Re: [PATCH v8 02/23] mm: Teach core mm about pte markers

Peter Xu
2022-04-12T20:01:42Z

Hi, Alistair,

On Tue, Apr 12, 2022 at 11:22:01AM +1000, Alistair Popple wrote:
> I've been reviewing existing pte_none() call sites and noticed the following:
>
> From khugepaged_scan_pmd():
>
> pte_t pteval = *_pte;
> if (is_swap_pte(pteval)) {
> if (++unmapped <= khugepaged_max_ptes_swap) {
> /*
> * Always be strict with uffd-wp
> * enabled swap entries. Please see
> * comment below for pte_uffd_wp().
> */
> if (pte_swp_uffd_wp(pteval)) {
> result = SCAN_PTE_UFFD_WP;
> goto out_unmap;
> }
> continue;
> } else {
> result = SCAN_EXCEED_SWAP_PTE;
> count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
> goto out_unmap;
> }
> }
> if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> if (!userfaultfd_armed(vma) &&
> ++none_or_zero <= khugepaged_max_ptes_none) {
> continue;
> } else {
> result = SCAN_EXCEED_NONE_PTE;
> count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> goto out_unmap;
> }
> }
>
> I think the above could encounter a marker PTE right? So the behviour would be
> wrong in that case. As I understand things the is_swap_pte() path will be taken
> rather than pte_none() but in the absence of any special handling shouldn't
> marker PTE's mostly be treated as pte_none() here?
>
> I think you need to s/pte_none/pte_none_mostly/ here and swap the order of
> conditionals around.

Isn't khugepaged_scan_pmd() only for anonymous?

The shmem side is covered by khugepaged_scan_file(), imho. We definitely
don't want to collapse shmem vma ranges that has uffd-wp registered, and
that's actually handled explicilty in "mm/khugepaged: Don't recycle vma
pgtable if uffd-wp registered". Please feel free to have a look.

Thanks,

--
Peter Xu