linux-kernel - Re: [PATCH net-next v4 12/19] xdp: add generic xdp_build_skb_from

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Message-ID: <e35afda3-a64a-432e-a69d-80519eb0ff33@intel.com>
Date: Tue, 12 Nov 2024 16:20:12 +0100
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Toke Høiland-Jørgensen <toke@...hat.com>
CC: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann
	<daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, "Andrii
 Nakryiko" <andrii@...nel.org>, Maciej Fijalkowski
	<maciej.fijalkowski@...el.com>, Stanislav Fomichev <sdf@...ichev.me>, "Magnus
 Karlsson" <magnus.karlsson@...el.com>,
	<nex.sw.ncis.osdt.itp.upstreaming@...el.com>, <bpf@...r.kernel.org>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v4 12/19] xdp: add generic
 xdp_build_skb_from_buff()

From: Toke Høiland-Jørgensen <toke@...hat.com>
Date: Mon, 11 Nov 2024 17:39:51 +0100

> Alexander Lobakin <aleksander.lobakin@...el.com> writes:
> 
>> The code which builds an skb from an &xdp_buff keeps multiplying itself
>> around the drivers with almost no changes. Let's try to stop that by
>> adding a generic function.
>> Unlike __xdp_build_skb_from_frame(), always allocate an skbuff head
>> using napi_build_skb() and make use of the available xdp_rxq pointer to
>> assign the Rx queue index. In case of PP-backed buffer, mark the skb to
>> be recycled, as every PP user's been switched to recycle skbs.
>>
>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>
>> ---
>>  include/net/xdp.h |  1 +
>>  net/core/xdp.c    | 55 +++++++++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 56 insertions(+)
>>
>> diff --git a/include/net/xdp.h b/include/net/xdp.h
>> index 4c19042adf80..b0a25b7060ff 100644
>> --- a/include/net/xdp.h
>> +++ b/include/net/xdp.h
>> @@ -330,6 +330,7 @@ xdp_update_skb_shared_info(struct sk_buff *skb, u8 nr_frags,
>>  void xdp_warn(const char *msg, const char *func, const int line);
>>  #define XDP_WARN(msg) xdp_warn(msg, __func__, __LINE__)
>>  
>> +struct sk_buff *xdp_build_skb_from_buff(const struct xdp_buff *xdp);
>>  struct xdp_frame *xdp_convert_zc_to_xdp_frame(struct xdp_buff *xdp);
>>  struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
>>  					   struct sk_buff *skb,
>> diff --git a/net/core/xdp.c b/net/core/xdp.c
>> index b1b426a9b146..3a9a3c14b080 100644
>> --- a/net/core/xdp.c
>> +++ b/net/core/xdp.c
>> @@ -624,6 +624,61 @@ int xdp_alloc_skb_bulk(void **skbs, int n_skb, gfp_t gfp)
>>  }
>>  EXPORT_SYMBOL_GPL(xdp_alloc_skb_bulk);
>>  
>> +/**
>> + * xdp_build_skb_from_buff - create an skb from an &xdp_buff
>> + * @xdp: &xdp_buff to convert to an skb
>> + *
>> + * Perform common operations to create a new skb to pass up the stack from
>> + * an &xdp_buff: allocate an skb head from the NAPI percpu cache, initialize
>> + * skb data pointers and offsets, set the recycle bit if the buff is PP-backed,
>> + * Rx queue index, protocol and update frags info.
>> + *
>> + * Return: new &sk_buff on success, %NULL on error.
>> + */
>> +struct sk_buff *xdp_build_skb_from_buff(const struct xdp_buff *xdp)
>> +{
>> +	const struct xdp_rxq_info *rxq = xdp->rxq;
>> +	const struct skb_shared_info *sinfo;
>> +	struct sk_buff *skb;
>> +	u32 nr_frags = 0;
>> +	int metalen;
>> +
>> +	if (unlikely(xdp_buff_has_frags(xdp))) {
>> +		sinfo = xdp_get_shared_info_from_buff(xdp);
>> +		nr_frags = sinfo->nr_frags;
>> +	}
> 
> Why this separate branch at the start of the function? nr_frags is no
> used until the other branch below, so why not just make that branch on
> xdp_buff_has_frags() and keep everything frags-related together in one
> block?

Because napi_build_skb() will call build_skb_around() which will
memset() a piece of shared info including nr_frags.
xdp_build_skb_from_frame() has the same logic. I'd be happy to have only
one block, but I can't =\

> 
> -Toke

Thanks,
Olek