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.

> 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