Re: [Patch mm-hotfixes v4] mm/page_vma_mapped: fix device-private PMD handling

[Lance Yang <lance.yang@linux.dev>]

On 2026/6/27 08:38, Wei Yang wrote:
[...]
>>
>> Might be good with this on top:
>>
>> ---8<---
>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>> index cfa1230c87bb..8b7c062bd81d 100644
>> --- a/mm/page_vma_mapped.c
>> +++ b/mm/page_vma_mapped.c
>> @@ -281,7 +281,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>> 					return not_found(pvmw);
>> 				return true;
>> 			}
>> -			/* THP pmd was split under us: handle on pte level */
>> +			/* THP/device-private pmd was split under us: handle on pte level */
> 
> As the comment in commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration
> support device-private entries") says:
> 
>    Add device-private THP support...
> 
> Per my understanding, we first already setup mapping and "migrate" to device
> memory. This looks a kind of place holder.
> 
> Not familiar with this. Just want to clarify, we want to treat device-private
> pmd as some sort of THP or not?

Not a regular THP, obviously. Just the PMD-sized device-private entry case.

It can be split under us too; see commit 146287290023 ("mm/huge_memory:
implement device-private THP splitting").

Nothing deeper meant here. After taking PTL, if that PMD-sized entry
is gone, just drop to the PTE walk.

> 
>> 			spin_unlock(pvmw->ptl);
>> 			pvmw->ptl = NULL;
>> 		} else if (!pmd_present(pmde)) {
>> --