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

["Morten Brørup" <mb@smartsharesystems.com>]

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.