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

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Date:	Fri, 23 Dec 2011 09:35:22 +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 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.

This change actually makes the size increase due to adding the
destructor small enough that a maximum MTU frame + shared_info still
fits in 2048 bytes (just!). I think I can see some scope for shaving off
a few more bytes as well but that'll have to wait for next year.

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

>From 75d3ee297af89b7f801bb0a13517f51181ef4d3c 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: document reason for 2 additional pages in MAX_SKB_FRAGS

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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