Subject: Re: [PATCH bpf-next 3/5] net: core: introduce build_skb_around

From: Ilias Apalodimas <[email-masked]>
Date: Thu, 11 Apr 2019 08:33:03 +0300

On Wed, Apr 10, 2019 at 01:43:47PM +0200, Jesper Dangaard Brouer wrote:
> The function build_skb() also have the responsibility to allocate and clear
> the SKB structure. Introduce a new function build_skb_around(), that moves
> the responsibility of allocation and clearing to the caller. This allows
> caller to use kmem_cache (slab/slub) bulk allocation API.
> 
> Next patch use this function combined with kmem_cache_alloc_bulk.
> 
> Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> ---
>  include/linux/skbuff.h |    2 +
>  net/core/skbuff.c      |   71 +++++++++++++++++++++++++++++++++++-------------
>  2 files changed, 54 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 9027a8c4219f..c40ffab8a9b0 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -1044,6 +1044,8 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t priority, int flags,
>  			    int node);
>  struct sk_buff *__build_skb(void *data, unsigned int frag_size);
>  struct sk_buff *build_skb(void *data, unsigned int frag_size);
> +struct sk_buff *build_skb_around(struct sk_buff *skb,
> +				 void *data, unsigned int frag_size);
>  
>  /**
>   * alloc_skb - allocate a network buffer
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 4782f9354dd1..d904b6e5fe08 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -258,6 +258,33 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
>  }
>  EXPORT_SYMBOL(__alloc_skb);
>  
> +/* Caller must provide SKB that is memset cleared */
> +static struct sk_buff *__build_skb_around(struct sk_buff *skb,
> +					  void *data, unsigned int frag_size)
> +{
> +	struct skb_shared_info *shinfo;
> +	unsigned int size = frag_size ? : ksize(data);
> +
> +	size -= SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> +
> +	/* Assumes caller memset cleared SKB */
> +	skb->truesize = SKB_TRUESIZE(size);
> +	refcount_set(&skb->users, 1);
> +	skb->head = data;
> +	skb->data = data;
> +	skb_reset_tail_pointer(skb);
> +	skb->end = skb->tail + size;
> +	skb->mac_header = (typeof(skb->mac_header))~0U;
> +	skb->transport_header = (typeof(skb->transport_header))~0U;
> +
> +	/* make sure we initialize shinfo sequentially */
> +	shinfo = skb_shinfo(skb);
> +	memset(shinfo, 0, offsetof(struct skb_shared_info, dataref));
> +	atomic_set(&shinfo->dataref, 1);
> +
> +	return skb;
> +}
> +
>  /**
>   * __build_skb - build a network buffer
>   * @data: data buffer provided by caller
> @@ -279,32 +306,15 @@ EXPORT_SYMBOL(__alloc_skb);
>   */
>  struct sk_buff *__build_skb(void *data, unsigned int frag_size)
>  {
> -	struct skb_shared_info *shinfo;
>  	struct sk_buff *skb;
> -	unsigned int size = frag_size ? : ksize(data);
>  
>  	skb = kmem_cache_alloc(skbuff_head_cache, GFP_ATOMIC);
> -	if (!skb)
> +	if (unlikely(!skb))
>  		return NULL;
>  
> -	size -= SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> -
>  	memset(skb, 0, offsetof(struct sk_buff, tail));
> -	skb->truesize = SKB_TRUESIZE(size);
> -	refcount_set(&skb->users, 1);
> -	skb->head = data;
> -	skb->data = data;
> -	skb_reset_tail_pointer(skb);
> -	skb->end = skb->tail + size;
> -	skb->mac_header = (typeof(skb->mac_header))~0U;
> -	skb->transport_header = (typeof(skb->transport_header))~0U;
>  
> -	/* make sure we initialize shinfo sequentially */
> -	shinfo = skb_shinfo(skb);
> -	memset(shinfo, 0, offsetof(struct skb_shared_info, dataref));
> -	atomic_set(&shinfo->dataref, 1);
> -
> -	return skb;
> +	return __build_skb_around(skb, data, frag_size);
>  }
>  
>  /* build_skb() is wrapper over __build_skb(), that specifically
> @@ -325,6 +335,29 @@ struct sk_buff *build_skb(void *data, unsigned int frag_size)
>  }
>  EXPORT_SYMBOL(build_skb);
>  
> +/**
> + * build_skb_around - build a network buffer around provided skb
> + * @skb: sk_buff provide by caller, must be memset cleared
> + * @data: data buffer provided by caller
> + * @frag_size: size of data, or 0 if head was kmalloced
> + */
> +struct sk_buff *build_skb_around(struct sk_buff *skb,
> +				 void *data, unsigned int frag_size)
> +{
> +	if (unlikely(!skb))
Maybe add a warning here, indicating the buffer *must* be there before calling
this?

> +		return NULL;
> +
> +	skb = __build_skb_around(skb, data, frag_size);
> +
> +	if (skb && frag_size) {
> +		skb->head_frag = 1;
> +		if (page_is_pfmemalloc(virt_to_head_page(data)))
> +			skb->pfmemalloc = 1;
> +	}
> +	return skb;
> +}
> +EXPORT_SYMBOL(build_skb_around);
> +
>  #define NAPI_SKB_CACHE_SIZE	64
>  
>  struct napi_alloc_cache {
> 

/Ilias