Re: [PATCH v3 13/15] mm/memory: optimize fork() with PTE-mapped THP

Mike Rapoport
2024-02-08T06:42:22Z

On Mon, Jan 29, 2024 at 01:46:47PM +0100, David Hildenbrand wrote:
> Let's implement PTE batching when consecutive (present) PTEs map
> consecutive pages of the same large folio, and all other PTE bits besides
> the PFNs are equal.
>
> We will optimize folio_pte_batch() separately, to ignore selected
> PTE bits. This patch is based on work by Ryan Roberts.
>
> Use __always_inline for __copy_present_ptes() and keep the handling for
> single PTEs completely separate from the multi-PTE case: we really want
> the compiler to optimize for the single-PTE case with small folios, to
> not degrade performance.
>
> Note that PTE batching will never exceed a single page table and will
> always stay within VMA boundaries.
>
> Further, processing PTE-mapped THP that maybe pinned and have
> PageAnonExclusive set on at least one subpage should work as expected,
> but there is room for improvement: We will repeatedly (1) detect a PTE
> batch (2) detect that we have to copy a page (3) fall back and allocate a
> single page to copy a single page. For now we won't care as pinned pages
> are a corner case, and we should rather look into maintaining only a
> single PageAnonExclusive bit for large folios.
>
> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>

