Re: [PATCH v3] ip_frag: add IPv4 fragment copy packet API

[Konstantin Ananyev <konstantin.v.ananyev@yandex.ru>]

22/07/2022 17:14, Morten Brørup пишет:
> From: Huichao Cai [mailto:chcchc88@163.com]
> Sent: Friday, 22 July 2022 17.59
> 
>> At 2022-07-22 23:52:28, "Morten Brørup" <mb@smartsharesystems.com> wrote:
>>>> From: Stephen Hemminger [mailto:stephen@networkplumber.org]
>>>> Sent: Friday, 22 July 2022 16.49
>>>>
>>>> On Fri, 22 Jul 2022 21:01:50 +0800
>>>> Huichao Cai <chcchc88@163.com> wrote:
>>>>
>>>>> Some NIC drivers support MBUF_FAST_FREE(Device supports optimization
>>>>> for fast release of mbufs. When set application must guarantee that
>>>>> per-queue all mbufs comes from the same mempool and has refcnt = 1)
>>>>> offload. In order to adapt to this offload function, add this API.
>>>>> Add some test data for this API.
>>>>>
>>>>> Signed-off-by: Huichao Cai <chcchc88@163.com>
>>>>
>>>> The code should just be checking that refcnt == 1 directly.
>>>>
>>>> There are cases where sender passes a cloned mbuf.  This is independent
>>>> of the fast free optimization.
>>>>
>>>> Similar to what Linux kernel does with skb_cow().
>>>
>>> Olivier just confirmed that MBUF_FAST_FREE requires that the mbufs are direct and non-segmented, although these requirements are not yet documented.
>>>
>>> This means that you should not generate segmented mbufs with this patch. I don't know what to do instead; probably fail with an appropriate errno.
>>
>> When the bnxt driver sends mbuf, it will take the mbuf segments apart and hang it to the tx_buf_ring, so there is no mbuf segments when it is released. Does this mean that there can be mbuf segments?
> 
> Only if the bnxt driver also resets the segmentation fields (nb_segs and next) in those mbufs, which I suppose it does, if it supports MBUF_FAST_FREE with segmented packets.
> 
> However, other Ethernet drivers don't do that, so a generic library function cannot rely on it. These missing requirements for MBUF_FAST_FREE is a bug, either in the MBUF_FAST_FREE documentation, or in the drivers where MBUF_FAST_FREE only works correctly with direct and non-segmented mbufs.
> 

I believe multi-segment packets work ok with MBUF_FAST_FREE
(as long as other requirements are met).