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

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

On Tue, May 12, 2026 at 08:55:47PM +0200, 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);
>> 
>> 		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.
>

Hi, David

I may not follow you. Just want to confirm whether you prefer this goes as a
fix first, or you prefer it goes as what you suggested here as a cleanup?

>-- 
>Cheers,
>
>David

-- 
Wei Yang
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