Re: [PATCH RFC 07/14] fs/dax: use vmf_insert_folio_pmd() to insert the huge zero folio

[Alistair Popple]

On Tue, Jun 17, 2025 at 05:43:38PM +0200, David Hildenbrand wrote:
> Let's convert to vmf_insert_folio_pmd().
> 
> In the unlikely case there is already something mapped, we'll now still
> call trace_dax_pmd_load_hole() and return VM_FAULT_NOPAGE.
> 
> That should probably be fine, no need to add special cases for that.

I'm not sure about that. Consider dax_iomap_pmd_fault() -> dax_fault_iter() ->
dax_pmd_load_hole(). It calls split_huge_pmd() in response to VM_FAULT_FALLBACK
which will no longer happen, what makes that ok?

> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  fs/dax.c | 47 ++++++++++-------------------------------------
>  1 file changed, 10 insertions(+), 37 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 4229513806bea..ae90706674a3f 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -1375,51 +1375,24 @@ static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
>  		const struct iomap_iter *iter, void **entry)
>  {
>  	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
> -	unsigned long pmd_addr = vmf->address & PMD_MASK;
> -	struct vm_area_struct *vma = vmf->vma;
>  	struct inode *inode = mapping->host;
> -	pgtable_t pgtable = NULL;
>  	struct folio *zero_folio;
> -	spinlock_t *ptl;
> -	pmd_t pmd_entry;
> -	unsigned long pfn;
> +	vm_fault_t ret;
>  
>  	zero_folio = mm_get_huge_zero_folio(vmf->vma->vm_mm);
>  
> -	if (unlikely(!zero_folio))
> -		goto fallback;
> -
> -	pfn = page_to_pfn(&zero_folio->page);
> -	*entry = dax_insert_entry(xas, vmf, iter, *entry, pfn,
> -				  DAX_PMD | DAX_ZERO_PAGE);
> -
> -	if (arch_needs_pgtable_deposit()) {
> -		pgtable = pte_alloc_one(vma->vm_mm);
> -		if (!pgtable)
> -			return VM_FAULT_OOM;
> -	}
> -
> -	ptl = pmd_lock(vmf->vma->vm_mm, vmf->pmd);
> -	if (!pmd_none(*(vmf->pmd))) {
> -		spin_unlock(ptl);
> -		goto fallback;
> +	if (unlikely(!zero_folio)) {
> +		trace_dax_pmd_load_hole_fallback(inode, vmf, zero_folio, *entry);
> +		return VM_FAULT_FALLBACK;
>  	}
>  
> -	if (pgtable) {
> -		pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
> -		mm_inc_nr_ptes(vma->vm_mm);
> -	}
> -	pmd_entry = folio_mk_pmd(zero_folio, vmf->vma->vm_page_prot);
> -	set_pmd_at(vmf->vma->vm_mm, pmd_addr, vmf->pmd, pmd_entry);
> -	spin_unlock(ptl);
> -	trace_dax_pmd_load_hole(inode, vmf, zero_folio, *entry);
> -	return VM_FAULT_NOPAGE;
> +	*entry = dax_insert_entry(xas, vmf, iter, *entry, folio_pfn(zero_folio),
> +				  DAX_PMD | DAX_ZERO_PAGE);
>  
> -fallback:
> -	if (pgtable)
> -		pte_free(vma->vm_mm, pgtable);
> -	trace_dax_pmd_load_hole_fallback(inode, vmf, zero_folio, *entry);
> -	return VM_FAULT_FALLBACK;
> +	ret = vmf_insert_folio_pmd(vmf, zero_folio, false);
> +	if (ret == VM_FAULT_NOPAGE)
> +		trace_dax_pmd_load_hole(inode, vmf, zero_folio, *entry);
> +	return ret;
>  }
>  #else
>  static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
> -- 
> 2.49.0
>