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

From: Jesper Dangaard Brouer <[email-masked]>
Date: Mon, 29 Mar 2021 17:02:09 +0200

On Mon, 29 Mar 2021 16:00:39 +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
> 
> So, this patch fixes the issue by adding "if (napi_direct)" condition
> to skip calling xdp_return_frame_no_direct() if napi_direct = false.
> 
> Fixes: 2539650fadbf ("xdp: Helpers for disabling napi_direct of xdp_return_frame")
> Signed-off-by: Ong Boon Leong <[email-masked]>
> ---

This looks correct to me.

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..4eaa28972af2 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)
> +			napi_direct &= !xdp_return_frame_no_direct();

if (napi_direct && xdp_return_frame_no_direct())
	napi_direct = false;

I wonder if this code would be easier to understand?

>  		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