Re: [net-next PATCH v4 3/3] net: reserve ports for applications usingfixed port numbers

[Cong Wang]

Octavian Purdila wrote:
> On Tuesday 16 February 2010 15:06:26 you wrote:
> > Octavian Purdila wrote:
> > > On Tuesday 16 February 2010 11:37:04 you wrote:
> > > > >  	BUILD_BUG_ON(sizeof(struct inet_skb_parm) > sizeof(dummy_skb->cb));
> > > > >
> > > > > +	sysctl_local_reserved_ports = kzalloc(65536 / 8, GFP_KERNEL);
> > > > > +	if (!sysctl_local_reserved_ports)
> > > > > +		goto out;
> > > > > +
> > > > I think we should also consider the ports in ip_local_port_range,
> > > > since we can only reserve the ports in that range.
> > > That is subject to changes at runtime, which means we will have to
> > > readjust the bitmap at runtime which introduces the need for additional
> > > synchronization operations which I would rather avoid.
> > Why? As long as the bitmap is global, this will not be hard.
>
> For the more important point see bellow, but with regard to reallocation, this 
> means we need to at least use rcu_read_lock() in the fast path to avoid races 
> between freeing the old bitmap and doing a read in progress. 
>
> Granted, that is a light operation, but would it makes things so much more 
> complicated just so that we save one memory page (assuming the range is the 
> default [32000 64000] one).


Why not just allocate the bitmap for all ports? 65535/8 bytes are
needed.

> > Consider that if one user writes a port number which is beyond
> > the ip_local_port_range into ip_local_reserved_ports, we should
> > not accept this, because it doesn't make any sense. But with your
> > patch, we do.
>
> I think it should be allowed. I see ip_local_reserved_ports and ip_local_range 
> as independent settings that can be change at any time.


According to the original purpose, they are not.

> That way I can flag port 8080 even if the current range is [32000, 64000] and 
> then later I can expand the range to [1024, 64000] without loosing the 8080 
> reservation.

Then its meaning is changed, bind(0) will never have chance to get 8080,
thus reserving 8080 for this purpose fails.

I want to always keep its original meaning, if the local_port_range goes
out, then local_reserved_port should be empty at the same time, you have
to reset it after changing local_port_range.
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