NVLink/NVLS Failure with Xid 31 MMU Fault Cascade
NVLS collective failures on Hopper/NVLINK4 nodes cascade from Fabric Manager restarts, producing Xid 31 MMU page faults and illegal memory accesses across the node.
NVLS collective failures on Hopper/NVLINK4 nodes cascade from Fabric Manager restarts, producing Xid 31 MMU page faults and illegal memory accesses across the node.
What this failure is
NVLink/NVLS Failure with Xid 31 MMU Fault Cascade is a Hardware failure seen during ML training runs. NVLS collective failures on Hopper/NVLINK4 nodes cascade from Fabric Manager restarts, producing Xid 31 MMU page faults and illegal memory accesses across the node. Common tags: Nvlink, Nvls, Xid 31, Mmu Fault.
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Why it happens (the mechanism)
Forceful Fabric Manager restarts leave NVLink/NVSwitch state inconsistent. MMU page-table (Xid 31) faults follow from the coherency loss. NVLS multicast mappings become invalid mid-operation. Taken together, these mechanisms explain why the failure is reproducible, why it tends to surface on specific workloads or scales, and why generic mitigation attempts often fall short without addressing the underlying cause.
What you'll observe
- NCCL/NVLS operations fail across the whole node
- Multiple frameworks (NCCL tests, HPL, TF) crash with related errors
- Failures appear after a Fabric Manager restart
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| transport/nvls.cc:244 NCCL WARN Cuda failure 1 'invalid argument' | Forceful Fabric Manager restarts leave NVLink/NVSwitch state inconsistent |
| Xid (PCI:...): 31 ... MMU Fault: ENGINE GRAPHICS ... FAULT_PDE ACCESS_TYPE_VIRT_WRITE | MMU page-table (Xid 31) faults follow from the coherency loss |
| an illegal memory access was encountered = 700 | NVLS multicast mappings become invalid mid-operation |
Which systems are affected
- 8x H100/H200 SXM nodes with NVLINK4 + NVSwitch
- Jobs using NVLink SHARP (NVLS)
- Nodes where Fabric Manager was restarted under load
How to confirm this is the problem
Use this checklist to test the hypothesis against a small reproduction. No single line proves the root cause, so preserve the preceding events and compare one variable at a time.
- ✓Reproduce the failure from a clean checkpoint/seed: the symptom must appear without warm-up state from a previous run.
- ✓Verified signal present: transport/nvls.cc:244 NCCL WARN Cuda failure 1 'invalid argument'
- ✓Verified signal present: Xid (PCI:...): 31 ... MMU Fault: ENGINE GRAPHICS ... FAULT_PDE ACCESS_TYPE_VIRT_WRITE
- ✓Verified signal present: an illegal memory access was encountered = 700
- ✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.
The fix and the prevention pattern
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Xid 31 in context
Xid 31 is one of a small set of codes the NVIDIA driver uses to report GPU faults, and the number is most of the diagnosis: it tells you whether you are looking at your own code, the driver, or a board that needs replacing.
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Root cause
- Forceful Fabric Manager restarts leave NVLink/NVSwitch state inconsistent
- MMU page-table (Xid 31) faults follow from the coherency loss
- NVLS multicast mappings become invalid mid-operation
The fix and how to prevent it
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