> ---
> include/linux/pgtable.h | 31 +++++++++++
> mm/memory.c | 112 +++++++++++++++++++++++++++++++++-------
> 2 files changed, 124 insertions(+), 19 deletions(-)
>
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 351cd9dc7194..aab227e12493 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -650,6 +650,37 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addres
> }
> #endif
>
> +#ifndef wrprotect_ptes
> +/**
> + * wrprotect_ptes - Write-protect 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 write-protect.
> + *
> + * May be overridden by the architecture; otherwise, implemented as a simple
> + * loop over ptep_set_wrprotect().
> + *
> + * 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 wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
> + pte_t *ptep, unsigned int nr)
> +{
> + for (;;) {
> + ptep_set_wrprotect(mm, addr, ptep);
> + if (--nr == 0)
> + break;
> + ptep++;
> + addr += PAGE_SIZE;
> + }
> +}
> +#endif
> +
> /*
> * On some architectures hardware does not set page access bit when accessing
> * memory page, it is responsibility of software setting this bit. It brings
> diff --git a/mm/memory.c b/mm/memory.c
> index 41b24da5be38..86f8a0021c8e 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -930,15 +930,15 @@ copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
> return 0;
> }
>
> -static inline void __copy_present_pte(struct vm_area_struct *dst_vma,
> +static __always_inline void __copy_present_ptes(struct vm_area_struct *dst_vma,
> struct vm_area_struct *src_vma, pte_t *dst_pte, pte_t *src_pte,
> - pte_t pte, unsigned long addr)
> + pte_t pte, unsigned long addr, int nr)
> {
> struct mm_struct *src_mm = src_vma->vm_mm;
>
> /* If it's a COW mapping, write protect it both processes. */
> if (is_cow_mapping(src_vma->vm_flags) && pte_write(pte)) {
> - ptep_set_wrprotect(src_mm, addr, src_pte);
> + wrprotect_ptes(src_mm, addr, src_pte, nr);
> pte = pte_wrprotect(pte);
> }
>
> @@ -950,26 +950,93 @@ static inline void __copy_present_pte(struct vm_area_struct *dst_vma,
> if (!userfaultfd_wp(dst_vma))
> pte = pte_clear_uffd_wp(pte);
>
> - set_pte_at(dst_vma->vm_mm, addr, dst_pte, pte);
> + set_ptes(dst_vma->vm_mm, addr, dst_pte, pte, nr);
> +}
> +
> +/*
> + * Detect a PTE batch: consecutive (present) PTEs that map consecutive
> + * pages of the same folio.
> + *
> + * All PTEs inside a PTE batch have the same PTE bits set, excluding the PFN.
> + */
> +static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
> + pte_t *start_ptep, pte_t pte, int max_nr)
> +{
> + unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
> + const pte_t *end_ptep = start_ptep + max_nr;
> + pte_t expected_pte = pte_next_pfn(pte);
> + pte_t *ptep = start_ptep + 1;
> +
> + VM_WARN_ON_FOLIO(!pte_present(pte), folio);
> +
> + while (ptep != end_ptep) {
> + pte = ptep_get(ptep);
> +
> + if (!pte_same(pte, expected_pte))
> + break;
> +
> + /*
> + * Stop immediately once we reached the end of the folio. In
> + * corner cases the next PFN might fall into a different
> + * folio.
> + */
> + if (pte_pfn(pte) == folio_end_pfn)
> + break;
> +
> + expected_pte = pte_next_pfn(expected_pte);
> + ptep++;
> + }
> +
> + return ptep - start_ptep;
> }
>
> /*
> - * Copy one pte. Returns 0 if succeeded, or -EAGAIN if one preallocated page
> - * is required to copy this pte.
> + * Copy one present PTE, trying to batch-process subsequent PTEs that map
> + * consecutive pages of the same folio by copying them as well.
> + *
> + * Returns -EAGAIN if one preallocated page is required to copy the next PTE.
> + * Otherwise, returns the number of copied PTEs (at least 1).
> */
> static inline int
> -copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
> +copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
> pte_t *dst_pte, pte_t *src_pte, pte_t pte, unsigned long addr,
> - int *rss, struct folio **prealloc)
> + int max_nr, int *rss, struct folio **prealloc)
> {
> struct page *page;
> struct folio *folio;
> + int err, nr;
>
> page = vm_normal_page(src_vma, addr, pte);
> if (unlikely(!page))
> goto copy_pte;
>
> folio = page_folio(page);
> +
> + /*
> + * If we likely have to copy, just don't bother with batching. Make
> + * sure that the common "small folio" case is as fast as possible
> + * by keeping the batching logic separate.
> + */
> + if (unlikely(!*prealloc && folio_test_large(folio) && max_nr != 1)) {
> + nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr);
> + folio_ref_add(folio, nr);
> + if (folio_test_anon(folio)) {
> + if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page,
> + nr, src_vma))) {
> + folio_ref_sub(folio, nr);
> + return -EAGAIN;
> + }
> + rss[MM_ANONPAGES] += nr;
> + VM_WARN_ON_FOLIO(PageAnonExclusive(page), folio);
> + } else {
> + folio_dup_file_rmap_ptes(folio, page, nr);
> + rss[mm_counter_file(folio)] += nr;
> + }
> + __copy_present_ptes(dst_vma, src_vma, dst_pte, src_pte, pte,
> + addr, nr);
> + return nr;
> + }
> +
> folio_get(folio);
> if (folio_test_anon(folio)) {
> /*
> @@ -981,8 +1048,9 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
> if (unlikely(folio_try_dup_anon_rmap_pte(folio, page, src_vma))) {
> /* Page may be pinned, we have to copy. */
> folio_put(folio);
> - return copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
> - addr, rss, prealloc, page);
> + err = copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
> + addr, rss, prealloc, page);
> + return err ? err : 1;
> }
> rss[MM_ANONPAGES]++;
> VM_WARN_ON_FOLIO(PageAnonExclusive(page), folio);
> @@ -992,8 +1060,8 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
> }
>
> copy_pte:
> - __copy_present_pte(dst_vma, src_vma, dst_pte, src_pte, pte, addr);
> - return 0;
> + __copy_present_ptes(dst_vma, src_vma, dst_pte, src_pte, pte, addr, 1);
> + return 1;
> }
>
> static inline struct folio *folio_prealloc(struct mm_struct *src_mm,
> @@ -1030,10 +1098,11 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
> pte_t *src_pte, *dst_pte;
> pte_t ptent;
> spinlock_t *src_ptl, *dst_ptl;
> - int progress, ret = 0;
> + int progress, max_nr, ret = 0;
> int rss[NR_MM_COUNTERS];
> swp_entry_t entry = (swp_entry_t){0};
> struct folio *prealloc = NULL;
> + int nr;
>
> again:
> progress = 0;
> @@ -1064,6 +1133,8 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
> arch_enter_lazy_mmu_mode();
>
> do {
> + nr = 1;
> +
> /*
> * We are holding two locks at this point - either of them
> * could generate latencies in another task on another CPU.
> @@ -1100,9 +1171,10 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
> */
> WARN_ON_ONCE(ret != -ENOENT);
> }
> - /* copy_present_pte() will clear `*prealloc' if consumed */
> - ret = copy_present_pte(dst_vma, src_vma, dst_pte, src_pte,
> - ptent, addr, rss, &prealloc);
> + /* copy_present_ptes() will clear `*prealloc' if consumed */
> + max_nr = (end - addr) / PAGE_SIZE;
> + ret = copy_present_ptes(dst_vma, src_vma, dst_pte, src_pte,
> + ptent, addr, max_nr, rss, &prealloc);
> /*
> * If we need a pre-allocated page for this pte, drop the
> * locks, allocate, and try again.
> @@ -1119,8 +1191,10 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
> folio_put(prealloc);
> prealloc = NULL;
> }
> - progress += 8;
> - } while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end);
> + nr = ret;
> + progress += 8 * nr;
> + } while (dst_pte += nr, src_pte += nr, addr += PAGE_SIZE * nr,
> + addr != end);
>
> arch_leave_lazy_mmu_mode();
> pte_unmap_unlock(orig_src_pte, src_ptl);
> @@ -1141,7 +1215,7 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
> prealloc = folio_prealloc(src_mm, src_vma, addr, false);
> if (!prealloc)
> return -ENOMEM;
> - } else if (ret) {
> + } else if (ret < 0) {
> VM_WARN_ON_ONCE(1);
> }
>
> --
> 2.43.0
>
>

--
Sincerely yours,
Mike.