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: Wed, 31 Mar 2021 10:53:06 +0200

Hi Ong,

Patch is in "Changes Requested".
Thus, please send a V2 patch with suggested code changes below.

--Jesper

On Mon, 29 Mar 2021 18:25:08 +0200
Toke Høiland-Jørgensen <[email-masked]> wrote:

> Jesper Dangaard Brouer <[email-masked]> writes:
> 
> > 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?  
> 
> Yes, IMO it would! :)
> 
> -Toke
> 

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