[ovs-dev] [PATCH v8 05/13] dp-packet: Fix data_len handling multi-seg mbufs.

Lam, Tiago tiago.lam at intel.com
Fri Jun 22 19:04:05 UTC 2018


On 18/06/2018 12:39, Eelco Chaudron wrote:
> 
> 
> On 11 Jun 2018, at 18:21, 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 | 56 
>> +++++++++++++++++++++++++++++++++++++++++++++-----------
>>  1 file changed, 45 insertions(+), 11 deletions(-)
>>
>> diff --git a/lib/dp-packet.h b/lib/dp-packet.h
>> index 4c104b6..c301ed5 100644
>> --- a/lib/dp-packet.h
>> +++ b/lib/dp-packet.h
>> @@ -429,17 +429,39 @@ 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) {
> 
> This function does not have any check to guarantee that the size set is 
> valid? i.e. is this not a trim only function? If we increase the size, 
> no additional mbufs get allocated (see below)

Indeed, it should be used as a trim function. If the size is increased
then the expectation is that enough space has been allocated. This is
the current behavior when using `DPBUF_MALLOC` dp_packets as well, and
is kept here for `DPBUF_DPDK` dp_packets.

> 
>> +        struct rte_mbuf *seg = &b->mbuf;
>> +        struct rte_mbuf *fmbuf = seg;
>> +        uint16_t pkt_len = v;
>> +        uint16_t seg_len;
>> +        uint16_t nb_segs = 0;
>> +
>> +        /* Trim 'v' length bytes from the end of the chained buffers, 
>> freeing
>> +           any buffers that may be left floating */
>> +        while (seg) {
>> +            seg_len = MIN(pkt_len, seg->data_len);
>> +            seg->data_len = seg_len;
>> +
>> +            pkt_len -= seg_len;
>> +            if (pkt_len <= 0) {
>> +                /* Free the rest of chained mbufs */
>> +                free_dpdk_buf((struct dp_packet *) seg->next);
>> +                seg->next = NULL;
>> +            } else if (!seg->next) {
>> +                seg->data_len = pkt_len;
> Here we could potentially set the size of the segment larger than the 
> allocated/available buffer.
> 
> Also if this is fine, we set it to the wrong value... Assume we have a 
> single buffer of 100 bytes and we call with v=400. This will result in 
> data_len being 300, and not 400, should probably be =pkt_len + seg_len
> 

Thanks for pointing that out. I've overlooked this, which together with
the leftover in patch 07/13 made it work correctly.

Tiago.


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