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 15:39, John Garry wrote:
> 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.

BTW, @Adrian, it would be good to run your test with the IOMMU in strict 
mode as well, just to give an idea if there is a problem there as well.