Segmentation fault when running MPRQ on testpmd

[Joni <canary.overflow@gmail.com>]

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Hi,

I hope this is the correct place to report these issues since it seems to
be related to DPDK codes. I've reported this to Nvidia a few days ago but
have yet to receive any response from them.

My server is currently using ConnectX5 MT27800 (mlx5_core 5.7-1.0.2) on
firmware 16.35.4506 (MT_0000000011). My DPDK library version is 22.11.

I ran the following testpmd command which resulted in segmentation fault (I
am currently running on filtered traffic with packets >1000 bytes to
increase the odds of hitting the segmentation fault):

dpdk-testpmd -l 1-5 -n 4 -a
0000:1f:00.0,rxq_comp_en=1,rxq_pkt_pad_en=1,rxqs_min_mprq=1,mprq_en=1,mprq_log_stride_num=6,mprq_log_stride_size=9,mprq_max_memcpy_len=64,rx_vec_en=1
-- -i --rxd=8192 --max-pkt-len=1700 --rxq=1 --total-num-mbufs=16384
--mbuf-size=3000 --enable_drop_en –enable_scatter

This segmentation fault goes away when I disable vectorization
(rx_vec_en=0). (Note that the segmentation fault does not occur in
forward-mode=rxonly). The segmentation fault also seems to happen with
higher chances when there is a rxnombuf.

Upon some investigation, I noticed that in DPDK’s source codes
drivers/net/mlx5/mlx5_rxtx_vec.c
(function rxq_copy_mprq_mbuf_v()), there is a possibility where the
consumed stride exceeds the stride number (64 in this case) which should
not be happening. I'm suspecting there's some CQE misalignment here upon
encountering rxnombuf.

rxq_copy_mprq_mbuf_v(...) {
    ...
    if(rxq->consumed_strd == strd_n) {
        // replenish WQE
    }
    ...
    strd_cnt = (elts[i]->pkt_len / strd_sz) +
               ((elts[i]->pkt_len % strd_sz) ? 1 : 0);

    rxq_code = mprq_buf_to_pkt(rxq, elts[i], elts[i]->pkt_len, buf,
rxq->consumed_strd, strd_cnt);
    rxq->consumed_strd += strd_cnt;       // encountering cases where
rxq->consumed_strd > strd_n
    ...
}

In addition, there were also cases in mprq_buf_to_pkt() where the allocated
seg address is exactly the same as the pkt (elts[i]) address passed in
which should not happen.

mprq_buf_to_pkt(...) {
    ...
    if(hdrm_overlap > 0) {
        MLX5_ASSERT(rxq->strd_scatter_en);

        struct rte_mbuf *seg = rte_pktmbuf_alloc(rxq->mp);
        if (unlikely(seg == NULL)) return MLX5_RXQ_CODE_NOMBUF;
        SET_DATA_OFF(seg, 0);

        // added debug statement
        DRV_LOG(DEBUG, "pkt %p seg %p", (void *)pkt, (void *)seg);

rte_memcpy(rte_pktmbuf_mtod(seg, void *), RTE_PTR_ADD(addr, len -
hdrm_overlap), hdrm_overlap); ... } }

I have tried upgrading my DPDK version to 24.11 but the segmentation fault
still persists.

In addition, there were also a few other issues that I've noticed:

   - max-pkt-len does not seem to work for values < 1500 even though "show
   port info X" showed that the MTU was set to the value I've passed in
   - In mprq_buf_to_pkt():
       - uint32_t seg_len = RTE_MIN(len, (uint32_t)(pkt->buf_len -
   RTE_PKTMBUF_HEADROOM)) --> seems unnecessary as to hit this code, len has
   to be greater than (uint32_t)(pkt->buf_len - RTE_PKTMBUF_HEADROOM) due to
   the if condition
       - If the allocation struct rte_mbuf *next =
   rte_pktmbuf_alloc(rxq->mp) fails and packet has more than 2 segs, the segs
   that were allocated previously do not get freed

    mprq_buf_to_pkt(...) {
        ...                } else if (rxq->strd_scatter_en) {

struct rte_mbuf *prev = pkt;

uint32_t seg_len = RTE_MIN(len, (uint32_t)

(pkt->buf_len - RTE_PKTMBUF_HEADROOM));

uint32_t rem_len = len - seg_len;


      rte_memcpy(rte_pktmbuf_mtod(pkt, void *), addr, seg_len);
      DATA_LEN(pkt) = seg_len;
      while (rem_len) {
         struct rte_mbuf *next = rte_pktmbuf_alloc(rxq->mp);


            if (unlikely(next == NULL))
                return MLX5_RXQ_CODE_NOMBUF;
            ...
    - In the external buffer attach case where hdrm_overlap > 0, the code
did not decrement the buffer refcnt if allocation struct rte_mbuf *next =
rte_pktmbuf_alloc(rxq->mp) fails

mprq_buf_to_pkt(...) {
    ...            if (hdrm_overlap > 0) {

        __atomic_add_fetch(&buf->refcnt, 1, __ATOMIC_RELAXED);
        ...
        MLX5_ASSERT(rxq->strd_scatter_en);
        struct rte_mbuf *seg = rte_pktmbuf_alloc(rxq->mp);
        if (unlikely(seg == NULL))
            return MLX5_RXQ_CODE_NOMBUF;
        SET_DATA_OFF(seg, 0);
        ...


Hope to hear from you soon!

With regards,
Joni

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