Re: [PATCH 4/5] mm/page_vma_mapped: use huge_ptep_get() for hugetlb

[Dev Jain]

On 26/06/26 7:40 pm, Lance Yang wrote:
> 
> On Fri, Jun 26, 2026 at 06:53:10PM +0530, Dev Jain wrote:
>>
>>
>> On 26/06/26 1:18 pm, Lance Yang wrote:
>>>
>>> On Thu, Jun 25, 2026 at 11:29:53AM +0000, Dev Jain wrote:
>>>> check_pte() is the final validation step in page_vma_mapped_walk().
>>>> It reads pvmw->pte with ptep_get() to decide whether the entry maps
>>>> the PFN range being walked. For hugetlb VMAs, that pointer refers
>>>> to a hugetlb entry.
>>>>
>>>> On arches which provide their own huge_ptep_get() to dereference a huge
>>>> pte pointer, accessing via ptep_get() would cause pte_pfn(),
>>>> pte_present() etc to misbehave.
>>>>
>>>> It is not clear whether this has a trivially visible effect to userspace.
>>>>
>>>> Use huge_ptep_get() to dereference a huge pte pointer.
>>>>
>>>> Fixes: ace71a19cec5 ("mm: introduce page_vma_mapped_walk()")
>>>> Cc: stable@xxxxxxxxxxxxxxx
>>>> Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
>>>> ---
>>>> mm/page_vma_mapped.c | 8 +++++++-
>>>> 1 file changed, 7 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>>>> index 2ccbabfb2cc17..18e1d341f463c 100644
>>>> --- a/mm/page_vma_mapped.c
>>>> +++ b/mm/page_vma_mapped.c
>>>> @@ -107,7 +107,13 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
>>>> static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
>>>> {
>>>> 	unsigned long pfn;
>>>> -	pte_t ptent = ptep_get(pvmw->pte);
>>>> +	pte_t ptent;
>>>> +
>>>> +	if (is_vm_hugetlb_page(pvmw->vma))
>>>> +		ptent = huge_ptep_get(pvmw->vma->vm_mm, pvmw->address,
>>>> +				      pvmw->pte);
>>>
>>> I think check_pte() can pass a wrong address to huge_ptep_get() ...
>>
>> Won't this be handled by rmap_walk_anon/rmap_walk_file - they are the ones
>> performing the rmap traversal and passing address to try_to_unmap_one/folio_referenced_one
>> etc ...
> 
> Right, that should cover the rmap callbacks. The bit I was worried about
> is page_mapped_in_vma() though.
> 
>>>
>>> Not sure that is wrong in the first place. For memory failure,
>>> page_mapped_in_vma() can be called with a poisoned tail page of a hugetlb
>>> folio. In that case, pvmw->address need not be hugepage-aligned.
>>>
>>> @Miaohe
> 
> For hugetlb memory failure we start with the poisoned PFN:
> 
> static int try_memory_failure_hugetlb(unsigned long pfn, int flags)
> {
> 	...
> 	struct page *p = pfn_to_page(pfn);
> 	struct folio *folio;
> 	...
> 
> 	folio = page_folio(p);
> 
> 	...
> 
> 	if (!hwpoison_user_mappings(folio, p, pfn, flags)) {
> 		...
> 	}
> 
> 	...
> }
> 
> and pass the same p down:
> 
> static bool hwpoison_user_mappings(struct folio *folio, struct page *p,
> 		unsigned long pfn, int flags)
> {
> 	...
> 
> 	collect_procs(folio, p, &tokill, flags & MF_ACTION_REQUIRED);
> 
> 	...
> }
> 
> static void collect_procs(const struct folio *folio, const struct page *page,
> 		struct list_head *tokill, int force_early)
> {
> 	...
> 
> 	if (unlikely(folio_test_ksm(folio)))
> 		...
> 	else if (folio_test_anon(folio))
> 		collect_procs_anon(folio, page, tokill, force_early);
> 	else
> 		...
> }
> 
> So collect_procs_anon() still gets the poisoned page, not &folio->page:
> 
> static void collect_procs_anon(const struct folio *folio,
> 		const struct page *page, struct list_head *to_kill,
> 		int force_early)
> {
> 	...
> 
> 	pgoff = page_pgoff(folio, page);
> 	rcu_read_lock();
> 	for_each_process(tsk) {
> 		...
> 		
> 		anon_vma_interval_tree_foreach(vmac, &av->rb_root,
> 					       pgoff, pgoff) {
> 			...
> 			addr = page_mapped_in_vma(page, vma);
> 			...
> 		}
> 	}
> 	rcu_read_unlock();
> 	anon_vma_unlock_read(av);
> }
> 
> page_mapped_in_vma() then builds pvmw for that page:
> 
> unsigned long page_mapped_in_vma(const struct page *page,
> 		struct vm_area_struct *vma)
> {
> 	const struct folio *folio = page_folio(page);
> 	struct page_vma_mapped_walk pvmw = {
> 		.pfn = page_to_pfn(page),
> 		.nr_pages = 1,
> 		.vma = vma,
> 		.flags = PVMW_SYNC,
> 	};
> 
> 	pvmw.address = vma_address(vma, page_pgoff(folio, page), 1);
> 	...
> }
> 
> and page_pgoff() includes the subpage index:
> 
> static inline pgoff_t page_pgoff(const struct folio *folio,
> 		const struct page *page)
> {
> 	return folio->index + folio_page_idx(folio, page);
> }
> 
> So if the poisoned PFN points to a tail page, pvmw->address can be offset
> from the start of the hugetlb mapping by
> 
> folio_page_idx(folio, page) << PAGE_SHIFT
> 
> Should check_pte() pass the hugepage-aligned address to huge_ptep_get()
> for that case?

