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

David Hildenbrand
2025-01-14T15:05:10Z

On 10.01.25 07:00, 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@nvidia.com>
> Suggested-by: Dan Williams <dan.j.williams@intel.com>
>
> ---
>
> 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;
>

Just drop "entry" and reuse "pteval"; even saves you from one bug below :)

pte_t pteval = ptep_get(pte);

> - 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;
> + }
> +
> /* Ok, finally just insert the thing.. */
> pteval = mk_pte(page, prot);
> if (unlikely(is_zero_folio(folio))) {
> pteval = pte_mkspecial(pteval);
> } else {
> folio_get(folio);
> + entry = mk_pte(page, prot);

we already do "pteval = mk_pte(page, prot);" above?

And I think your change here does not do what you want, because you
modify the new "entry" but we do

set_pte_at(vma->vm_mm, addr, pte, pteval);

below ...

> + if (mkwrite) {
> + entry = pte_mkyoung(entry);
> + entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> + }

So again, better just reuse pteval :)

--
Cheers,

David / dhildenb