Re: [PATCH v6 15/26] huge_memory: Add vmf_insert_folio_pud()

[David Hildenbrand]

On 10.01.25 07:00, Alistair Popple wrote:
> Currently DAX folio/page reference counts are managed differently to
> normal pages. To allow these to be managed the same as normal pages
> introduce vmf_insert_folio_pud. This will map the entire PUD-sized folio
> and take references as it would for a normally mapped page.
>
> This is distinct from the current mechanism, vmf_insert_pfn_pud, which
> simply inserts a special devmap PUD entry into the page table without
> holding a reference to the page for the mapping.
>
> Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>

[...]

> +/**
> + * vmf_insert_folio_pud - insert a pud size folio mapped by a pud entry
> + * @vmf: Structure describing the fault
> + * @folio: folio to insert
> + * @write: whether it's a write fault
> + *
> + * Return: vm_fault_t value.
> + */
> +vm_fault_t vmf_insert_folio_pud(struct vm_fault *vmf, struct folio *folio, bool write)
> +{
> +	struct vm_area_struct *vma = vmf->vma;
> +	unsigned long addr = vmf->address & PUD_MASK;
> +	pud_t *pud = vmf->pud;
> +	struct mm_struct *mm = vma->vm_mm;
> +	spinlock_t *ptl;
> +
> +	if (addr < vma->vm_start || addr >= vma->vm_end)
> +		return VM_FAULT_SIGBUS;
> +
> +	if (WARN_ON_ONCE(folio_order(folio) != PUD_ORDER))
> +		return VM_FAULT_SIGBUS;
> +
> +	ptl = pud_lock(mm, pud);
> +	if (pud_none(*vmf->pud)) {
> +		folio_get(folio);
> +		folio_add_file_rmap_pud(folio, &folio->page, vma);
> +		add_mm_counter(mm, mm_counter_file(folio), HPAGE_PUD_NR);
> +	}
> +	insert_pfn_pud(vma, addr, vmf->pud, pfn_to_pfn_t(folio_pfn(folio)), write);

This looks scary at first (inserting something when not taking a 
reference), but insert_pfn_pud() seems to handle that. A comment here 
would have been nice.

It's weird, though, that if there is already something else, that we 
only WARN but don't actually return an error. So ...

> +	spin_unlock(ptl);
> +
> +	return VM_FAULT_NOPAGE;

I assume always returning VM_FAULT_NOPAGE, even when something went 
wrong, is the right thing to do?

Apart from that LGTM.


--
Cheers,

David / dhildenb