From: Huichao Cai [mailto:chcch...@163.com] Sent: Friday, 22 July 2022 17.59
> At 2022-07-22 23:52:28, "Morten Brørup" <m...@smartsharesystems.com> wrote: > >> From: Stephen Hemminger [mailto:step...@networkplumber.org] > >> Sent: Friday, 22 July 2022 16.49 > >> > >> On Fri, 22 Jul 2022 21:01:50 +0800 > >> Huichao Cai <chcch...@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 <chcch...@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.
