Re: [PATCH v2 10/10] mm/memory: optimize unmap/zap with PTE-mapped THP

Ryan Roberts
2024-02-12T09:37:22Z

On 09/02/2024 22:15, David Hildenbrand wrote:
> Similar to how we optimized fork(), let's implement PTE batching when
> consecutive (present) PTEs map consecutive pages of the same large
> folio.
>
> Most infrastructure we need for batching (mmu gather, rmap) is already
> there. We only have to add get_and_clear_full_ptes() and
> clear_full_ptes(). Similarly, extend zap_install_uffd_wp_if_needed() to
> process a PTE range.
>
> We won't bother sanity-checking the mapcount of all subpages, but only
> check the mapcount of the first subpage we process. If there is a real
> problem hiding somewhere, we can trigger it simply by using small
> folios, or when we zap single pages of a large folio. Ideally, we had
> that check in rmap code (including for delayed rmap), but then we cannot
> print the PTE. Let's keep it simple for now. If we ever have a cheap
> folio_mapcount(), we might just want to check for underflows there.
>
> To keep small folios as fast as possible force inlining of a specialized
> variant using __always_inline with nr=1.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>

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

> ---
> include/linux/pgtable.h | 70 +++++++++++++++++++++++++++++++
> mm/memory.c | 92 +++++++++++++++++++++++++++++------------
> 2 files changed, 136 insertions(+), 26 deletions(-)
>
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index aab227e12493..49ab1f73b5c2 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -580,6 +580,76 @@ static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm,
> }
> #endif
>
> +#ifndef get_and_clear_full_ptes
> +/**
> + * get_and_clear_full_ptes - Clear present PTEs that map consecutive pages of
> + * the same folio, collecting dirty/accessed bits.
> + * @mm: Address space the pages are mapped into.
> + * @addr: Address the first page is mapped at.
> + * @ptep: Page table pointer for the first entry.
> + * @nr: Number of entries to clear.
> + * @full: Whether we are clearing a full mm.
> + *
> + * May be overridden by the architecture; otherwise, implemented as a simple
> + * loop over ptep_get_and_clear_full(), merging dirty/accessed bits into the
> + * returned PTE.
> + *
> + * Note that PTE bits in the PTE range besides the PFN can differ. For example,
> + * some PTEs might be write-protected.
> + *
> + * Context: The caller holds the page table lock. The PTEs map consecutive
> + * pages that belong to the same folio. The PTEs are all in the same PMD.
> + */
> +static inline pte_t get_and_clear_full_ptes(struct mm_struct *mm,
> + unsigned long addr, pte_t *ptep, unsigned int nr, int full)
> +{
> + pte_t pte, tmp_pte;
> +
> + pte = ptep_get_and_clear_full(mm, addr, ptep, full);
> + while (--nr) {
> + ptep++;
> + addr += PAGE_SIZE;
> + tmp_pte = ptep_get_and_clear_full(mm, addr, ptep, full);
> + if (pte_dirty(tmp_pte))
> + pte = pte_mkdirty(pte);
> + if (pte_young(tmp_pte))
> + pte = pte_mkyoung(pte);
> + }
> + return pte;
> +}
> +#endif
> +
> +#ifndef clear_full_ptes
> +/**
> + * clear_full_ptes - Clear present PTEs that map consecutive pages of the same
> + * folio.
> + * @mm: Address space the pages are mapped into.
> + * @addr: Address the first page is mapped at.
> + * @ptep: Page table pointer for the first entry.
> + * @nr: Number of entries to clear.
> + * @full: Whether we are clearing a full mm.
> + *
> + * May be overridden by the architecture; otherwise, implemented as a simple
> + * loop over ptep_get_and_clear_full().
> + *
> + * Note that PTE bits in the PTE range besides the PFN can differ. For example,
> + * some PTEs might be write-protected.
> + *
> + * Context: The caller holds the page table lock. The PTEs map consecutive
> + * pages that belong to the same folio. The PTEs are all in the same PMD.
