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

[yang_y_yi <yang_y_yi@163.com>]

At 2020-10-19 11:17:48, "Hu, Jiayu" <jiayu.hu@intel.com> wrote:
>
>
>> -----Original Message-----
>> From: Ananyev, Konstantin <konstantin.ananyev@intel.com>
>> Sent: Friday, October 16, 2020 4:31 PM
>> 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
>> 
>> 
>> 
>> >
>> > > > > >
>> > > > > > 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;
>> >         }
>> 
>> Yes, definitely. I am not arguing about that.
>> My question was to the author of the original patch:
>> He suggests not to free input packet inside gso function and leave it to the
>> user.
>> So, in his proposition, would it also become user responsibility to determine
>> when input packet can be freed (it is not present in pkt_out[]) or not?
>
>@Yi, I am OK with the both designs. If you think it's better to free the input pkt by
>users, you can keep the original design. But one thing to notice is that you need
>to update definition of rte_gso_segment() in rte_gso.h too.
>
>Thanks,
>Jiayu

Ok, I prefer to handle it by users, this is incremental patch for rte_gso_segment description. Is it ok to you?

diff --git a/lib/librte_gso/rte_gso.h b/lib/librte_gso/rte_gso.h
index 3aab297..3762eba 100644
--- a/lib/librte_gso/rte_gso.h
+++ b/lib/librte_gso/rte_gso.h
@@ -89,8 +89,11 @@ struct rte_gso_ctx {
  * the GSO segments are sent to should support transmission of multi-segment
  * packets.
  *
- * If the input packet is GSO'd, its mbuf refcnt reduces by 1. Therefore,
- * when all GSO segments are freed, the input packet is freed automatically.
+ * If the input packet is GSO'd, all the indirect segments are attached to the
+ * input packet.
+ *
+ * rte_gso_segment() will not free the input packet no matter whether it is
+ * GSO'd or not, the application should free it after call rte_gso_segment().
  *
  * If the memory space in pkts_out or MBUF pools is insufficient, this
  * function fails, and it returns (-1) * errno. Otherwise, GSO succeeds,

>> 
>> >
>> > Thanks,
>> > Jiayu
>> > >
>> > >
>> > >
>> >
>>