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: Tuesday, October 13, 2020 11:39 PM
> To: Hu, Jiayu <jiayu.hu@intel.com>; yang_y_yi@163.com; dev@dpdk.org
> Cc: mark.b.kavanagh@intel.com; olivier.matz@6wind.com;
> thomas@monjalon.net; yangyi01@inspur.com
> Subject: RE: [PATCH] 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.
> >
> > GSO doesn't support the input pktmbuf has external buffer.
> > Indeed, requiring users to free the input pktmbuf can avoid
> > memory leak, but I'm afraid that it also changes the semantic
> > of rte_gso_segment() which is defined in rte_gso.h.
> >
> > @Konstantin, any suggestions?
> 
> Probably, a stupid question, but why can't we call mbuf_free()
> here instead fo decrementing refcnt manually:
> if (ret > 1)
> rte_pktmbuf_free(pkt);
> else if ...
> ?

You are right. Freeing mbuf inside GSO is a better way to solve
the problem.

Thanks,
Jiayu
> 
> 
> 
> >
> > Thanks,
> > Jiayu
> > >
> > > Fixes: 119583797b6a ("gso: support TCP/IPv4 GSO")
> > > Signed-off-by: Yi Yang <yangyi01@inspur.com>
> > > ---
> > >  doc/guides/prog_guide/generic_segmentation_offload_lib.rst | 7 +++++-
> -
> > >  lib/librte_gso/rte_gso.c                                   | 9 +--------
> > >  2 files changed, 6 insertions(+), 10 deletions(-)
> > >
> > > 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()``.
> 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;
> > >  }
> > > --
> > > 1.8.3.1
> >
>