Re: [RFC][PATCH 0/3] TCP/IP Critical socket communication mechanism

[Stephen Hemminger <shemminger@osdl.org>]

On Thu, 15 Dec 2005 18:09:22 -0800
Sridhar Samudrala <sri@us.ibm.com> wrote:

> On Thu, 2005-12-15 at 00:21 -0800, David S. Miller wrote:
> > From: Sridhar Samudrala <sri@us.ibm.com>
> > Date: Wed, 14 Dec 2005 23:37:37 -0800 (PST)
> > 
> > > Instead, you seem to be suggesting in_emergency to be set dynamically
> > > when we are about to run out of ATOMIC memory. Is this right?
> > 
> > Not when we run out, but rather when we reach some low water mark, the
> > "critical sockets" would still use GFP_ATOMIC memory but only
> > "critical sockets" would be allowed to do so.
> > 
> > But even this has faults, consider the IPSEC scenerio I mentioned, and
> > this applies to any kind of encapsulation actually, even simple
> > tunneling examples can be concocted which make the "critical socket"
> > idea fail.
> > 
> > The knee jerk reaction is "mark IPSEC's sockets critical, and mark the
> > tunneling allocations critical, and... and..."  well you have
> > GFP_ATOMIC then my friend.
> 
> I would like to mention another reason why we need to have a new 
> GFP_CRITICAL flag for an allocation request. When we are in emergency,
> even the GFP_KERNEL allocations for a critical socket should not 
> sleep. This is because the swap device may have failed and we would
> like to communicate this event to a management server over the 
> critical socket so that it can initiate the failover.
> 
> We are not trying to solve swapping over network problem. It is much
> simpler. The critical sockets are to be used only to send/receive
> a few critical messages reliably during a short period of emergency.
> 

If it is only one place, why not pre-allocate one "I'm sick now"
skb and hold onto it. Any bigger solution seems to snowball into
a huge mess.


-- 
Stephen Hemminger <shemminger@osdl.org>
OSDL http://developer.osdl.org/~shemminger