Thanks! This looks correct.

I can indeed fix this up in check_pte. But in the memory-failure path
it has always been confusing to me for hugetlb folios why we are bothering
with the tail page. I am sure that area can also be simplified. But for
now I'll just do a simple fix here itself.


> 
> Cheers, Lance
> 
>>>
>>> For arm64, CONT_PMD_SIZE is one supported HugeTLB size. With such a VMA,
>>> page_vma_mapped_walk() passes that size to hugetlb_walk():
>>>
>>> bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>> {
>>> 	...
>>> 	if (unlikely(is_vm_hugetlb_page(vma))) {
>>> 		...
>>> 		pvmw->pte = hugetlb_walk(vma, pvmw->address, size);
>>> 		...
>>> 	}
>>> 	...
>>> }
>>>
>>> hugetlb_walk() then calls arm64 huge_pte_offset(mm, addr, sz). For
>>> sz == CONT_PMD_SIZE, huge_pte_offset() aligns its local addr before
>>> calculating pmdp:
>>>
>>> pte_t *huge_pte_offset(struct mm_struct *mm,
>>> 		       unsigned long addr, unsigned long sz)
>>> {
>>> 	...
>>> 	if (sz == CONT_PMD_SIZE)
>>> 		addr &= CONT_PMD_MASK;
>>>
>>> 	pmdp = pmd_offset(pudp, addr);
>>> 	pmd = READ_ONCE(*pmdp);
>>> 	...
>>> }
>>>
>>> So for that case, pvmw->pte is calculated from the aligned addr, not
>>> necessarily from the original pvmw->address. But check_pte() passes the
>>> original address together with pvmw->pte:
>>>
>>> +		ptent = huge_ptep_get(pvmw->vma->vm_mm, pvmw->address,
>>> +				      pvmw->pte);
>>>
>>> arm64 then uses that addr again to choose ncontig:
>>>
>>> pte_t huge_ptep_get(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
>>> {
>>> 	...
>>> 	ncontig = find_num_contig(mm, addr, ptep, &pgsize);
>>> 	for (i = 0; i < ncontig; i++, ptep++) {
>>> 		...
>>> 	}
>>> 	return orig_pte;
>>> }
>>>
>>> static int find_num_contig(struct mm_struct *mm, unsigned long addr,
>>> 			   pte_t *ptep, size_t *pgsize)
>>> {
>>> 	pgd_t *pgdp = pgd_offset(mm, addr);
>>> 	p4d_t *p4dp;
>>> 	pud_t *pudp;
>>> 	pmd_t *pmdp;
>>>
>>> 	*pgsize = PAGE_SIZE;
>>> 	p4dp = p4d_offset(pgdp, addr);
>>> 	pudp = pud_offset(p4dp, addr);
>>> 	pmdp = pmd_offset(pudp, addr);
>>> 	if ((pte_t *)pmdp == ptep) {
>>> 		*pgsize = PMD_SIZE;
>>> 		return CONT_PMDS;
>>> 	}
>>> 	return CONT_PTES;
>>> }
>>>
>>> With a tail address, pmdp may no longer point at pvmw->pte, so
>>> find_num_contig() can return CONT_PTES for a CONT_PMD HugeTLB mapping.
>>>
>>> On 16K arm64, that changes ncontig from 32 to 128. So huge_ptep_get()
>>> can walk past the CONT_PMD entries, and possibly past the PMD table.
>>>
>>> Should check_pte() pass the address matching pvmw->pte, sth like:
>>>
>>> ---8<---
>>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>>> index 406fd50bbd8f..58463493bd3d 100644
>>> --- a/mm/page_vma_mapped.c
>>> +++ b/mm/page_vma_mapped.c
>>> @@ -109,11 +109,14 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
>>>  	unsigned long pfn;
>>>  	pte_t ptent;
>>>
>>> -	if (is_vm_hugetlb_page(pvmw->vma))
>>> -		ptent = huge_ptep_get(pvmw->vma->vm_mm, pvmw->address,
>>> -				      pvmw->pte);
>>> -	else
>>> +	if (is_vm_hugetlb_page(pvmw->vma)) {
>>> +		struct hstate *hstate = hstate_vma(pvmw->vma);
>>> +		unsigned long haddr = pvmw->address & huge_page_mask(hstate);
>>> +
>>> +		ptent = huge_ptep_get(pvmw->vma->vm_mm, haddr, pvmw->pte);
>>> +	} else {
>>>  		ptent = ptep_get(pvmw->pte);
>>> +	}
>>>
>>>  	if (pvmw->flags & PVMW_MIGRATION) {
>>>  		const softleaf_t entry = softleaf_from_pte(ptent);
>>> --
>>>
>>> while leaving pvmw->address unchanged for page_mapped_in_vma()?
>>>
>>> Cheers, Lance
>>>
>>>> +	else
>>>> +		ptent = ptep_get(pvmw->pte);
>>>>
>>>> 	if (pvmw->flags & PVMW_MIGRATION) {
>>>> 		const softleaf_t entry = softleaf_from_pte(ptent);
>>>> -- 
>>>> 2.43.0
>>>>
>>>>
>>
>>
>