Re: [Patch v2] mm/page_vma_mapped: revalidate and do proper check before return device-private pmd

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/16/26 14:30, Lance Yang wrote:
> 
> On Tue, Jun 16, 2026 at 06:34:36AM +0000, Wei Yang wrote:
> [...]
>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>> index 2ccbabfb2cc1..21635fab209c 100644
>> --- a/mm/page_vma_mapped.c
>> +++ b/mm/page_vma_mapped.c
>> @@ -243,40 +243,28 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>> 		 */
>> 		pmde = pmdp_get_lockless(pvmw->pmd);
>>
>> -		if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde)) {
>> -			pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>> -			pmde = *pvmw->pmd;
>> -			if (!pmd_present(pmde)) {
>> -				softleaf_t entry;
>> -
>> -				if (!thp_migration_supported() ||
>> -				    !(pvmw->flags & PVMW_MIGRATION))
>> -					return not_found(pvmw);
>> -				entry = softleaf_from_pmd(pmde);
>> -
>> -				if (!softleaf_is_migration(entry) ||
>> -				    !check_pmd(softleaf_to_pfn(entry), pvmw))
>> -					return not_found(pvmw);
>> -				return true;
>> -			}
>> -			if (likely(pmd_trans_huge(pmde))) {
>> -				if (pvmw->flags & PVMW_MIGRATION)
>> -					return not_found(pvmw);
>> -				if (!check_pmd(pmd_pfn(pmde), pvmw))
>> -					return not_found(pvmw);
>> -				return true;
>> -			}
>> -			/* THP pmd was split under us: handle on pte level */
>> -			spin_unlock(pvmw->ptl);
>> -			pvmw->ptl = NULL;
>> -		} else if (!pmd_present(pmde)) {
>> -			const softleaf_t entry = softleaf_from_pmd(pmde);
>> -
>> -			if (softleaf_is_device_private(entry)) {
>> -				pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>> -				return true;
>> -			}
>> +		if (pmd_present(pmde)) {
>> +			if (!pmd_leaf(pmde))
>> +				goto pte_table;
>> +			if (pvmw->flags & PVMW_MIGRATION)
>> +				return not_found(pvmw);
>> +			if (!check_pmd(pmd_pfn(pmde), pvmw))
>> +				return not_found(pvmw);
>> +		} else if (pmd_is_migration_entry(pmde)) {
>> +			softleaf_t entry = softleaf_from_pmd(pmde);
>> +
>> +			if (!(pvmw->flags & PVMW_MIGRATION))
>> +				return not_found(pvmw);
> 
> Looked at history a bit, and I wonder if this changed something old
> here ...
> 
> Since 616b8371539a ("mm: thp: enable thp migration in generic path"), PMD
> migration handling took PTL before doing PVMW_MIGRATION/PFN checks,
> including not_found() cases. So lockless PMD read was just a filter ...
> 
> With this fix, true case gets final pmd_same() check, but this
> not_found() case happens before taking PTL.
> 
> So a !PVMW_MIGRATION walker could race with someone, e.g.
> remove_migration_pmd(): we make the not_found() decision from old PMD
> value that still says "migration", while real *pvmw->pmd may already be
> present again. We return without ever taking PTL :)
> 
> Not sure about practical fallout, but should these PMD-level not_found()
> cases also take PTL and restart if PMD changed?
I was hoping that we could so something similar to the PTE case.

In map_pte(), we check whether the PMD changed, which is slightly different.

The actual check happens in check_pte() after grabbing the PTL.

For the case you describe, map_pte() would find !pte_none(ptent) ...
!pte_present(ptent) and !is_migration, and effectively grab the lock and proceed
to check_pte().

In check_pte() we re-check under lock indeed.

-- 
Cheers,

David