netdev - Re: [PATCH v2] net: avoid the unnecessary kmalloc

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Date:	Tue, 30 Nov 2010 07:52:01 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH v2] net: avoid the unnecessary kmalloc

Le mardi 30 novembre 2010 à 08:40 +0800, Changli Gao a écrit :
> If the old memory allocated by kmalloc() is larger than the new requested,
> pskb_expand_head() doesn't need to allocate a new one, unless the skb->head
> is shared.
> 
> Signed-off-by: Changli Gao <xiaosuo@...il.com>

Seems fine to me, but patch title and changelog are a bit uninformative.

skb head being allocated by kmalloc(), it might be larger than what
actually requested because of discrete kmem caches sizes. Before
reallocating a new skb head, check if the current one has the needed
extra size.

Do this check only if skb head is not shared.

> +
> +	if (fastpath &&
> +	    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;
> +	}
> +

I suggest doing the max possible resize at this stage ?

Ie moving skb_shared_info at the edge of memory block.

Maybe its not necessary, and a given skb is not expanded multiple times
in our stack, I dont really know.

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