Re: [dpdk-dev] [PATCH v2] 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: Thursday, October 22, 2020 11:34 PM
> To: yang_y_yi@163.com; dev@dpdk.org
> Cc: Hu, Jiayu <jiayu.hu@intel.com>; techboard@dpdk.org;
> thomas@monjalon.net; yangyi01@inspur.com
> Subject: RE: [PATCH v2] gso: fix free issue of mbuf gso segments attach to
> 
> 
> > >
> > > rte_gso_segment decreased refcnt of pkt by one, but
> > > it is wrong if pkt is external mbuf, pkt won't be
> > > freed because of incorrect refcnt, the result is
> > > application can't allocate mbuf from mempool because
> > > mbufs in mempool are run out of.
> > >
> > > One correct way is application should call
> > > rte_pktmbuf_free after calling rte_gso_segment to free
> > > pkt explicitly. rte_gso_segment mustn't handle it, this
> > > should be responsibility of application.
> >
> > Probably needs to be stated clearly:
> > It is a change in functional behaviour.
> > Without deprecation note in advance.
> > TB members: please provide your opinion on that patch.
> >
> > >
> > > Fixes: 119583797b6a ("gso: support TCP/IPv4 GSO")
> > > Signed-off-by: Yi Yang <yangyi01@inspur.com>
> > > ---
> > > Changelog:
> > >
> > > v1->v2:
> > >   - update description of rte_gso_segment().
> > >   - change code which calls rte_gso_segment() to
> > >     fix free issue.
> > >
> > > ---
> > >  app/test-pmd/csumonly.c                                    | 3 ++-
> > >  doc/guides/prog_guide/generic_segmentation_offload_lib.rst | 7 +++++-
> -
> >
> > I think release notes also have to be updated.
> >
> > >  lib/librte_gso/rte_gso.c                                   | 9 +--------
> > >  lib/librte_gso/rte_gso.h                                   | 7 +++++--
> > >  4 files changed, 13 insertions(+), 13 deletions(-)
> > >
> > > diff --git a/app/test-pmd/csumonly.c b/app/test-pmd/csumonly.c
> > > index 3d7d244..829e07f 100644
> > > --- a/app/test-pmd/csumonly.c
> > > +++ b/app/test-pmd/csumonly.c
> > > @@ -1080,11 +1080,12 @@ struct simple_gre_hdr {
> > >  ret = rte_gso_segment(pkts_burst[i], gso_ctx,
> > >  &gso_segments[nb_segments],
> > >  GSO_MAX_PKT_BURST - nb_segments);
> > > +/* pkts_burst[i] can be freed safely here. */
> > > +rte_pktmbuf_free(pkts_burst[i]);
> >
> > It doesn't look correct to me.
> > I think it should be:
> > If (ret > 1) rte_pktmbuf_free(pkts_burst[i]);
> >
> > >  if (ret >= 0)
> > >  nb_segments += ret;
> > >  else {
> > >  TESTPMD_LOG(DEBUG, "Unable to segment packet");
> > > -rte_pktmbuf_free(pkts_burst[i]);
> > >  }
> > >  }
> >
> >
> > About drivers/net/tap/rte_eth_tap.c:
> > I think it has to be modified too, as here:
> >
> > /* free original mbuf */
> >                 rte_pktmbuf_free(mbuf_in);
> >                 /* free tso mbufs */
> >                 if (num_tso_mbufs > 0)
> >                         rte_pktmbuf_free_bulk(mbuf, num_tso_mbufs);
> >
> > if mbuf[0] == mbuf_in
> > Will have a double free() for the same mbuf.
> >
> > >
> > > diff --git a/doc/guides/prog_guide/generic_segmentation_offload_lib.rst
> > b/doc/guides/prog_guide/generic_segmentation_offload_lib.rst
> > > index 205cb8a..8577572 100644
> > > --- a/doc/guides/prog_guide/generic_segmentation_offload_lib.rst
> > > +++ b/doc/guides/prog_guide/generic_segmentation_offload_lib.rst
> > > @@ -25,8 +25,9 @@ Bearing that in mind, the GSO library enables DPDK
> applications to segment
> > >  packets in software. Note however, that GSO is implemented as a
> standalone
> > >  library, and not via a 'fallback' mechanism (i.e. for when TSO is
> unsupported
> > >  in the underlying hardware); that is, applications must explicitly invoke
> the
> > > -GSO library to segment packets. The size of GSO segments ``(segsz)`` is
> > > -configurable by the application.
> > > +GSO library to segment packets, they also must call
> ``rte_pktmbuf_free()`` to
> > > +free mbuf GSO segments attach to after calling ``rte_gso_segment()``.
> >
> > Probably worth to mention that if return code == 1, then
> > output mbuf will point to input mbuf and extra care with free() is required.
> 
> Another possibility - change gso_segment() behaviour even further,
> and don't put input_pkt into pkt_out[] when no segmentation happened.
> Might be even a bit cleaner and less error prone.

Agree. Pkts_out[] better not to contain input_pkt when GSO doesn't happen.

BTW, @Yi, you also need to update the comment of describing what rte_gso_segment()
returns in rte_gso.h.

Thanks,
Jiayu
> 
> >
> > > The size
> > > +of GSO segments ``(segsz)`` is configurable by the application.
> > >
> > >  Limitations
> > >  -----------
> > > @@ -233,6 +234,8 @@ To segment an outgoing packet, an application
> must:
> > >
> > >  #. Invoke the GSO segmentation API, ``rte_gso_segment()``.
> > >
> > > +#. Call ``rte_pktmbuf_free()`` to free mbuf ``rte_gso_segment()``
> segments.
> > > +
> > >  #. If required, update the L3 and L4 checksums of the newly-created
> segments.
> > >     For tunneled packets, the outer IPv4 headers' checksums should also
> be
> > >     updated. Alternatively, the application may offload checksum
> calculation
> > > diff --git a/lib/librte_gso/rte_gso.c b/lib/librte_gso/rte_gso.c
> > > index 751b5b6..0d6cae5 100644
> > > --- a/lib/librte_gso/rte_gso.c
> > > +++ b/lib/librte_gso/rte_gso.c
> > > @@ -30,7 +30,6 @@
> > >  uint16_t nb_pkts_out)
> > >  {
> > >  struct rte_mempool *direct_pool, *indirect_pool;
> > > -struct rte_mbuf *pkt_seg;
> > >  uint64_t ol_flags;
> > >  uint16_t gso_size;
> > >  uint8_t ipid_delta;
> > > @@ -80,13 +79,7 @@
> > >  return 1;
> > >  }
> > >
> > > -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 < 0) {
> > >  /* Revert the ol_flags in the event of failure. */
> > >  pkt->ol_flags = ol_flags;
> > >  }
> > > diff --git a/lib/librte_gso/rte_gso.h b/lib/librte_gso/rte_gso.h
> > > index 3aab297..f6694f9 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,
> > > --
> > > 1.8.3.1
>