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From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To:     Jesper Dangaard Brouer <brouer@redhat.com>,
        Ong Boon Leong <boon.leong.ong@intel.com>
Cc:     brouer@redhat.com, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        "David S . Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH net 1/1] xdp: fix xdp_return_frame() kernel BUG throw
 for page_pool memory model
In-Reply-To: <20210329170209.6db77c3d@carbon>
References: <20210329080039.32753-1-boon.leong.ong@intel.com>
 <20210329170209.6db77c3d@carbon>
Date:   Mon, 29 Mar 2021 18:25:08 +0200
Message-ID: <87lfa6rkpn.fsf@toke.dk>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Jesper Dangaard Brouer <brouer@redhat.com> writes:

> On Mon, 29 Mar 2021 16:00:39 +0800
> Ong Boon Leong <boon.leong.ong@intel.com> 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 <boon.leong.ong@intel.com>
>> ---
>
> This looks correct to me.
>
> Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
>
>
>>  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