Re: [PATCH v6 12/26] mm/memory: Enhance insert_page_into_pte_locked() to create writable mappings

[Alistair Popple]

On Mon, Jan 13, 2025 at 05:08:31PM -0800, Dan Williams wrote:
> Alistair Popple wrote:
> > In preparation for using insert_page() for DAX, enhance
> > insert_page_into_pte_locked() to handle establishing writable
> > mappings.  Recall that DAX returns VM_FAULT_NOPAGE after installing a
> > PTE which bypasses the typical set_pte_range() in finish_fault.
> > 
> > Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
> > Suggested-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > 
> > ---
> > 
> > Changes for v5:
> >  - Minor comment/formatting fixes suggested by David Hildenbrand
> > 
> > Changes since v2:
> > 
> >  - New patch split out from "mm/memory: Add dax_insert_pfn"
> > ---
> >  mm/memory.c | 37 +++++++++++++++++++++++++++++--------
> >  1 file changed, 29 insertions(+), 8 deletions(-)
> > 
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 06bb29e..8531acb 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -2126,19 +2126,40 @@ static int validate_page_before_insert(struct vm_area_struct *vma,
> >  }
> >  
> >  static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
> > -			unsigned long addr, struct page *page, pgprot_t prot)
> > +				unsigned long addr, struct page *page,
> > +				pgprot_t prot, bool mkwrite)
> >  {
> >  	struct folio *folio = page_folio(page);
> > +	pte_t entry = ptep_get(pte);
> >  	pte_t pteval;
> >  
> > -	if (!pte_none(ptep_get(pte)))
> > -		return -EBUSY;
> > +	if (!pte_none(entry)) {
> > +		if (!mkwrite)
> > +			return -EBUSY;
> > +
> > +		/* see insert_pfn(). */
> > +		if (pte_pfn(entry) != page_to_pfn(page)) {
> > +			WARN_ON_ONCE(!is_zero_pfn(pte_pfn(entry)));
> > +			return -EFAULT;
> > +		}
> > +		entry = maybe_mkwrite(entry, vma);
> > +		entry = pte_mkyoung(entry);
> > +		if (ptep_set_access_flags(vma, addr, pte, entry, 1))
> > +			update_mmu_cache(vma, addr, pte);
> > +		return 0;
> > +	}
> 
> This hunk feels like it is begging to be unified with insert_pfn() after
> pfn_t dies. Perhaps a TODO to remember to come back and unify them, or
> you can go append that work to your pfn_t removal series?

No one has complained about removing pfn_t so I do intend to clean that series
up once this has all been merged somewhere, so I will just go append this
work there.

> Other than that you can add:
> 
> Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>