Subject: Re: [PATCH net v2 1/1] xdp: fix xdp_return_frame() kernel BUG throw for page_pool memory model

From: Jesper Dangaard Brouer <[email-masked]>
Date: Wed, 31 Mar 2021 15:38:03 +0200

On Wed, 31 Mar 2021 21:25:03 +0800
Ong Boon Leong <[email-masked]> wrote:

> xdp_return_frame() may be called outside of NAPI context to return
> xdpf back to page_pool. xdp_return_frame() calls __xdp_return() with
> napi_direct = false. For page_pool memory model, __xdp_return() calls
> xdp_return_frame_no_direct() unconditionally and below false negative
> kernel BUG throw happened under preempt-rt build:
> 
> [  430.450355] BUG: using smp_processor_id() in preemptible [00000000] code: modprobe/3884
> [  430.451678] caller is __xdp_return+0x1ff/0x2e0
> [  430.452111] CPU: 0 PID: 3884 Comm: modprobe Tainted: G     U      E     5.12.0-rc2+ #45
> 
> Changes in v2:
>  - This patch fixes the issue by making xdp_return_frame_no_direct() is
>    only called if napi_direct = true, as recommended for better by
>    Jesper Dangaard Brouer. Thanks!
> 
> Fixes: 2539650fadbf ("xdp: Helpers for disabling napi_direct of xdp_return_frame")
> Signed-off-by: Ong Boon Leong <[email-masked]>

Acked-by: Jesper Dangaard Brouer <[email-masked]>

> ---
>  net/core/xdp.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index 05354976c1fc..858276e72c68 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -350,7 +350,8 @@ 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);
> -		napi_direct &= !xdp_return_frame_no_direct();
> +		if (napi_direct && xdp_return_frame_no_direct())
> +			napi_direct = false;
>  		page_pool_put_full_page(xa->page_pool, page, napi_direct);
>  		rcu_read_unlock();
>  		break;

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