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

[yang_y_yi <yang_y_yi@163.com>]

At 2020-10-22 23:33:42, "Ananyev, Konstantin" <konstantin.ananyev@intel.com> wrote:
>
>> >
>> > 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, I didn't notice it before.
 
>
>> 
>> > 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