[External] Re: [PATCH] Revert "nvme-pci: clamp max_hw_sectors based on DMA optimized limitation"

[Christoph Hellwig]

On Mon, Oct 12, 2026 at 09:42:04AM +0100, John Garry wrote:
> And now we seem to have a conflict, as Christoph wants block size > IOVA 
> rcache limit but you find it may cause this soft lockup.
>
>> 2. Another NVME with mdts=5 (maximum data transfer size = 128KB)
>>    2.1 The issue does not reproduce regardless of the 'bs' configuration
>> 3. The condition '128 KB' is identified because the IOMMU scalable IOVA mechanism supports a maximum transfer size of 128 KB.
>> 4. IOMMU pass-through mode does not trigger the issue with the 'mdts=10' nvme disk.
>> 5. The issue only appears with high-core-count servers
>>    5.1 Reproduced servers: 448-core server and 384-core server
>>    5.2 Non-reproduced server: 128-core server
>
> How long does it take for the lockup to trigger? Pretty quickly, I guess.
>
> Does changing the number of CPUs affect throughput (before the lockup)? Are 
> you using just a single disk?

If the iova cache locks up under a perfectly normal workloads it
is broken.  We really need to fix the code there.