Re: [PATCH net-next v5 12/19] xdp: add generic xdp_build_skb_from_buff()

[Alexander Lobakin <aleksander.lobakin@intel.com>]

From: Amit Cohen <amcohen@nvidia.com>
Date: Sun, 17 Nov 2024 12:42:11 +0000

> 
> 
>> -----Original Message-----
>> From: Alexander Lobakin <aleksander.lobakin@intel.com>
>> Sent: Friday, 15 November 2024 16:35
>> To: Ido Schimmel <idosch@idosch.org>
>> Cc: David S. Miller <davem@davemloft.net>; Eric Dumazet <edumazet@google.com>; Jakub Kicinski <kuba@kernel.org>; Paolo Abeni
>> <pabeni@redhat.com>; Toke Høiland-Jørgensen <toke@redhat.com>; Alexei Starovoitov <ast@kernel.org>; Daniel Borkmann
>> <daniel@iogearbox.net>; John Fastabend <john.fastabend@gmail.com>; Andrii Nakryiko <andrii@kernel.org>; Maciej Fijalkowski
>> <maciej.fijalkowski@intel.com>; Stanislav Fomichev <sdf@fomichev.me>; Magnus Karlsson <magnus.karlsson@intel.com>;
>> nex.sw.ncis.osdt.itp.upstreaming@intel.com; bpf@vger.kernel.org; netdev@vger.kernel.org; linux-kernel@vger.kernel.org
>> Subject: Re: [PATCH net-next v5 12/19] xdp: add generic xdp_build_skb_from_buff()
>>
>> From: Ido Schimmel <idosch@idosch.org>
>> Date: Thu, 14 Nov 2024 17:16:44 +0200
>>
>>> On Thu, Nov 14, 2024 at 05:06:06PM +0200, Ido Schimmel wrote:
>>>> Looks good (no objections to the patch), but I have a question. See
>>>> below.
>>>>
>>>> On Wed, Nov 13, 2024 at 04:24:35PM +0100, Alexander Lobakin wrote:
>>>>> 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.
>>>>>
>>>>> Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
>>>>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
>>>>
>>>> Reviewed-by: Ido Schimmel <idosch@nvidia.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;
>>>>> +	}
>>>>> +
>>>>> +	skb = napi_build_skb(xdp->data_hard_start, xdp->frame_sz);
>>>>> +	if (unlikely(!skb))
>>>>> +		return NULL;
>>>>> +
>>>>> +	skb_reserve(skb, xdp->data - xdp->data_hard_start);
>>>>> +	__skb_put(skb, xdp->data_end - xdp->data);
>>>>> +
>>>>> +	metalen = xdp->data - xdp->data_meta;
>>>>> +	if (metalen > 0)
>>>>> +		skb_metadata_set(skb, metalen);
>>>>> +
>>>>> +	if (is_page_pool_compiled_in() && rxq->mem.type == MEM_TYPE_PAGE_POOL)
>>>>> +		skb_mark_for_recycle(skb);
>>>>> +
>>>>> +	skb_record_rx_queue(skb, rxq->queue_index);
>>>>> +
>>>>> +	if (unlikely(nr_frags)) {
>>>>> +		u32 tsize;
>>>>> +
>>>>> +		tsize = sinfo->xdp_frags_truesize ? : nr_frags * xdp->frame_sz;
>>>>> +		xdp_update_skb_shared_info(skb, nr_frags,
>>>>> +					   sinfo->xdp_frags_size, tsize,
>>>>> +					   xdp_buff_is_frag_pfmemalloc(xdp));
>>>>> +	}
>>>>> +
>>>>> +	skb->protocol = eth_type_trans(skb, rxq->dev);
>>>>
>>>> The device we are working with has more ports (net devices) than Rx
>>>> queues, so each queue can receive packets from different net devices.
>>>> Currently, each Rx queue has its own NAPI instance and its own page
>>>> pool. All the Rx NAPI instances are initialized using the same dummy net
>>>> device which is allocated using alloc_netdev_dummy().
>>>>
>>>> What are our options with regards to the XDP Rx queue info structure? As
>>>> evident by this patch, it does not seem valid to register one such
>>>> structure per Rx queue and pass the dummy net device. Would it be valid
>>>> to register one such structure per port (net device) and pass zero for
>>>> the queue index and NAPI ID?
>>>
>>> Actually, this does not seem to be valid either as we need to associate
>>> an XDP Rx queue info with the correct page pool :/
>>
>> Right.
>> But I'd say, this assoc slowly becomes redundant. For example, idpf has
>> up to 4 page_pools per queue and I only pass 1 of them to rxq_info as
>> there are no other options. Regardless, its frames get processed
>> correctly thanks to that we have struct page::pp pointer + patch 9 from
>> this series which teaches put_page_bulk() to handle mixed bulks.
>>
>> Regarding your usecase -- after calling this function, you are free to
>> overwrite any skb fields as this helper doesn't pass it up the stack.
>> For example, in ice driver we have port reps and sometimes we need to
>> pass a different net_device, not the one saved in rxq_info. So when
>> switching to this function, we'll do eth_type_trans() once again (it's
>> either way under unlikely() in our code as it's swichdev slowpath).
>> Same for the queue number in rxq_info.
> 
> With this series, maintaining 'struct xdp_mem_allocator' in hash-table looks unnecessary.
> If so, xdp_reg_mem_model() does not need 'allocator' when mem_type is Page-Pool.
> 
> Is there a reason for not removing 'mem_id_ht'? With this patch, the nodes are no longer used.

Let me review this once again since I need to rebase it anyway.
Maybe we really could drop more code.

Thanks,
Olek