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

[David Hildenbrand (Arm)]

>> 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?

-- 
Cheers,

David