Re: Segmentation fault when running MPRQ on testpmd

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

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Hi Dariusz,

Sure!

#0  0x0000000001c34912 in __rte_pktmbuf_free_extbuf ()
#1  0x0000000001c36a10 in rte_pktmbuf_detach ()
#2  0x0000000001c4a9ec in rxq_copy_mprq_mbuf_v ()
#3  0x0000000001c4d63b in rxq_burst_mprq_v ()
#4  0x0000000001c4d7a7 in mlx5_rx_burst_mprq_vec ()
#5  0x000000000050be66 in rte_eth_rx_burst ()
#6  0x000000000050c53d in pkt_burst_io_forward ()
#7  0x00000000005427b4 in run_pkt_fwd_on_lcore ()
#8  0x000000000054289b in start_pkt_forward_on_core ()
#9  0x0000000000a473c9 in eal_thread_loop ()
#10 0x00007ffff60061ca in start_thread () from /lib64/libpthread.so.0
#11 0x00007ffff5c72e73 in clone () from /lib64/libc.so.6

I've raised the bugs as instructed (ID 1776, 1777, 1778 and 1779) and
included the stack trace there too.


With regards,
Joni


On Wed, Aug 20, 2025 at 8:04 PM Dariusz Sosnowski <dsosnowski@nvidia.com>
wrote:

> 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
>

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