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

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Date:	Wed, 21 Dec 2011 11:18:10 +0000
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Eric Dumazet <eric.dumazet@...il.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

On Wed, 2011-12-21 at 11:08 +0000, Eric Dumazet wrote:
> 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.

I did say hypothesis ;-) Do you know what that + 2 is actually all
about?

If no one knows what it is for maybe we should just remove it directly
instead of what I proposed?

> 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.

I think I must misunderstand the terminology.

An order 1 allocation is in multiples of PAGE_SIZE, isn't it, even
though they happen to be contiguous?

Or are you considering the possibility of kmalloc returning a super page
of some description? Isn't that to some extent transparent to the caller
who (assuming PAGE_SIZE 4096) doesn't know if kmalloc(16384) returned
4*4096 contiguous pages or 1*16384 page?

Anyway I take your underlying point that 2*PAGE_SIZE is no kind of limit
on the size of the linear region.

Ian.

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