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

[Wei Yang <richard.weiyang@gmail.com>]

On Wed, May 13, 2026 at 09:03:47AM +1000, Balbir Singh wrote:
>On 5/13/26 04:55, David Hildenbrand (Arm) wrote:
>> On 5/12/26 16:35, Wei Yang wrote:
>>> On Tue, May 12, 2026 at 02:43:54PM +0200, David Hildenbrand (Arm) wrote:
>>>> On 5/9/26 00:48, Balbir Singh wrote:
>>>>>
>>>>> Could you elaborate a more on the improper situation?
>>>>>
>>>>>
>>>>> Do we need to check softleaf_is_device_private() twice, can't we hold the pmd
>>>>> lock and check once?
>>>>
>>>> I think what we try to do here is, is to only grab the lock if we verified that there is something of interest in there.
>>>>
>>>> I wonder if we should rewrite that whole thing to just do a pmd_same() check after grabbing the lock.
>>>>
>>>> Something a lot cleaner like:
>>>>
>>>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>>>> index a4d52fdb3056..de6a255cc847 100644
>>>> --- a/mm/page_vma_mapped.c
>>>> +++ b/mm/page_vma_mapped.c
>>>> @@ -242,40 +242,28 @@ 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);
>>>> +                       if (!check_pmd(softleaf_to_pfn(entry), pvmw))
>>>> +                               return not_found(pvmw);
>>>> +               } else if (pmd_is_device_private_entry(pmde)) {
>>>> +                       softleaf_t entry = softleaf_from_pmd(pmde);
>>>>
>>>> +                       if (pvmw->flags & PVMW_MIGRATION)
>>>> +                               return not_found(pvmw);
>>>> +                       if (!check_pmd(softleaf_to_pfn(entry), pvmw))
>>>> +                               return not_found(pvmw);
>>>> +               } else {
>>>>                        if ((pvmw->flags & PVMW_SYNC) &&
>>>>                            thp_vma_suitable_order(vma, pvmw->address,
>>>>                                                   PMD_ORDER) &&
>>>> @@ -285,6 +273,15 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>>>                        step_forward(pvmw, PMD_SIZE);
>>>>                        continue;
>>>>                }
>>>> +
>>>> +               /* Double-check under PTL that the PMD didn't change. */
>>>> +               pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>>>> +               if (pmd_same(pmde, pmdp_get(pvmw->pmd)))
>>>> +                       return true;
>>>> +               spin_unlock(pvmw->ptl);
>>>> +               pvmw->ptl = NULL;
>>>> +               goto restart;
>>>> +pte_table:
>>>>                if (!map_pte(pvmw, &pmde, &ptl)) {
>>>>                        if (!pvmw->pte)
>>>>
>>>>
>>>>
>>>>
>>>> There is likely room to clean this up / compress it further.
>>>
>>> I tried to compress above logic like this, hope it could look cleaner.
>>>
>>> 	if (pmd_trans_huge(pmde) || pmd_is_valid_softleaf(pmde)) {
>>> 		unsigned long pfn;
>>> 		bool is_migration = pmd_is_migration_entry(pmde);
>>> 		bool for_migration = !!(pvmw->flags & PVMW_MIGRATION);
>>>
>>> 		if (is_migration != for_migration)
>>> 			return not_found(pvmw);
>>>
>
>I got some time to look at PVMW_MIGRATION, remove_migration_ptes
>is invoked for device private pages, would we want them to skip
>device private pmd pages?
>

Not get you clearly.

You mean skip device-private pmd page in remove_migration_ptes()?

>>> 		if (pmd_trans_huge(pmde))
>>> 			pfn = pmd_pfn(pmde);
>>> 		else
>>> 			pfn = softleaf_to_pfn(softleaf_from_pmd(pmde));
>>>
>>> 		if (!check_pmd(pfn, pvmw))
>>> 			return not_found(pvmw);
>>> 	} else if (!pmd_present(pmde)) {
>> 
>> It's more compact, but not necessarily cleaner. In particular, I detest
>> pmd_trans_huge(), we should phase it out.
>> 
>> if (pmd_present(pmde) && !pmd_leaf(pmde)) {
>> 	goto pte_table;
>> } else if (pmd_present(pmde) || pmd_is_valid_softleaf(pmde))
>> 
>> ...
>> 
>> Might work as well. But once we add support for other softleaf types, we'll have
>> to touch it again. So I'd rather just list what we actually expect.
>> 
>
>Balbir

-- 
Wei Yang
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