Segmentation fault when running MPRQ on testpmd

[Dariusz Sosnowski]

Hi,

On Wed, Aug 20, 2025 at 04:40:16PM +0800, Joni wrote:
> 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.

Thank you for reporting and for the analysis.

Could you please open a bug on https://bugs.dpdk.org/ with all the
details?

Do you happen to have a stack trace from the segmentation fault?

Slava: Could you please take a look at the issue described by Joni in this mail?

> 
> 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

Best regards,
Dariusz Sosnowski