Subject: Re: [RFC][PATCH 0/3] TCP/IP Critical socket communication mechanism
From: Sridhar Samudrala <sri@us.ibm.com>
To: "David S. Miller" <davem@davemloft.net>
Cc: mpm@selenic.com, ak@suse.de, linux-kernel@vger.kernel.org,
       netdev@vger.kernel.org
In-Reply-To: <20051215.002120.133621586.davem@davemloft.net>
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Date: Thu, 15 Dec 2005 18:09:22 -0800
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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.

Thanks
Sridhar


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