Re: [v3 03/11] mm/migrate_device: THP migration of zone device pages

[Balbir Singh]

On 8/15/25 10:04, Matthew Brost wrote:
> On Fri, Aug 15, 2025 at 08:51:21AM +1000, Balbir Singh wrote:
>> On 8/13/25 10:07, Mika Penttilä wrote:
>>>
>>> On 8/13/25 02:36, Balbir Singh wrote:
>>>
>>>> On 8/12/25 15:35, Mika Penttilä wrote:
>>>>> Hi,
>>>>>
>>>>> On 8/12/25 05:40, Balbir Singh wrote:
...

>> I've not run into this with my testing, let me try with more mTHP sizes enabled. I'll wait on Matthew
>> to post his test case or any results, issues seen
>>
> 
> I’ve hit this. In the code I shared privately, I split THPs in the
> page-collection path. You omitted that in v2 and v3; I believe you’ll
> need those changes. The code I'm referring to had the below comment.
> 
>  416         /*
>  417          * XXX: No clean way to support higher-order folios that don't match PMD
>  418          * boundaries for now — split them instead. Once mTHP support lands, add
>  419          * proper support for this case.
>  420          *
>  421          * The test, which exposed this as problematic, remapped (memremap) a
>  422          * large folio to an unaligned address, resulting in the folio being
>  423          * found in the middle of the PTEs. The requested number of pages was
>  424          * less than the folio size. Likely to be handled gracefully by upper
>  425          * layers eventually, but not yet.
>  426          */
> 
> I triggered it by doing some odd mremap operations, which caused the CPU
> page-fault handler to spin indefinitely iirc. In that case, a large device
> folio had been moved into the middle of a PMD.
> 
> Upstream could see the same problem if the device fault handler enforces
> a must-migrate-to-device policy and mremap moves a large CPU folio into
> the middle of a PMD.
> 
> I’m in the middle of other work; when I circle back, I’ll try to create
> a selftest to reproduce this. My current test is a fairly convoluted IGT
> with a bunch of threads doing remap nonsense, but I’ll try to distill it
> into a concise selftest.
> 

I ran into this while doing some testing as well, I fixed it in a manner similar
to split_folio() for partial unmaps. I will consolidate the folio splits into
a single helper and post it with v4.


Balbir Singh