netdev - Re: [PATCH net-next] net: netdev_alloc_skb() use build

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Date:	Mon, 04 Jun 2012 21:24:02 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Willy Tarreau <w@....eu>, David Miller <davem@...emloft.net>,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: netdev_alloc_skb() use build_skb()

On Mon, 2012-06-04 at 21:16 +0300, Michael S. Tsirkin wrote:

> Will take a look, thanks.
> By the way, this comment at build_skb:
> * @frag_size: size of fragment, or 0 if head was kmalloced
> 
> is not very clear to me. Could you clarify what exactly size
> of fragment means in this context?
> 


If your driver did :

data = kmalloc(100) then you use @frag_size=0, so that build_skb() does
the ksize(data) to fetch real size (It can depend on slab/slub/sob
allocator)


If you used netdev_alloc_frag(128), then you use 128 because there is no
way build_skb() can guess the size of the fragment. Its also how we
signal to build_skb() that skb->head_frag is set to 1.

__netdev_alloc_skb() for example does :

void *data = netdev_alloc_frag(fragsz);
skb = build_skb(data, fragsz);



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