netdev - Re: net: unix: Align send data_len up to PAGE

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Date:	Thu, 15 May 2014 12:54:40 +0400
From:	Kirill Tkhai <ktkhai@...allels.com>
To:	David Miller <davem@...emloft.net>
CC:	<netdev@...r.kernel.org>, <edumazet@...gle.com>
Subject: Re: net: unix: Align send data_len up to PAGE_SIZE

В Ср, 14/05/2014 в 15:15 -0400, David Miller пишет:
> From: Kirill Tkhai <ktkhai@...allels.com>
> Date: Mon, 12 May 2014 18:52:12 +0400
> 
> >  		data_len = min_t(size_t,
> >  				 len - SKB_MAX_ALLOC,
> >  				 MAX_SKB_FRAGS * PAGE_SIZE);
> > +		data_len = min_t(size_t,
> > +				 len,
> > +				 PAGE_ALIGN(data_len));
> > +	}
> 
> When I see:
> 
> 		x = min(y - N, z1);
> 		x = min(y, z2);
> 
> I'm a bit suspicious.  Why are you not subtracting the constant
> factor out of the first variable in the second min() call?

Because, in the most cases (len - SKB_MAX_ALLOC) < PAGE_ALIGN(data_len),
and the only payload of the patch is it tries to fix that :)
We use unused space of allocated page. For mem cache it's easier
to allocate

(len - PAGE_ALIGN(x)) than (len - x).


Here really should be 

 		data_len = min_t(size_t,
 				 len - SKB_MAX_ALLOC,
 				 MAX_SKB_FRAGS * PAGE_SIZE);
+		data_len = PAGE_ALIGN(data_len));


I added the second min, because I was afraid somebody plays with
SKB_MAX_ALLOC size in debug purposes. Not sure now.

Ok, yes, simple PAGE_ALIGN is much better :)

+ data_len = PAGE_ALIGN(data_len));

And I assumed it's OK to allocate skbs with zero header size like this:

sock_alloc_send_pskb(sk, 0, data_len)

Please say, if it's wrong.

Thanks!
Kirill

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