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

[John Garry <john.garry@linux.dev>]

On 10/12/26 13:17, Christoph Hellwig wrote:
> 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.

Sure, but it would help to first fully understand the conditions to fix it.