Re: [PATCH] mm/page_vma_mapped_walk: add missing pgtable entry accessors

[David Hildenbrand (Arm)]

On 4/29/26 15:47, Alexander Gordeev wrote:
> On Mon, Apr 27, 2026 at 11:02:50AM +0200, David Hildenbrand (Arm) wrote:
>> On 4/27/26 07:20, Alexander Gordeev wrote:
>>> Convert pgtable direct entry dereferences to the corresponding
>>> pXdp_get() accessors. Use ptep_get_lockless() variant for PTE
>>> reads when no lock is taken.
>>>
>>> Signed-off-by: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
>>> ---
>>>  mm/page_vma_mapped.c | 12 ++++++------
>>>  1 file changed, 6 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>>> index b38a1d00c971..a4520bb10d2a 100644
>>> --- a/mm/page_vma_mapped.c
>>> +++ b/mm/page_vma_mapped.c
>>> @@ -41,7 +41,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
>>>  	if (!pvmw->pte)
>>>  		return false;
>>>  
>>> -	ptent = ptep_get(pvmw->pte);
>>> +	ptent = ptep_get_lockless(pvmw->pte);
> 
> (*)
> 
>>>  	if (pte_none(ptent)) {
>>>  		return false;
>>> @@ -219,17 +219,17 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>>  restart:
>>>  	do {
>>>  		pgd = pgd_offset(mm, pvmw->address);
>>> -		if (!pgd_present(*pgd)) {
>>> +		if (!pgd_present(pgdp_get(pgd))) {
>>>  			step_forward(pvmw, PGDIR_SIZE);
>>>  			continue;
>>>  		}
>>>  		p4d = p4d_offset(pgd, pvmw->address);
>>> -		if (!p4d_present(*p4d)) {
>>> +		if (!p4d_present(p4dp_get(p4d))) {
>>>  			step_forward(pvmw, P4D_SIZE);
>>>  			continue;
>>>  		}
>>>  		pud = pud_offset(p4d, pvmw->address);
>>> -		if (!pud_present(*pud)) {
>>> +		if (!pud_present(pudp_get(pud))) {
>>>  			step_forward(pvmw, PUD_SIZE);
>>>  			continue;
>>
>> Wasn't there a problem with folded page tables, where we would no longer be able
>> to optimize out the folded page table accesses?
> 
> Ok, that is a different topic. I actually tried to resolve the lockless
> issue(s) in (*) while the direct dereferences just seemed to be relevant
> to go along (similarly to the GUP patch).
> 
> But I would rather resend ptep_get() => ptep_get_lockless() fixes for
> this patch.
> 
> Would it work?

You mean only sending {*) ?

-- 
Cheers,

David