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

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> 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?

P.S. Please don't reply on the top.    

Konstantin
 
> 
> 
> 
> 
> At 2020-10-14 09:00:12, "Hu, Jiayu" <mailto:jiayu.hu@intel.com> wrote:
> >
> >
> >> -----Original Message-----
> >> From: Ananyev, Konstantin <mailto:konstantin.ananyev@intel.com>
> >> Sent: Tuesday, October 13, 2020 11:39 PM
> >> To: Hu, Jiayu <mailto:jiayu.hu@intel.com>; mailto:yang_y_yi@163.com; mailto:dev@dpdk.org
> >> Cc: mailto:mark.b.kavanagh@intel.com; mailto:olivier.matz@6wind.com;
> >> mailto:thomas@monjalon.net; mailto: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 <mailto: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
> >> >
> >>
> 
>