> + */
> +static inline void clear_full_ptes(struct mm_struct *mm, unsigned long addr,
> + pte_t *ptep, unsigned int nr, int full)
> +{
> + for (;;) {
> + ptep_get_and_clear_full(mm, addr, ptep, full);
> + if (--nr == 0)
> + break;
> + ptep++;
> + addr += PAGE_SIZE;
> + }
> +}
> +#endif
>
> /*
> * If two threads concurrently fault at the same page, the thread that
> diff --git a/mm/memory.c b/mm/memory.c
> index a3efc4da258a..3b8e56eb08a3 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1515,7 +1515,7 @@ static inline bool zap_drop_file_uffd_wp(struct zap_details *details)
> */
> static inline void
> zap_install_uffd_wp_if_needed(struct vm_area_struct *vma,
> - unsigned long addr, pte_t *pte,
> + unsigned long addr, pte_t *pte, int nr,
> struct zap_details *details, pte_t pteval)
> {
> /* Zap on anonymous always means dropping everything */
> @@ -1525,20 +1525,27 @@ zap_install_uffd_wp_if_needed(struct vm_area_struct *vma,
> if (zap_drop_file_uffd_wp(details))
> return;
>
> - pte_install_uffd_wp_if_needed(vma, addr, pte, pteval);
> + for (;;) {
> + /* the PFN in the PTE is irrelevant. */
> + pte_install_uffd_wp_if_needed(vma, addr, pte, pteval);
> + if (--nr == 0)
> + break;
> + pte++;
> + addr += PAGE_SIZE;
> + }
> }
>
> -static inline void zap_present_folio_pte(struct mmu_gather *tlb,
> +static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb,
> struct vm_area_struct *vma, struct folio *folio,
> - struct page *page, pte_t *pte, pte_t ptent, unsigned long addr,
> - struct zap_details *details, int *rss, bool *force_flush,
> - bool *force_break)
> + struct page *page, pte_t *pte, pte_t ptent, unsigned int nr,
> + unsigned long addr, struct zap_details *details, int *rss,
> + bool *force_flush, bool *force_break)
> {
> struct mm_struct *mm = tlb->mm;
> bool delay_rmap = false;
>
> if (!folio_test_anon(folio)) {
> - ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
> + ptent = get_and_clear_full_ptes(mm, addr, pte, nr, tlb->fullmm);
> if (pte_dirty(ptent)) {
> folio_mark_dirty(folio);
> if (tlb_delay_rmap(tlb)) {
> @@ -1548,36 +1555,49 @@ static inline void zap_present_folio_pte(struct mmu_gather *tlb,
> }
> if (pte_young(ptent) && likely(vma_has_recency(vma)))
> folio_mark_accessed(folio);
> - rss[mm_counter(folio)]--;
> + rss[mm_counter(folio)] -= nr;
> } else {
> /* We don't need up-to-date accessed/dirty bits. */
> - ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
> - rss[MM_ANONPAGES]--;
> + clear_full_ptes(mm, addr, pte, nr, tlb->fullmm);
> + rss[MM_ANONPAGES] -= nr;
> }
> + /* Checking a single PTE in a batch is sufficient. */
> arch_check_zapped_pte(vma, ptent);
> - tlb_remove_tlb_entry(tlb, pte, addr);
> + tlb_remove_tlb_entries(tlb, pte, nr, addr);
> if (unlikely(userfaultfd_pte_wp(vma, ptent)))
> - zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent);
> + zap_install_uffd_wp_if_needed(vma, addr, pte, nr, details,
> + ptent);
>
> if (!delay_rmap) {
> - folio_remove_rmap_pte(folio, page, vma);
> + folio_remove_rmap_ptes(folio, page, nr, vma);
> +
> + /* Only sanity-check the first page in a batch. */
> if (unlikely(page_mapcount(page) < 0))
> print_bad_pte(vma, addr, ptent, page);
> }
> - if (unlikely(__tlb_remove_page(tlb, page, delay_rmap))) {
> + if (unlikely(__tlb_remove_folio_pages(tlb, page, nr, delay_rmap))) {
> *force_flush = true;
> *force_break = true;
> }
> }
>
> -static inline void zap_present_pte(struct mmu_gather *tlb,
> +/*
> + * Zap or skip at least one present PTE, trying to batch-process subsequent
> + * PTEs that map consecutive pages of the same folio.
> + *
> + * Returns the number of processed (skipped or zapped) PTEs (at least 1).
> + */
> +static inline int zap_present_ptes(struct mmu_gather *tlb,
> struct vm_area_struct *vma, pte_t *pte, pte_t ptent,
> - unsigned long addr, struct zap_details *details,
> - int *rss, bool *force_flush, bool *force_break)
> + unsigned int max_nr, unsigned long addr,
> + struct zap_details *details, int *rss, bool *force_flush,
> + bool *force_break)
> {
> + const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> struct mm_struct *mm = tlb->mm;
> struct folio *folio;
> struct page *page;
> + int nr;
>
> page = vm_normal_page(vma, addr, ptent);
> if (!page) {
> @@ -1587,14 +1607,29 @@ static inline void zap_present_pte(struct mmu_gather *tlb,
> tlb_remove_tlb_entry(tlb, pte, addr);
> VM_WARN_ON_ONCE(userfaultfd_wp(vma));
> ksm_might_unmap_zero_page(mm, ptent);
> - return;
> + return 1;
> }
>
> folio = page_folio(page);
> if (unlikely(!should_zap_folio(details, folio)))
> - return;
> - zap_present_folio_pte(tlb, vma, folio, page, pte, ptent, addr, details,
> - rss, force_flush, force_break);
> + return 1;
> +
> + /*
> + * Make sure that the common "small folio" case is as fast as possible
> + * by keeping the batching logic separate.
> + */
> + if (unlikely(folio_test_large(folio) && max_nr != 1)) {
> + nr = folio_pte_batch(folio, addr, pte, ptent, max_nr, fpb_flags,
> + NULL);
> +
> + zap_present_folio_ptes(tlb, vma, folio, page, pte, ptent, nr,
> + addr, details, rss, force_flush,
> + force_break);
> + return nr;
> + }
> + zap_present_folio_ptes(tlb, vma, folio, page, pte, ptent, 1, addr,
> + details, rss, force_flush, force_break);
> + return 1;
> }
>
> static unsigned long zap_pte_range(struct mmu_gather *tlb,
> @@ -1609,6 +1644,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
> pte_t *start_pte;
> pte_t *pte;
> swp_entry_t entry;
> + int nr;
>
> tlb_change_page_size(tlb, PAGE_SIZE);
> init_rss_vec(rss);
> @@ -1622,7 +1658,9 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
> pte_t ptent = ptep_get(pte);
> struct folio *folio;
> struct page *page;
> + int max_nr;
>
> + nr = 1;
> if (pte_none(ptent))
> continue;
>
> @@ -1630,10 +1668,12 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
> break;
>
> if (pte_present(ptent)) {
> - zap_present_pte(tlb, vma, pte, ptent, addr, details,
> - rss, &force_flush, &force_break);
> + max_nr = (end - addr) / PAGE_SIZE;
> + nr = zap_present_ptes(tlb, vma, pte, ptent, max_nr,
> + addr, details, rss, &force_flush,
> + &force_break);
> if (unlikely(force_break)) {
> - addr += PAGE_SIZE;
> + addr += nr * PAGE_SIZE;
> break;
> }
> continue;
> @@ -1687,8 +1727,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
> WARN_ON_ONCE(1);
> }
> pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
> - zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent);
> - } while (pte++, addr += PAGE_SIZE, addr != end);
> + zap_install_uffd_wp_if_needed(vma, addr, pte, 1, details, ptent);
> + } while (pte += nr, addr += PAGE_SIZE * nr, addr != end);
>
> add_mm_rss_vec(mm, rss);
> arch_leave_lazy_mmu_mode();