netdev - Re: [PATCH 0/4] skb paged fragment destructors

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Date:	Wed, 21 Dec 2011 12:08:44 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ian Campbell <Ian.Campbell@...rix.com>
Cc:	David Miller <davem@...emloft.net>,
	"jesse.brandeburg@...el.com" <jesse.brandeburg@...el.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH 0/4] skb paged fragment destructors

Le mercredi 21 décembre 2011 à 11:03 +0000, Ian Campbell a écrit :
> On Fri, 2011-12-09 at 18:34 +0000, David Miller wrote:
> > From: Ian Campbell <Ian.Campbell@...rix.com>
> > Date: Fri, 9 Dec 2011 13:47:07 +0000
> > 
> > > If we had a concept like MAX_SKB_PAGES then it would perhaps make
> > > sense to have + 2 there, but AFAICT drivers etc are already
> > > accounting for this appropriately by adding a further + 2 (or
> > > sometimes + 1) to MAX_SKB_FRAGS.
> > 
> > Any kind of code like this, including the "+ 2" in the skbuff header,
> > should be coded to use some kind of macro so we can track this
> > dependency instead of stumbling onto it and accidently breaking lots
> > of stuff if we want to change this "2" value.
> 
> Agreed.
> 
> Part of the problem is that no one seems to have any idea what this
> particular + 2 means. My best hypothesis is that it accounts for the
> pages used by the linear area (which potentially crosses a page
> boundary).

I dont understand the point.

linear data is allocated with kmalloc(), so technically speaking its
located in a single page, but page order can be 0, 1, 2, 3, ...
MAX_ORDER.



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