Re: [dpdk-dev] [PATCH v2] gso: fix free issue of mbuf gso segments attach to

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

> >
> > 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. 

> 
> > 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