Re: [PATCH v2 7/9] mm/memory: factor out common code from vm_normal_page_*()

Lorenzo Stoakes
2025-07-30T13:25:50Z

On Wed, Jul 30, 2025 at 02:54:46PM +0200, David Hildenbrand wrote:
> On 18.07.25 14:43, Lorenzo Stoakes wrote:
> > On Thu, Jul 17, 2025 at 10:03:44PM +0200, David Hildenbrand wrote:
> > > On 17.07.25 21:55, Lorenzo Stoakes wrote:
> > > > On Thu, Jul 17, 2025 at 08:51:51PM +0100, Lorenzo Stoakes wrote:
> > > > > > @@ -721,37 +772,21 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
> > > > > > print_bad_page_map(vma, addr, pmd_val(pmd), NULL);
> > > > > > return NULL;
> > > > > > }
> > > > > > -
> > > > > > - if (unlikely(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP))) {
> > > > > > - if (vma->vm_flags & VM_MIXEDMAP) {
> > > > > > - if (!pfn_valid(pfn))
> > > > > > - return NULL;
> > > > > > - goto out;
> > > > > > - } else {
> > > > > > - unsigned long off;
> > > > > > - off = (addr - vma->vm_start) >> PAGE_SHIFT;
> > > > > > - if (pfn == vma->vm_pgoff + off)
> > > > > > - return NULL;
> > > > > > - if (!is_cow_mapping(vma->vm_flags))
> > > > > > - return NULL;
> > > > > > - }
> > > > > > - }
> > > > > > -
> > > > > > - if (is_huge_zero_pfn(pfn))
> > > > > > - return NULL;
> > > > > > - if (unlikely(pfn > highest_memmap_pfn)) {
> > > > > > - print_bad_page_map(vma, addr, pmd_val(pmd), NULL);
> > > > > > - return NULL;
> > > > > > - }
> > > > > > -
> > > > > > - /*
> > > > > > - * NOTE! We still have PageReserved() pages in the page tables.
> > > > > > - * eg. VDSO mappings can cause them to exist.
> > > > > > - */
> > > > > > -out:
> > > > > > - return pfn_to_page(pfn);
> > > > > > + return vm_normal_page_pfn(vma, addr, pfn, pmd_val(pmd));
> > > > >
> > > > > Hmm this seems broken, because you're now making these special on arches with
> > > > > pte_special() right? But then you're invoking the not-special function?
> > > > >
> > > > > Also for non-pte_special() arches you're kind of implying they _maybe_ could be
> > > > > special.
> > > >
> > > > OK sorry the diff caught me out here, you explicitly handle the pmd_special()
> > > > case here, duplicatively (yuck).
> > > >
> > > > Maybe you fix this up in a later patch :)
> > >
> > > I had that, but the conditions depend on the level, meaning: unnecessary
> > > checks for pte/pmd/pud level.
> > >
> > > I had a variant where I would pass "bool special" into vm_normal_page_pfn(),
> > > but I didn't like it.
> > >
> > > To optimize out, I would have to provide a "level" argument, and did not
> > > convince myself yet that that is a good idea at this point.
> >
> > Yeah fair enough. That probably isn't worth it or might end up making things
> > even more ugly.
>
> So, I decided to add the level arguments, but not use them to optimize the checks,
> only to forward it to the new print_bad_pte().
>
> So the new helper will be
>
> /**
> * __vm_normal_page() - Get the "struct page" associated with a page table entry.
> * @vma: The VMA mapping the page table entry.
> * @addr: The address where the page table entry is mapped.
> * @pfn: The PFN stored in the page table entry.
> * @special: Whether the page table entry is marked "special".
> * @level: The page table level for error reporting purposes only.
> * @entry: The page table entry value for error reporting purposes only.
> ...
> */
> static inline struct page *__vm_normal_page(struct vm_area_struct *vma,
> unsigned long addr, unsigned long pfn, bool special,
> unsigned long long entry, enum pgtable_level level)
> ...
>
>
> And vm_nomal_page() will for example be
>
> /**
> * vm_normal_page() - Get the "struct page" associated with a PTE
> * @vma: The VMA mapping the @pte.
> * @addr: The address where the @pte is mapped.
> * @pte: The PTE.
> *
> * Get the "struct page" associated with a PTE. See __vm_normal_page()
> * for details on "normal" and "special" mappings.
> *
> * Return: Returns the "struct page" if this is a "normal" mapping. Returns
> * NULL if this is a "special" mapping.
> */
> struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
> pte_t pte)
> {
> return __vm_normal_page(vma, addr, pte_pfn(pte), pte_special(pte),
> pte_val(pte), PGTABLE_LEVEL_PTE);
> }
>

OK that could work out well actually, cool thank you!