Subject: Re: [PATCH v5 bpf-next 7/9] xdp: Helpers for disabling napi_direct of xdp_return_frame

From: Jesper Dangaard Brouer <[email-masked]>
Date: Mon, 30 Jul 2018 14:33:49 +0200

On Thu, 26 Jul 2018 23:40:30 +0900
Toshiaki Makita <[email-masked]> wrote:

> From: Toshiaki Makita <[email-masked]>
> 
> We need some mechanism to disable napi_direct on calling
> xdp_return_frame_rx_napi() from some context.
> When veth gets support of XDP_REDIRECT, it will redirects packets which
> are redirected from other devices. On redirection veth will reuse
> xdp_mem_info of the redirection source device to make return_frame work.
> But in this case .ndo_xdp_xmit() called from veth redirection uses
> xdp_mem_info which is not guarded by NAPI, because the .ndo_xdp_xmit()
> is not called directly from the rxq which owns the xdp_mem_info.

Hmm "not guarded by NAPI" sounds scary to me, as XDP depends heavily on
being protected by NAPI. But it does look like you handle this is
earlier patches...

> This approach introduces a flag in bpf_redirect_info to indicate that
> napi_direct should be disabled even when _rx_napi variant is used as
> well as helper functions to use it.
> 
> A NAPI handler who wants to use this flag needs to call
> xdp_set_return_frame_no_direct() before processing packets, and call
> xdp_clear_return_frame_no_direct() after xdp_do_flush_map() before
> exiting NAPI.

Okay, so it still runs under NAPI. It should be okay then.  Also such
that the bpf_redirect_info is "stable", in the sense that we cannot
change the CPU we are running on, given bpf_redirect_info is a per-cpu thing.

> v4:
> - Use bpf_redirect_info for storing the flag instead of xdp_mem_info to
>   avoid per-frame copy cost.
> 
> Signed-off-by: Toshiaki Makita <[email-masked]>
> ---
>  include/linux/filter.h | 25 +++++++++++++++++++++++++
>  net/core/xdp.c         |  6 ++++--
>  2 files changed, 29 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 4717af8b95e6..2b072dab32c0 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -543,10 +543,14 @@ struct bpf_redirect_info {
>  	struct bpf_map *map;
>  	struct bpf_map *map_to_flush;
>  	unsigned long   map_owner;
> +	u32 kern_flags;
>  };
>  
>  DECLARE_PER_CPU(struct bpf_redirect_info, bpf_redirect_info);
>  
> +/* flags for bpf_redirect_info kern_flags */
> +#define BPF_RI_F_RF_NO_DIRECT	BIT(0)	/* no napi_direct on return_frame */
> +
>  /* Compute the linear packet data range [data, data_end) which
>   * will be accessed by various program types (cls_bpf, act_bpf,
>   * lwt, ...). Subsystems allowing direct data access must (!)
> @@ -775,6 +779,27 @@ static inline bool bpf_dump_raw_ok(void)
>  struct bpf_prog *bpf_patch_insn_single(struct bpf_prog *prog, u32 off,
>  				       const struct bpf_insn *patch, u32 len);
>  
> +static inline bool xdp_return_frame_no_direct(void)
> +{
> +	struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
> +
> +	return ri->kern_flags & BPF_RI_F_RF_NO_DIRECT;
> +}
> +
> +static inline void xdp_set_return_frame_no_direct(void)
> +{
> +	struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
> +
> +	ri->kern_flags |= BPF_RI_F_RF_NO_DIRECT;
> +}
> +
> +static inline void xdp_clear_return_frame_no_direct(void)
> +{
> +	struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
> +
> +	ri->kern_flags &= ~BPF_RI_F_RF_NO_DIRECT;
> +}
> +
>  static inline int xdp_ok_fwd_dev(const struct net_device *fwd,
>  				 unsigned int pktlen)
>  {
> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index 57285383ed00..3dd99e1c04f5 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -330,10 +330,12 @@ static void __xdp_return(void *data, struct xdp_mem_info *mem, bool napi_direct,
>  		/* mem->id is valid, checked in xdp_rxq_info_reg_mem_model() */
>  		xa = rhashtable_lookup(mem_id_ht, &mem->id, mem_id_rht_params);
>  		page = virt_to_head_page(data);
> -		if (xa)
> +		if (xa) {
> +			napi_direct &= !xdp_return_frame_no_direct();
>  			page_pool_put_page(xa->page_pool, page, napi_direct);
> -		else
> +		} else {
>  			put_page(page);
> +		}
>  		rcu_read_unlock();
>  		break;
>  	case MEM_TYPE_PAGE_SHARED:

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer