Re: [PATCH v1 2/9] mm/memory: handle !page case in zap_present_pte() separately

Ryan Roberts
2024-01-30T08:20:07Z

On 29/01/2024 14:32, David Hildenbrand wrote:
> We don't need uptodate accessed/dirty bits, so in theory we could
> replace ptep_get_and_clear_full() by an optimized ptep_clear_full()
> function. Let's rely on the provided pte.
>
> Further, there is no scenario where we would have to insert uffd-wp
> markers when zapping something that is not a normal page (i.e., zeropage).
> Add a sanity check to make sure this remains true.
>
> should_zap_folio() no longer has to handle NULL pointers. This change
> replaces 2/3 "!page/!folio" checks by a single "!page" one.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>

> ---
> mm/memory.c | 20 ++++++++++----------
> 1 file changed, 10 insertions(+), 10 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 50a6c79c78fc..69502cdc0a7d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1497,10 +1497,6 @@ static inline bool should_zap_folio(struct zap_details *details,
> if (should_zap_cows(details))
> return true;
>
> - /* E.g. the caller passes NULL for the case of a zero folio */
> - if (!folio)
> - return true;
> -
> /* Otherwise we should only zap non-anon folios */
> return !folio_test_anon(folio);
> }
> @@ -1543,19 +1539,23 @@ static inline void zap_present_pte(struct mmu_gather *tlb,
> struct page *page;
>
> page = vm_normal_page(vma, addr, ptent);
> - if (page)
> - folio = page_folio(page);
> + if (!page) {
> + /* We don't need up-to-date accessed/dirty bits. */
> + ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
> + arch_check_zapped_pte(vma, ptent);
> + tlb_remove_tlb_entry(tlb, pte, addr);
> + VM_WARN_ON_ONCE(userfaultfd_wp(vma));
> + ksm_might_unmap_zero_page(mm, ptent);
> + return;
> + }
>
> + folio = page_folio(page);
> if (unlikely(!should_zap_folio(details, folio)))
> return;
> ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
> arch_check_zapped_pte(vma, ptent);
> tlb_remove_tlb_entry(tlb, pte, addr);
> zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent);
> - if (unlikely(!page)) {
> - ksm_might_unmap_zero_page(mm, ptent);
> - return;
> - }
>
> if (!folio_test_anon(folio)) {
> if (pte_dirty(ptent)) {