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

[Sridhar Samudrala <sri@us.ibm.com>]

On Wed, 2005-12-14 at 11:17 +0000, Alan Cox wrote:
> On Mer, 2005-12-14 at 01:12 -0800, Sridhar Samudrala wrote:
> > Pass __GFP_CRITICAL flag with all allocation requests that are critical.
> > - All allocations needed to process incoming packets are marked as CRITICAL.
> >   This includes the allocations
> >      - made by the driver to receive incoming packets
> >      - to process and send ARP packets
> >      - to create new routes for incoming packets
> 
> 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. In
> which case I'm not clear why it is any better than the current
> GFP_ATOMIC approach.

I am not referring to routes that are added by user-space, but the allocations
needed for cached routes stored in skb->dst in ip_route_input() path.

> > +#define SK_CRIT_ALLOC(sk, flags) ((sk->sk_allocation & __GFP_CRITICAL) | flags)
> 
> Lots of hidden conditional logic on critical paths. Also sk should be in
> brackets so that the macro evaluation order is defined as should flags
> 
> > +#define CRIT_ALLOC(flags) (__GFP_CRITICAL | flags)
> 
> Pointless obfuscation

The only reason i made these macros is that i would expect this to a compile
time configurable option so that there is zero overhead for regular users.

#ifdef CONFIG_CRIT_SOCKET
#define SK_CRIT_ALLOC(sk, flags) ((sk->sk_allocation & __GFP_CRITICAL) | flags)
#define CRIT_ALLOC(flags) (__GFP_CRITICAL | flags)
#else
#define SK_CRIT_ALLOC(sk, flags) flags
#define CRIT_ALLOC(flags) flags
#endif

Thanks
Sridhar

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