Messages in this thread | | | From | Кулешов Алексей <> | Subject | sk_buff: why field 'head' contains skb_shared_info? | Date | Sat, 22 Feb 2014 21:08:27 +0400 |
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Subj.
Look at this core function: net/core/skbuff.c: __alloc_skb ... size = SKB_DATA_ALIGN(size); size += SKB_DATA_ALIGN(sizeof(struct skb_shared_info)); data = kmalloc_reserve(size, gfp_mask, node, &pfmemalloc); ... shinfo = skb_shinfo(skb); memset(shinfo, 0, offsetof(struct skb_shared_info, dataref)); ...
It looks like it was made purposely, but for what?
Why this question has risen. I found for my needs a very nice (for the first look) function <build_skb> from skbuff.c: > * build_skb - build a network buffer > * @data: data buffer provided by caller > ... > * Notes : > * Before IO, driver allocates only data buffer where NIC put incoming frame
Everything looks delicious, until this place: > * Driver should add room at head (NET_SKB_PAD) and > * MUST add room at tail (SKB_DATA_ALIGN(skb_shared_info))
And what if NIC is not a regular net_device and it doesn't need sk_buff at all. But for virtual ethernet device "Ethernet-over-NIC" sk_buff is crucial. So driver for NIC can't just kmalloc(NICs_RCV_BUF_SIZE, ...) for incoming buffers, it has to kmalloc(NICs_RCV_BUF_SIZE + <skb_shared_info>, ...) to cover attached (if any) to NIC virtual ethernet device needs. Or virtual ethernet device has to do netdev_alloc_skb + memcpy, which is obviously not so good.
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