netdev - Re: [RFC] Kernel unaligned access at __skb_flow

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Date:	Fri, 29 Jan 2016 11:39:58 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	sowmini.varadhan@...cle.com, netdev@...r.kernel.org
Subject: Re: [RFC] Kernel unaligned access at __skb_flow_dissect

On Fri, 2016-01-29 at 11:08 -0800, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Fri, 29 Jan 2016 10:40:10 -0800
> 
> > I would try following ixgbe fix (sorry, totally untested, but you get
> > the idea...)
> > 
> > diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > index c4003a88bbf6..7ba64ed463a6 100644
> > --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > @@ -1525,7 +1525,7 @@ static bool ixgbe_alloc_mapped_page(struct ixgbe_ring *rx_ring,
> >  
> >  	bi->dma = dma;
> >  	bi->page = page;
> > -	bi->page_offset = 0;
> > +	bi->page_offset = NET_IP_ALIGN;
> >  
> >  	return true;
> >  }
> 
> Hmmm, wouldn't this waste 2 bytes on all pages?  Or just the head one with the headers?

On arches where NET_IP_ALIGN = 2, where it makes sense to get a proper
alignement, even for the copy of headers into skb->head, with same
source/destination alignment.

We 'waste' already 66 bytes for typical ethernet+IPv4+TCP headers.

Lets remind we allocate half a page (2048 bytes) for an ethernet frame
anyway. We have plenty of room.