diff mbox

[0/4] skb paged fragment destructors

Message ID 1324632922.7877.103.camel@zakaz.uk.xensource.com
State Superseded, archived
Delegated to: David Miller
Headers show

Commit Message

Ian Campbell Dec. 23, 2011, 9:35 a.m. UTC 
On Thu, 2011-12-22 at 18:20 +0000, David Miller wrote:
> From: Ian Campbell <Ian.Campbell@citrix.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@gmail.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@citrix.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@citrix.com>
---
 include/linux/skbuff.h |   12 ++++++++----
 1 files changed, 8 insertions(+), 4 deletions(-)

Comments

David Laight Jan. 3, 2012, 9:36 a.m. UTC | #1 
> -#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

Wouldn't it be better to only have the expression once?
Either using a MAX() define or maybe:

#define MAX_SKB_FRAGS (65536UL/PAGE_SIZE + 1)
#if MAX_SKB_FRAGS < 16UL
#undef MAX_SKB_FRAGS
#define MAX_SKB_FRAGS 16UL
#endif

Although that replicates the 16 instead :-(

	David


--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
diff mbox

Patch

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;