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

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

On 2026/6/25 18:37, David Hildenbrand (Arm) wrote:
> 
>>> CPU0: pmde = pmdp_get_lockless();   // sees PMD migration entry
>>>
>>> CPU1: remove_migration_ptes(src, dst /* device-private */)
>>>         ... via rmap_walk(dst) ...
>>>         page_vma_mapped_walk(&pvmw /* src, PVMW_MIGRATION */)
>>>           returns with PTL held for the PMD migration entry
>>>         remove_migration_pmd(new = dst page)
>>>           installs a device-private PMD
>>>         next page_vma_mapped_walk()
>>>           drops PTL via not_found()
>>>
>>> CPU0: takes PTL
>>>       pmde = *pvmw->pmd;            // now device-private PMD
>>>
>>> So when PVMW_MIGRATION is not set, current code can return not_found()
>>> before we even decode the locked PMD as a device-private entry.
>>>
>>> Commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support
>>> device-private entries") made the
>>>
>>> device-private PMD <-> PMD migration
>>>
>>> transition possible.
> 
> Doesn't the folio lock help here already?

Ah, yeah, I was too focused on the PTL and missed the folio lock ...
Don't have a caller like that :) Went over the fix again, nothing
else jumped out.