Re: [dpdk-dev] [PATCH] gso: fix free issue of mbuf gso segments attach to

["Hu, Jiayu" <jiayu.hu@intel.com>]

> -----Original Message-----
> From: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> Sent: Friday, October 16, 2020 12:16 AM
> To: Hu, Jiayu <jiayu.hu@intel.com>; yang_y_yi <yang_y_yi@163.com>
> Cc: dev@dpdk.org; olivier.matz@6wind.com; thomas@monjalon.net;
> yangyi01@inspur.com
> Subject: RE: Re:RE: [PATCH] gso: fix free issue of mbuf gso segments attach to
> 
> 
> 
> >
> > > -----Original Message-----
> > > From: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> > > Sent: Wednesday, October 14, 2020 8:06 PM
> > > To: yang_y_yi <yang_y_yi@163.com>; Hu, Jiayu <jiayu.hu@intel.com>
> > > Cc: dev@dpdk.org; olivier.matz@6wind.com; thomas@monjalon.net;
> > > yangyi01@inspur.com
> > > Subject: RE: Re:RE: [PATCH] gso: fix free issue of mbuf gso segments
> attach to
> > >
> > >
> > > > From: yang_y_yi <yang_y_yi@163.com>
> > > > Sent: Wednesday, October 14, 2020 3:56 AM
> > > > To: Hu, Jiayu <jiayu.hu@intel.com>
> > > > Cc: Ananyev, Konstantin <konstantin.ananyev@intel.com>;
> dev@dpdk.org;
> > > olivier.matz@6wind.com; thomas@monjalon.net;
> > > > yangyi01@inspur.com
> > > > Subject: Re:RE: [PATCH] gso: fix free issue of mbuf gso segments attach
> to
> > > >
> > > > I think it isn't a good idea to free it in rte_gso_segment, maybe
> application
> > > will continue to use this pkt for other purpose, rte_gso_segment
> > > > can't make decision for application without any notice, it is better to
> return
> > > this decision right backt to application.
> > > >
> > >
> > > I think, if user wants to keep original packet, he can always call
> > > rte_pktmbuf_refcnt_update(pkt, 1)
> > > just before calling gso function.
> > >
> > > Also, as I remember in some cases it is not safe to do free() for input
> packet
> > > (as pkt_out[] can contain input pkt itself). Would it also be user
> responsibility
> > > to determine
> > > such situations?
> >
> > In what case will pkt_out[] contain the input pkt? Can you give an example?
> 
> As I can read the code, whenever gso code decides that
> no segmentation is not really needed, or it is not capable
> of doing it properly.
> Let say:
> 
> gso_tcp4_segment(struct rte_mbuf *pkt,
>                 uint16_t gso_size,
>                 uint8_t ipid_delta,
>                 struct rte_mempool *direct_pool,
>                 struct rte_mempool *indirect_pool,
>                 struct rte_mbuf **pkts_out,
>                 uint16_t nb_pkts_out)
> {
> ...
> /* Don't process the fragmented packet */
>         ipv4_hdr = (struct rte_ipv4_hdr *)(rte_pktmbuf_mtod(pkt, char *) +
>                         pkt->l2_len);
>         frag_off = rte_be_to_cpu_16(ipv4_hdr->fragment_offset);
>         if (unlikely(IS_FRAGMENTED(frag_off))) {
>                 pkts_out[0] = pkt;
>                 return 1;
>         }
> 
>         /* Don't process the packet without data */
>         hdr_offset = pkt->l2_len + pkt->l3_len + pkt->l4_len;
>         if (unlikely(hdr_offset >= pkt->pkt_len)) {
>                 pkts_out[0] = pkt;
>                 return 1;
>         }
> 
> That's why in rte_gso_segment() we update refcnt only when ret > 1.

But in these cases, the value of ret is 1. So we can free input pkt only when
ret > 1. Like:

-       if (ret > 1) {
-               pkt_seg = pkt;
-               while (pkt_seg) {
-                       rte_mbuf_refcnt_update(pkt_seg, -1);
-                       pkt_seg = pkt_seg->next;
-               }
-       } else if (ret < 0) {
+       if (ret > 1)
+               rte_pktmbuf_free(pkt);
+       else if (ret < 0) {
                /* Revert the ol_flags in the event of failure. */
                pkt->ol_flags = ol_flags;
        }

Thanks,
Jiayu
> 
> 
>