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Date: Sat, 05 May 2012 07:35:30 +0200
From: Eric Dumazet <eric.dumazet@...il.com>
To: Alexander Duyck <alexander.h.duyck@...el.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net,
jeffrey.t.kirsher@...el.com
Subject: Re: [PATCH 1/3] skb: Drop bad code from pskb_expand_head
On Fri, 2012-05-04 at 17:26 -0700, Alexander Duyck wrote:
> The fast-path for pskb_expand_head contains a check where the size plus the
> unaligned size of skb_shared_info is compared against the size of the data
> buffer. This code path has two issues. First is the fact that after the
> recent changes by Eric Dumazet to __alloc_skb and build_skb the shared info
> is always placed in the optimal spot for a buffer size making this check
> unnecessary. The second issue is the fact that the check doesn't take into
> account the aligned size of shared info. As a result the code burns cycles
> doing a memcpy with nothing actually being shifted.
>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
> ---
>
> net/core/skbuff.c | 12 ------------
> 1 files changed, 0 insertions(+), 12 deletions(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index c199aa4..4d085d4 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -951,17 +951,6 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
> fastpath = atomic_read(&skb_shinfo(skb)->dataref) == delta;
> }
>
> - if (fastpath && !skb->head_frag &&
> - size + sizeof(struct skb_shared_info) <= ksize(skb->head)) {
> - memmove(skb->head + size, skb_shinfo(skb),
> - offsetof(struct skb_shared_info,
> - frags[skb_shinfo(skb)->nr_frags]));
> - memmove(skb->head + nhead, skb->head,
> - skb_tail_pointer(skb) - skb->head);
> - off = nhead;
> - goto adjust_others;
> - }
> -
> data = kmalloc(size + SKB_DATA_ALIGN(sizeof(struct skb_shared_info)),
> gfp_mask);
> if (!data)
> @@ -997,7 +986,6 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
>
> skb->head = data;
> skb->head_frag = 0;
> -adjust_others:
> skb->data += off;
> #ifdef NET_SKBUFF_DATA_USES_OFFSET
> skb->end = size;
>
I totally agree this code is no longer needed, we already have the
skb_shared_info at the end of the buffer.
Acked-by: Eric Dumazet <edumazet@...gle.com>
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