0200, Pablo Neira Ayuso wrote:
> > On x86_64, each notification results in one skbuff allocation which
> > consumes at least 768 bytes due to the skbuff overhead.
> > 
> > This patch coalesces several notifications into one single skbuff, so
> > each notification consumes at least ~211 bytes, that ~3.5 times less
> > memory consumption. As a result, this is reducing the chances to exhaust
> > the netlink socket receive buffer.
> > 
> > Rule of thumb is that each notification batch only contains netlink
> > messages whose report flag is the same, nfnetlink_send() requires this
> > to do appropriately delivery to userspace, either via unicast (echo
> > mode) or multicast (monitor mode).
> > 
> > The skbuff control buffer is used to annotate the report flag for later
> > handling at the new coalescing routine.
> > 
> > The batch skbuff notification size is NLMSG_GOODSIZE, using a larger
> > skbuff would allow for more socket receiver buffer savings (to amortize
> > the cost of the skbuff even more), however, going over that size might
> > break userspace applications, so let's be conservative and stick to
> > NLMSG_GOODSIZE.
> > 
> > Reported-by: Phil Sutter <phil@nwl.cc>
> > Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
> 
> Acked-by: Phil Sutter <phil@nwl.cc>

Thanks, I'll place this into nf.git

BTW, I assume this mitigates the problem that Eric reported? Is it
not so easy to trigger the problem after this patch?

I forgot to say, probably it would be good to monitor
/proc/net/netlink to catch how busy the socket receive buffer is
getting with your firewalld ruleset.

Thanks.