[ovs-dev] [PATCH v2 05/14] dp-packet: Fix data_len handling multi-seg mbufs.

[Lam, Tiago]

On 05/07/2018 09:59, Eelco Chaudron wrote:
> 
> 
> On 4 Jul 2018, at 20:06, Tiago Lam wrote:
> 
>> When a dp_packet is from a DPDK source, and it contains multi-segment
>> mbufs, the data_len is not equal to the packet size, pkt_len. Instead,
>> the data_len of each mbuf in the chain should be considered while
>> distributing the new (provided) size.
>>
>> To account for the above dp_packet_set_size() has been changed so 
>> that,
>> in the multi-segment mbufs case, only the data_len on the last mbuf of
>> the chain and the total size of the packet, pkt_len, are changed. The
>> data_len on the intermediate mbufs preceeding the last mbuf is not
>> changed by dp_packet_set_size(). Furthermore, in some cases
>> dp_packet_set_size() may be used to set a smaller size than the 
>> current
>> packet size, thus effectively trimming the end of the packet. In the
>> multi-segment mbufs case this may lead to lingering mbufs that may 
>> need
>> freeing.
>>
>> __dp_packet_set_data() now also updates an mbufs' data_len after 
>> setting
>> the data offset. This is so that both fields are always in sync for 
>> each
>> mbuf in a chain.
>>
>> Co-authored-by: Michael Qiu <qiudayu at chinac.com>
>> Co-authored-by: Mark Kavanagh <mark.b.kavanagh at intel.com>
>> Co-authored-by: Przemyslaw Lal <przemyslawx.lal at intel.com>
>> Co-authored-by: Marcin Ksiadz <mksiadz at gmail.com>
>> Co-authored-by: Yuanhan Liu <yliu at fridaylinux.org>
>>
>> Signed-off-by: Michael Qiu <qiudayu at chinac.com>
>> Signed-off-by: Mark Kavanagh <mark.b.kavanagh at intel.com>
>> Signed-off-by: Przemyslaw Lal <przemyslawx.lal at intel.com>
>> Signed-off-by: Marcin Ksiadz <mksiadz at gmail.com>
>> Signed-off-by: Yuanhan Liu <yliu at fridaylinux.org>
>> Signed-off-by: Tiago Lam <tiago.lam at intel.com>
>> ---
>>  lib/dp-packet.h | 76 
>> ++++++++++++++++++++++++++++++++++++++++++++++++---------
>>  1 file changed, 64 insertions(+), 12 deletions(-)
>>
>> diff --git a/lib/dp-packet.h b/lib/dp-packet.h
>> index c612ad4..fcbaf60 100644
>> --- a/lib/dp-packet.h
>> +++ b/lib/dp-packet.h
>> @@ -429,17 +429,49 @@ dp_packet_size(const struct dp_packet *b)
>>  static inline void
>>  dp_packet_set_size(struct dp_packet *b, uint32_t v)
>>  {
>> -    /* netdev-dpdk does not currently support segmentation; 
>> consequently, for
>> -     * all intents and purposes, 'data_len' (16 bit) and 'pkt_len' 
>> (32 bit) may
>> -     * be used interchangably.
>> -     *
>> -     * On the datapath, it is expected that the size of packets
>> -     * (and thus 'v') will always be <= UINT16_MAX; this means that 
>> there is no
>> -     * loss of accuracy in assigning 'v' to 'data_len'.
>> -     */
>> -    b->mbuf.data_len = (uint16_t)v;  /* Current seg length. */
>> -    b->mbuf.pkt_len = v;             /* Total length of all segments 
>> linked to
>> -                                      * this segment. */
>> +    if (b->source == DPBUF_DPDK) {
>> +        struct rte_mbuf *mbuf = &b->mbuf;
>> +        uint16_t new_len = v;
>> +        uint16_t data_len;
>> +        uint16_t nb_segs = 0;
>> +        uint16_t pkt_len = 0;
>> +
>> +        /* Trim 'v' length bytes from the end of the chained buffers, 
>> freeing
>> +           any buffers that may be left floating */
>> +        while (mbuf) {
>> +            data_len = MIN(new_len, mbuf->data_len);
>> +            mbuf->data_len = data_len;
>> +
>> +            if (new_len - data_len <= 0) {
>> +                /* Free the rest of chained mbufs */
>> +                free_dpdk_buf(CONTAINER_OF(mbuf->next, struct 
>> dp_packet,
>> +                                           mbuf));
>> +                mbuf->next = NULL;
>> +            } else if (!mbuf->next) {
>> +                /* Don't assign more than what we have available */
>> +                mbuf->data_len = MIN(new_len,
>> +                                     mbuf->buf_len - mbuf->data_off);
>> +            }
>> +
>> +            new_len -= data_len;
>> +            nb_segs += 1;
>> +            pkt_len += mbuf->data_len;
>> +            mbuf = mbuf->next;
>> +        }
>> +
>> +        /* pkt_len != v would effectively mean that pkt_len < than 
>> 'v' (as
>> +         * being bigger is logically impossible). Being < than 'v' 
>> would mean
>> +         * the 'v' provided was bigger than the available room, which 
>> is the
>> +         * responsibility of the caller to make sure there is enough 
>> room */
>> +        ovs_assert(pkt_len == v);
>> +
>> +        b->mbuf.nb_segs = nb_segs;
>> +        b->mbuf.pkt_len = pkt_len;
>> +    } else {
>> +        b->mbuf.data_len = v;
>> +        /* Total length of all segments linked to this segment. */
>> +        b->mbuf.pkt_len = v;
>> +    }
>>  }
>>
>>  static inline uint16_t
>> @@ -451,7 +483,27 @@ __packet_data(const struct dp_packet *b)
>>  static inline void
>>  __packet_set_data(struct dp_packet *b, uint16_t v)
>>  {
>> -    b->mbuf.data_off = v;
>> +    if (b->source == DPBUF_DPDK) {
>> +        /* Moving data_off away from the first mbuf in the chain is 
>> not a
>> +         * possibility using DPBUF_DPDK dp_packets */
>> +        ovs_assert(v == UINT16_MAX || v <= b->mbuf.buf_len - 
>> b->mbuf.data_off);
> 
> Are you ok this check is ok? Should it not just be “.. || v<= 
> b->mbuf.buf_len”?
> Assuming you have a 1K buffer, data_off is currently 520, now you want 
> to set the offset to 530?
> 
> 530 <= 1024 - 520 —> ASSERT!
> 

You're right, this won't assert correctly. I somehow forgot that 'v' is
already passed as an offset to the base address, and not a value to be
incremented to `data_off`, hence why I was checking on the available
tailroom.

I'll prepare v3 and run some more tests with it before sending here.

Thanks,
Tiago.