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

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Date:	Fri, 23 Dec 2011 09:39:14 +0000
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	David Miller <davem@...emloft.net>
CC:	"eric.dumazet@...il.com" <eric.dumazet@...il.com>,
	"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 Fri, 2011-12-23 at 09:35 +0000, Ian Campbell wrote:
> On Thu, 2011-12-22 at 18:20 +0000, David Miller wrote:
> > From: Ian Campbell <Ian.Campbell@...rix.com>
> > Date: Thu, 22 Dec 2011 10:33:36 +0000
> > 
> > > On Wed, 2011-12-21 at 19:28 +0000, David Miller wrote:
> > >> From: Eric Dumazet <eric.dumazet@...il.com>
> > >> Date: Wed, 21 Dec 2011 15:02:18 +0100
> > >> 
> > >> > No idea on this +2 point.
> > >> 
> > >> I think I know, and I believe I instructed Alexey Kuznetsov to do
> > >> this.
> > >> 
> > >> When sendfile() is performed, we might start the SKB with the last few
> > >> bytes of one page, and end the SKB with the first few bytes of another
> > >> page.
> > >> 
> > >> In order to fit a full 64K frame into an SKB in this situation we have
> > >> to accomodate this case.
> > > 
> > > Thanks David, that makes sense.
> > > 
> > > However I think you only actually need 1 extra page for that. If the
> > > data in frag[0] starts at $offset then frag[16] will need to have
> > > $offset bytes in it. e.g.
> > > 	4096-$offset + 4096*15 + $offset = 65536
> > > which == 17 pages rather than 18.
> > > 
> > > The following documents the status quo but I could update to switch to +
> > > 1 instead if there are no flaws in the above logic...
> > 
> > Indeed, you're right.  Please change this to 1 and document it, and we
> > can put that change into net-next, thanks a lot!
> 
> Please see below.

-ENOCOFFEE. I forgot to update the changelog message :-( sorry. Another
attempt follows.

8<-----------------------------------------

>From 333b3165ad464b68c8fca87a759adea83f6ff6a6 Mon Sep 17 00:00:00 2001
From: Ian Campbell <ian.campbell@...rix.com>
Date: Thu, 22 Dec 2011 10:07:19 +0000
Subject: [PATCH] net: only use a single page of slop in MAX_SKB_FRAGS

In order to accommodate a 64K buffer we need 64K/PAGE_SIZE plus one more page
in order to allow for a buffer which does not start on a page boundary.

Signed-off-by: Ian Campbell <ian.campbell@...rix.com>
---
 include/linux/skbuff.h |   12 ++++++++----
 1 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index fe86488..0592b3d 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -128,13 +128,17 @@ struct sk_buff_head {
 
 struct sk_buff;
 
-/* To allow 64K frame to be packed as single skb without frag_list. Since
- * GRO uses frags we allocate at least 16 regardless of page size.
+/* To allow 64K frame to be packed as single skb without frag_list we
+ * require 64K/PAGE_SIZE pages plus 1 additional page to allow for
+ * buffers which do not start on a page boundary.
+ *
+ * Since GRO uses frags we allocate at least 16 regardless of page
+ * size.
  */
-#if (65536/PAGE_SIZE + 2) < 16
+#if (65536/PAGE_SIZE + 1) < 16
 #define MAX_SKB_FRAGS 16UL
 #else
-#define MAX_SKB_FRAGS (65536/PAGE_SIZE + 2)
+#define MAX_SKB_FRAGS (65536/PAGE_SIZE + 1)
 #endif
 
 typedef struct skb_frag_struct skb_frag_t;
-- 
1.7.2.5



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