Re: [RFC PATCH net-next v1 1/2] net: mirror skb frag ref/unref helpers

[Dragos Tatulea <dtatulea@nvidia.com>]

On Wed, 2024-03-06 at 15:59 -0800, Mina Almasry wrote:
> Refactor some of the skb frag ref/unref helpers for improved clarity.
> 
> Implement napi_pp_get_page() to be the mirror counterpart of
> napi_pp_put_page().
> 
> Implement napi_frag_ref() to be the mirror counterpart of
> napi_frag_unref().
> 
> Improve __skb_frag_ref() to become a mirror counterpart of
> __skb_frag_unref(). Previously unref could handle pp & non-pp pages,
> while the ref could only handle non-pp pages. Now both the ref & unref
> helpers can correctly handle both pp & non-pp pages.
> 
> Now that __skb_frag_ref() can handle both pp & non-pp pages, remove
> skb_pp_frag_ref(), and use __skb_frag_ref() instead.  This lets us
> remove pp specific handling from skb_try_coalesce.
> 
> Signed-off-by: Mina Almasry <almasrymina@google.com>
> 
> ---
>  include/linux/skbuff.h | 24 +++++++++++++++---
>  net/core/skbuff.c      | 56 ++++++++++++++----------------------------
>  2 files changed, 39 insertions(+), 41 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index d577e0bee18d..51316b0e20bc 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -3477,15 +3477,31 @@ static inline struct page *skb_frag_page(const skb_frag_t *frag)
>  	return netmem_to_page(frag->netmem);
>  }
>  
> +bool napi_pp_get_page(struct page *page);
> +
> +static inline void napi_frag_ref(skb_frag_t *frag, bool recycle)
> +{
> +#ifdef CONFIG_PAGE_POOL
> +	struct page *page = skb_frag_page(frag);
> +
Move assignment out of ifdef.

> +	if (recycle && napi_pp_get_page(page))
> +		return;
> +#endif
> +	get_page(page);
> +}
> +
>  /**
>   * __skb_frag_ref - take an addition reference on a paged fragment.
>   * @frag: the paged fragment
> + * @recycle: skb->pp_recycle param of the parent skb.
>   *
> - * Takes an additional reference on the paged fragment @frag.
> + * Takes an additional reference on the paged fragment @frag. Obtains the
> + * correct reference count depending on whether skb->pp_recycle is set and
> + * whether the frag is a page pool frag.
>   */
> -static inline void __skb_frag_ref(skb_frag_t *frag)
> +static inline void __skb_frag_ref(skb_frag_t *frag, bool recycle)
>  {
> -	get_page(skb_frag_page(frag));
> +	napi_frag_ref(frag, recycle);
>  }
>  
>  /**
> @@ -3497,7 +3513,7 @@ static inline void __skb_frag_ref(skb_frag_t *frag)
>   */
>  static inline void skb_frag_ref(struct sk_buff *skb, int f)
>  {
> -	__skb_frag_ref(&skb_shinfo(skb)->frags[f]);
> +	__skb_frag_ref(&skb_shinfo(skb)->frags[f], skb->pp_recycle);
>  }
>  
>  int skb_pp_cow_data(struct page_pool *pool, struct sk_buff **pskb,
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 1f918e602bc4..6d234faa9d9e 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -1006,6 +1006,21 @@ int skb_cow_data_for_xdp(struct page_pool *pool, struct sk_buff **pskb,
>  EXPORT_SYMBOL(skb_cow_data_for_xdp);
>  
>  #if IS_ENABLED(CONFIG_PAGE_POOL)
> +bool napi_pp_get_page(struct page *page)
> +{
> +
> +	struct page *head_page;
> +
> +	head_page = compound_head(page);
> +
> +	if (!is_pp_page(page))
> +		return false;
> +
> +	page_pool_ref_page(head_page);
> +	return true;
> +}
> +EXPORT_SYMBOL(napi_pp_get_page);
> +
>  bool napi_pp_put_page(struct page *page, bool napi_safe)
>  {
>  	bool allow_direct = false;
> @@ -1058,37 +1073,6 @@ static bool skb_pp_recycle(struct sk_buff *skb, void *data, bool napi_safe)
>  	return napi_pp_put_page(virt_to_page(data), napi_safe);
>  }
>  
> -/**
> - * skb_pp_frag_ref() - Increase fragment references of a page pool aware skb
> - * @skb:	page pool aware skb
> - *
> - * Increase the fragment reference count (pp_ref_count) of a skb. This is
> - * intended to gain fragment references only for page pool aware skbs,
> - * i.e. when skb->pp_recycle is true, and not for fragments in a
> - * non-pp-recycling skb. It has a fallback to increase references on normal
> - * pages, as page pool aware skbs may also have normal page fragments.
> - */
> -static int skb_pp_frag_ref(struct sk_buff *skb)
> -{
> -	struct skb_shared_info *shinfo;
> -	struct page *head_page;
> -	int i;
> -
> -	if (!skb->pp_recycle)
> -		return -EINVAL;
> -
> -	shinfo = skb_shinfo(skb);
> -
> -	for (i = 0; i < shinfo->nr_frags; i++) {
> -		head_page = compound_head(skb_frag_page(&shinfo->frags[i]));
> -		if (likely(is_pp_page(head_page)))
> -			page_pool_ref_page(head_page);
> -		else
> -			page_ref_inc(head_page);
> -	}
> -	return 0;
> -}
> -
>  static void skb_kfree_head(void *head, unsigned int end_offset)
>  {
>  	if (end_offset == SKB_SMALL_HEAD_HEADROOM)
> @@ -4199,7 +4183,7 @@ int skb_shift(struct sk_buff *tgt, struct sk_buff *skb, int shiftlen)
>  			to++;
>  
>  		} else {
> -			__skb_frag_ref(fragfrom);
> +			__skb_frag_ref(fragfrom, skb->pp_recycle);
>  			skb_frag_page_copy(fragto, fragfrom);
>  			skb_frag_off_copy(fragto, fragfrom);
>  			skb_frag_size_set(fragto, todo);
> @@ -4849,7 +4833,7 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>  			}
>  
>  			*nskb_frag = (i < 0) ? skb_head_frag_to_page_desc(frag_skb) : *frag;
> -			__skb_frag_ref(nskb_frag);
> +			__skb_frag_ref(nskb_frag, nskb->pp_recycle);
>  			size = skb_frag_size(nskb_frag);
>  
>  			if (pos < offset) {
> @@ -5980,10 +5964,8 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
>  	/* if the skb is not cloned this does nothing
>  	 * since we set nr_frags to 0.
>  	 */
> -	if (skb_pp_frag_ref(from)) {
> -		for (i = 0; i < from_shinfo->nr_frags; i++)
> -			__skb_frag_ref(&from_shinfo->frags[i]);
> -	}
> +	for (i = 0; i < from_shinfo->nr_frags; i++)
> +		__skb_frag_ref(&from_shinfo->frags[i], from->pp_recycle);
>  
>  	to->truesize += delta;
>  	to->len += len;