linux-kernel - Re: [RFC 05/12] net: optimise page get/free for bvec zc

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Date:   Wed, 1 Dec 2021 20:17:34 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     io-uring@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [RFC 05/12] net: optimise page get/free for bvec zc

On 12/1/21 19:20, Jonathan Lemon wrote:
> On Tue, Nov 30, 2021 at 03:18:53PM +0000, Pavel Begunkov wrote:
>> get_page() in __zerocopy_sg_from_bvec() and matching put_page()s are
>> expensive. However, we can avoid it if the caller can guarantee that
>> pages stay alive until the corresponding ubuf_info is not released.
>> In particular, it targets io_uring with fixed buffers following the
>> described contract.
>>
>> Assuming that nobody yet uses bvec together with zerocopy, make all
>> calls with bvec iterators follow this model.
>>
>> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
>> ---
[...]
>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>> index b23db60ea6f9..b7b087815539 100644
>> --- a/net/core/skbuff.c
>> +++ b/net/core/skbuff.c
>> @@ -666,10 +666,14 @@ static void skb_release_data(struct sk_buff *skb)
>>   			      &shinfo->dataref))
>>   		goto exit;
>>   
>> -	skb_zcopy_clear(skb, true);
>> +	if (!(shinfo->flags & SKBFL_MANAGED_FRAGS)) {
>> +		for (i = 0; i < shinfo->nr_frags; i++)
>> +			__skb_frag_unref(&shinfo->frags[i], skb->pp_recycle);
>> +	} else {
>> +		shinfo->flags &= ~SKBFL_MANAGED_FRAGS;
>> +	}
>>   
>> -	for (i = 0; i < shinfo->nr_frags; i++)
>> -		__skb_frag_unref(&shinfo->frags[i], skb->pp_recycle);
>> +	skb_zcopy_clear(skb, true);
> 
> It would be better if all of this logic was moved into the callback
> under zcopy_clear.  Note that this path is called for both TX and RX.

Yeah, can be done and removes the extra if from non-zc path.
Thanks!

-- 
Pavel Begunkov