netdev - Re: [PATCH bpf-next V2 2/4] net: core: introduce build_skb

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Date:   Tue, 16 Apr 2019 20:01:38 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Daniel Borkmann <borkmann@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Song Liu <songliubraving@...com>,
        Toke Høiland-Jørgensen <toke@...e.dk>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Edward Cree <ecree@...arflare.com>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next V2 2/4] net: core: introduce build_skb_around

On Fri, Apr 12, 2019 at 8:07 AM Jesper Dangaard Brouer
<brouer@...hat.com> 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 <brouer@...hat.com>
> ---
>  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 a06275a618f0..e81f2b0e8a83 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -1042,6 +1042,8 @@ 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 9901f5322852..087622298d77 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);
>  }

It looks good to me, but I'd like to see Eric's Ack before applying.