Subject: Re: [PATCH net-next v2 11/18] xdp: add generic xdp_buff_add_frag()

From: Maciej Fijalkowski <[email-masked]>
Date: Thu, 17 Oct 2024 14:26:48 +0200

On Tue, Oct 15, 2024 at 04:53:43PM +0200, Alexander Lobakin wrote:
> The code piece which would attach a frag to &xdp_buff is almost
> identical across the drivers supporting XDP multi-buffer on Rx.
> Make it a generic elegant onelner.

oneliner

> Also, I see lots of drivers calculating frags_truesize as
> `xdp->frame_sz * nr_frags`. I can't say this is fully correct, since
> frags might be backed by chunks of different sizes, especially with
> stuff like the header split. Even page_pool_alloc() can give you two
> different truesizes on two subsequent requests to allocate the same
> buffer size. Add a field to &skb_shared_info (unionized as there's no
> free slot currently on x6_64) to track the "true" truesize. It can be

x86_64

> used later when updating an skb.

I also agree that xdp->frame_sz * nr_frags for truesize might be an
over-assumption.

Reviewed-by: Maciej Fijalkowski <[email-masked]>

two small nits/questions below that might be ignored.

> 
> Signed-off-by: Alexander Lobakin <[email-masked]>
> ---
>  include/linux/skbuff.h | 16 ++++++--
>  include/net/xdp.h      | 90 +++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 101 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index c867df5b1051..6ec78c1598fe 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -607,11 +607,19 @@ struct skb_shared_info {
>  	 * Warning : all fields before dataref are cleared in __alloc_skb()
>  	 */
>  	atomic_t	dataref;
> -	unsigned int	xdp_frags_size;
>  
> -	/* Intermediate layers must ensure that destructor_arg
> -	 * remains valid until skb destructor */
> -	void *		destructor_arg;
> +	union {
> +		struct {
> +			u32		xdp_frags_size;
> +			u32		xdp_frags_truesize;
> +		};
> +
> +		/*
> +		 * Intermediate layers must ensure that destructor_arg
> +		 * remains valid until skb destructor.
> +		 */
> +		void		*destructor_arg;
> +	};
>  
>  	/* must be last field, see pskb_expand_head() */
>  	skb_frag_t	frags[MAX_SKB_FRAGS];
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index c4b408d22669..19d2b283b845 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -167,6 +167,88 @@ xdp_get_buff_len(const struct xdp_buff *xdp)
>  	return len;
>  }
>  
> +/**
> + * __xdp_buff_add_frag - attach a frag to an &xdp_buff
> + * @xdp: XDP buffer to attach the frag to
> + * @page: page containing the frag
> + * @offset: page offset at which the frag starts
> + * @size: size of the frag
> + * @truesize: truesize (page / page frag size) of the frag
> + * @try_coalesce: whether to try coalescing the frags
> + *
> + * Attach a frag to an XDP buffer. If it currently has no frags attached,
> + * initialize the related fields, otherwise check that the frag number
> + * didn't reach the limit of ``MAX_SKB_FRAGS``. If possible, try coalescing
> + * the frag with the previous one.
> + * The function doesn't check/update the pfmemalloc bit. Please use the
> + * non-underscored wrapper in drivers.
> + *
> + * Return: true on success, false if there's no space for the frag in
> + * the shared info struct.
> + */
> +static inline bool __xdp_buff_add_frag(struct xdp_buff *xdp, struct page *page,
> +				       u32 offset, u32 size, u32 truesize,
> +				       bool try_coalesce)
> +{
> +	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
> +	skb_frag_t *prev;
> +	u32 nr_frags;
> +
> +	if (!xdp_buff_has_frags(xdp)) {
> +		xdp_buff_set_frags_flag(xdp);
> +
> +		nr_frags = 0;
> +		sinfo->xdp_frags_size = 0;
> +		sinfo->xdp_frags_truesize = 0;
> +
> +		goto fill;
> +	}
> +
> +	nr_frags = sinfo->nr_frags;
> +	if (unlikely(nr_frags == MAX_SKB_FRAGS))
> +		return false;
> +
> +	prev = &sinfo->frags[nr_frags - 1];
> +	if (try_coalesce && page == skb_frag_page(prev) &&
> +	    offset == skb_frag_off(prev) + skb_frag_size(prev))
> +		skb_frag_size_add(prev, size);
> +	else
> +fill:
> +		__skb_fill_page_desc_noacc(sinfo, nr_frags++, page,
> +					   offset, size);
> +
> +	sinfo->nr_frags = nr_frags;

is it really necessary to work on local nr_frags instead of directly
update it from sinfo?

> +	sinfo->xdp_frags_size += size;
> +	sinfo->xdp_frags_truesize += truesize;
> +
> +	return true;
> +}
> +
> +/**
> + * xdp_buff_add_frag - attach a frag to an &xdp_buff
> + * @xdp: XDP buffer to attach the frag to
> + * @page: page containing the frag
> + * @offset: page offset at which the frag starts
> + * @size: size of the frag
> + * @truesize: truesize (page / page frag size) of the frag
> + *
> + * Version of __xdp_buff_add_frag() which takes care of the pfmemalloc bit.
> + *
> + * Return: true on success, false if there's no space for the frag in
> + * the shared info struct.
> + */
> +static inline bool xdp_buff_add_frag(struct xdp_buff *xdp, struct page *page,
> +				     u32 offset, u32 size, u32 truesize)
> +{
> +	if (!__xdp_buff_add_frag(xdp, page, offset, size, truesize, true))
> +		return false;
> +
> +	if (unlikely(page_is_pfmemalloc(page)))
> +		xdp_buff_set_frag_pfmemalloc(xdp);
> +
> +	return true;
> +}
> +
>  struct xdp_frame {
>  	void *data;
>  	u32 len;
> @@ -230,7 +312,13 @@ xdp_update_skb_shared_info(struct sk_buff *skb, u8 nr_frags,
>  			   unsigned int size, unsigned int truesize,
>  			   bool pfmemalloc)
>  {
> -	skb_shinfo(skb)->nr_frags = nr_frags;
> +	struct skb_shared_info *sinfo = skb_shinfo(skb);
> +
> +	sinfo->nr_frags = nr_frags;
> +	/* ``destructor_arg`` is unionized with ``xdp_frags_{,true}size``,
> +	 * reset it after that these fields aren't used anymore.
> +	 */
> +	sinfo->destructor_arg = NULL;

wouldn't clearing size and truesize from union be more obvious?
OTOH it's one write vs two :)

>  	skb->len += size;
>  	skb->data_len += size;
> -- 
> 2.46.2