netdev - [PATCH net-next-2.6] net: skb_frag_t can be smaller on small arches

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Message-ID: <1285254414.2509.50.camel@edumazet-laptop>
Date:	Thu, 23 Sep 2010 17:06:54 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>
Subject: [PATCH net-next-2.6] net: skb_frag_t can be smaller on small arches

David

I remember you reacted on my three years old comment about
commit a309bb072b96bfe (Page offsets and lengths need to be __u32.)

http://www.mail-archive.com/netdev@vger.kernel.org/msg43999.html

Could we reconsider the thing now and allow to shrink
sizeof(skb_shared_info) from 0x104 to 0xbc, considering nothing happened
about scatterlist work ?

This saves 128 bytes on i386 because of alignments, and can be reverted
pretty fast if needed.

Thanks !

[PATCH net-next-2.6] net: skb_frag_t can be smaller on small arches

On 32bit arches, if PAGE_SIZE is smaller than 65536, we can use 16bit
offset and size fields. This patch saves 72 bytes per skb on i386, or
128 bytes after rounding.

Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
---
 include/linux/skbuff.h |    5 +++++
 1 files changed, 5 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 9e8085a..9a7ea70 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -129,8 +129,13 @@ typedef struct skb_frag_struct skb_frag_t;
 
 struct skb_frag_struct {
 	struct page *page;
+#if (BITS_PER_LONG > 32) || (PAGE_SIZE >= 65536)
 	__u32 page_offset;
 	__u32 size;
+#else
+	__u16 page_offset;
+	__u16 size;
+#endif
 };
 
 #define HAVE_HW_TIME_STAMP


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