Re: [PATCH v13 bpf-next 05/18] net: xdp: add xdp_update_skb_shared_info utility routine

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

On 07/09/2021 14.35, Lorenzo Bianconi wrote:
> Introduce xdp_update_skb_shared_info routine to update frags array
> metadata in skb_shared_info data structure converting to a skb from
> a xdp_buff or xdp_frame.
> According to the current skb_shared_info architecture in
> xdp_frame/xdp_buff and to the xdp multi-buff support, there is
> no need to run skb_add_rx_frag() and reset frags array converting the buffer
> to a skb since the frag array will be in the same position for xdp_buff/xdp_frame
> and for the skb, we just need to update memory metadata.
> Introduce XDP_FLAGS_PF_MEMALLOC flag in xdp_buff_flags in order to mark
> the xdp_buff or xdp_frame as under memory-pressure if pages of the frags array
> are under memory pressure. Doing so we can avoid looping over all fragments in
> xdp_update_skb_shared_info routine. The driver is expected to set the
> flag constructing the xdp_buffer using xdp_buff_set_frag_pfmemalloc
> utility routine.
> Rely on xdp_update_skb_shared_info in __xdp_build_skb_from_frame routine
> converting the multi-buff xdp_frame to a skb after performing a XDP_REDIRECT.
> 
> Acked-by: John Fastabend <john.fastabend@gmail.com>
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> ---
>   include/net/xdp.h | 33 ++++++++++++++++++++++++++++++++-
>   net/core/xdp.c    | 17 +++++++++++++++++
>   2 files changed, 49 insertions(+), 1 deletion(-)
> 
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index ed5ea784fd45..53cccdc9528c 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -67,7 +67,10 @@ struct xdp_txq_info {
>   };
>   
>   enum xdp_buff_flags {
> -	XDP_FLAGS_MULTI_BUFF	= BIT(0), /* non-linear xdp buff */
> +	XDP_FLAGS_MULTI_BUFF		= BIT(0), /* non-linear xdp buff */
> +	XDP_FLAGS_FRAGS_PF_MEMALLOC	= BIT(1), /* xdp multi-buff paged memory
> +						   * is under pressure
> +						   */
>   };
>   
>   struct xdp_buff {
> @@ -96,6 +99,16 @@ static __always_inline void xdp_buff_clear_mb(struct xdp_buff *xdp)
>   	xdp->flags &= ~XDP_FLAGS_MULTI_BUFF;
>   }
>   
> +static __always_inline bool xdp_buff_is_frag_pfmemalloc(struct xdp_buff *xdp)
> +{
> +	return !!(xdp->flags & XDP_FLAGS_FRAGS_PF_MEMALLOC);
> +}
> +
> +static __always_inline void xdp_buff_set_frag_pfmemalloc(struct xdp_buff *xdp)
> +{
> +	xdp->flags |= XDP_FLAGS_FRAGS_PF_MEMALLOC;
> +}
> +
>   static __always_inline void
>   xdp_init_buff(struct xdp_buff *xdp, u32 frame_sz, struct xdp_rxq_info *rxq)
>   {
> @@ -151,6 +164,11 @@ static __always_inline bool xdp_frame_is_mb(struct xdp_frame *frame)
>   	return !!(frame->flags & XDP_FLAGS_MULTI_BUFF);
>   }
>   
> +static __always_inline bool xdp_frame_is_frag_pfmemalloc(struct xdp_frame *frame)
> +{
> +	return !!(frame->flags & XDP_FLAGS_FRAGS_PF_MEMALLOC);
> +}
> +
>   #define XDP_BULK_QUEUE_SIZE	16
>   struct xdp_frame_bulk {
>   	int count;
> @@ -186,6 +204,19 @@ static inline void xdp_scrub_frame(struct xdp_frame *frame)
>   	frame->dev_rx = NULL;
>   }
>   
> +static inline void
> +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;
> +
> +	skb->len += size;
> +	skb->data_len += size;
> +	skb->truesize += truesize;
> +	skb->pfmemalloc |= pfmemalloc;

Do we need to clear gso_type here as it is shared/union with 
xdp_frags_size ?
(see below ... it is already cleared before call)


> +}
> +
>   /* Avoids inlining WARN macro in fast-path */
>   void xdp_warn(const char *msg, const char *func, const int line);
>   #define XDP_WARN(msg) xdp_warn(msg, __func__, __LINE__)
> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index cc92ccb38432..504be3ce3ca9 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -531,8 +531,20 @@ struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
>   					   struct sk_buff *skb,
>   					   struct net_device *dev)
>   {
> +	unsigned int frag_size, frag_tsize;
>   	unsigned int headroom, frame_size;
>   	void *hard_start;
> +	u8 nr_frags;
> +
> +	/* xdp multi-buff frame */
> +	if (unlikely(xdp_frame_is_mb(xdpf))) {
> +		struct skb_shared_info *sinfo;
> +
> +		sinfo = xdp_get_shared_info_from_frame(xdpf);
> +		frag_tsize = sinfo->xdp_frags_tsize;
> +		frag_size = sinfo->xdp_frags_size;
> +		nr_frags = sinfo->nr_frags;
> +	}
>   
>   	/* Part of headroom was reserved to xdpf */
>   	headroom = sizeof(*xdpf) + xdpf->headroom;
> @@ -552,6 +564,11 @@ struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
>   	if (xdpf->metasize)
>   		skb_metadata_set(skb, xdpf->metasize);
>   
> +	if (unlikely(xdp_frame_is_mb(xdpf)))
> +		xdp_update_skb_shared_info(skb, nr_frags,
> +					   frag_size, frag_tsize,
> +					   xdp_frame_is_frag_pfmemalloc(xdpf));
> +

There is a build_skb_around() call before this call, which via 
__build_skb_around() will clear top part of skb_shared_info.
(Thus, clearing gso_type not needed ... see above)

>   	/* Essential SKB info: protocol and skb->dev */
>   	skb->protocol = eth_type_trans(skb, dev);
>   


Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>