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Subject: Re: [PATCH] net: eliminate meaningless memcpy to data in
 pskb_carve_inside_nonlinear()
To:     linmiaohe <linmiaohe@huawei.com>,
        Eric Dumazet <eric.dumazet@gmail.com>
Cc:     "davem@davemloft.net" <davem@davemloft.net>,
        "kuba@kernel.org" <kuba@kernel.org>,
        "pshelar@ovn.org" <pshelar@ovn.org>,
        "martin.varghese@nokia.com" <martin.varghese@nokia.com>,
        "fw@strlen.de" <fw@strlen.de>,
        "dcaratti@redhat.com" <dcaratti@redhat.com>,
        "edumazet@google.com" <edumazet@google.com>,
        "steffen.klassert@secunet.com" <steffen.klassert@secunet.com>,
        "pabeni@redhat.com" <pabeni@redhat.com>,
        "shmulik@metanetworks.com" <shmulik@metanetworks.com>,
        "kyk.segfault@gmail.com" <kyk.segfault@gmail.com>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <40a0b4ba22ff499686a2521998767ae5@huawei.com>
From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <9684c26c-9f26-5a7f-3d17-d180afff432c@gmail.com>
Date:   Tue, 11 Aug 2020 08:35:07 -0700
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On 8/11/20 5:10 AM, linmiaohe wrote:
> Eric Dumazet <eric.dumazet@gmail.com> wrote:
>> On 8/10/20 5:28 AM, Miaohe Lin wrote:
>>> The skb_shared_info part of the data is assigned in the following 
>>> loop. It is meaningless to do a memcpy here.
>>>
>>
>> Reminder : net-next is CLOSED.
>>
> 
> Thanks for your remind. I would wait for it open.
> 
>> This is not correct. We still have to copy _something_
>>
>> Something like :
>>
>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c index 2828f6d5ba898a5e50ccce45589bf1370e474b0f..1c0519426c7ba4b04377fc8054c4223c135879ab 100644
>> --- a/net/core/skbuff.c
>> +++ b/net/core/skbuff.c
>> @@ -5953,8 +5953,8 @@ static int pskb_carve_inside_nonlinear(struct sk_buff *skb, const u32 off,
>>        size = SKB_WITH_OVERHEAD(ksize(data));
>>
>>        memcpy((struct skb_shared_info *)(data + size),
>> -              skb_shinfo(skb), offsetof(struct skb_shared_info,
>> -                                        frags[skb_shinfo(skb)->nr_frags]));
>> +              skb_shinfo(skb), offsetof(struct skb_shared_info, 
>> + frags[0]));
>> +
>>        if (skb_orphan_frags(skb, gfp_mask)) {
>>                kfree(data);
>>                return -ENOMEM;
>>
> 
> This looks good. Will send a patch v2 soon. May I add a suggested-by tag of you ?

I would advise not using Suggested-by, as this would imply I suggested the idea of changing
this function in the first place.

I will add a Reviewed-by:  eventually if your v2 submission looks fine to me.

Thanks.


> Many thanks.
>