Subject: Re: [RFC][PATCH 3/3] TCP/IP Critical socket communication mechanism
From: Sridhar Samudrala <sri@us.ibm.com>
To: Mitchell Blank Jr <mitch@sfgoth.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>, linux-kernel@vger.kernel.org,
       netdev@vger.kernel.org
In-Reply-To: <20051214121253.GB23393@gaz.sfgoth.com>
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	 <20051214121253.GB23393@gaz.sfgoth.com>
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Date: Wed, 14 Dec 2005 10:29:27 -0800
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On Wed, 2005-12-14 at 04:12 -0800, Mitchell Blank Jr wrote:
> Alan Cox wrote:
> > But your user space that would add the routes is not so protected so I'm
> > not sure this is actually a solution, more of an extended fudge.
> 
> Yes, there's no 100% solution -- no matter how much memory you reserve and
> how many paths you protect if you try hard enough you can come up
> with cases where it'll fail.  ("I'm swapping to NFS across a tun/tap
> interface to a custom userland SSL tunnel to a server across a BGP route...")
> 
> However, if the 'extended fundge' pushes a problem from "can happen, even
> in a very normal setup" territory to "only happens if you're doing something
> pretty weird" then is it really such a bad thing?  I think the cost in code
> complexity looks pretty reasonable.

Yes. This should work fine for cases where you need a limited number of
critical allocation requests to succeed for a short period of time.

> > > +#define SK_CRIT_ALLOC(sk, flags) ((sk->sk_allocation & __GFP_CRITICAL) | flags)
> > 
> > Lots of hidden conditional logic on critical paths.
> 
> How expensive is it compared to the allocation itself?

Also, as i said in my other response we could make it a compile-time
configurable option with zero overhead when turned off.

Thanks
Sridhar

> 
> > > +#define CRIT_ALLOC(flags) (__GFP_CRITICAL | flags)
> > 
> > Pointless obfuscation
> 
> Fully agree.
> 
> -Mitch

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