On 17 Jul 2023, at 10:31, Ryan Roberts wrote:
> This allows batching the rmap removal with folio_remove_rmap_range(),
> which means we avoid spuriously adding a partially unmapped folio to the
> deferrred split queue in the common case, which reduces split queue lock
> contention.
>
> Previously each page was removed from the rmap individually with
> page_remove_rmap(). If the first page belonged to a large folio, this
> would cause page_remove_rmap() to conclude that the folio was now
> partially mapped and add the folio to the deferred split queue. But
> subsequent calls would cause the folio to become fully unmapped, meaning
> there is no value to adding it to the split queue.
>
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> ---
> mm/memory.c | 119 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 119 insertions(+)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 01f39e8144ef..6facb8c8807a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1391,6 +1391,95 @@ zap_install_uffd_wp_if_needed(struct vm_area_struct *vma,
> pte_install_uffd_wp_if_needed(vma, addr, pte, pteval);
> }
>
> +static inline unsigned long page_addr(struct page *page,
> + struct page *anchor, unsigned long anchor_addr)
> +{
> + unsigned long offset;
> + unsigned long addr;
> +
> + offset = (page_to_pfn(page) - page_to_pfn(anchor)) << PAGE_SHIFT;
> + addr = anchor_addr + offset;
> +
> + if (anchor > page) {
> + if (addr > anchor_addr)
> + return 0;
> + } else {
> + if (addr < anchor_addr)
> + return ULONG_MAX;
> + }
> +
> + return addr;
> +}
> +
> +static int calc_anon_folio_map_pgcount(struct folio *folio,
> + struct page *page, pte_t *pte,
> + unsigned long addr, unsigned long end)
> +{
> + pte_t ptent;
> + int floops;
> + int i;
> + unsigned long pfn;
> +
> + end = min(page_addr(&folio->page + folio_nr_pages(folio), page, addr),
> + end);
> + floops = (end - addr) >> PAGE_SHIFT;
> + pfn = page_to_pfn(page);
> + pfn++;
> + pte++;
> +
> + for (i = 1; i < floops; i++) {
> + ptent = ptep_get(pte);
> +
> + if (!pte_present(ptent) ||
> + pte_pfn(ptent) != pfn) {
> + return i;
> + }
> +
> + pfn++;
> + pte++;
> + }
> +
> + return floops;
> +}
> +
> +static unsigned long zap_anon_pte_range(struct mmu_gather *tlb,
> + struct vm_area_struct *vma,
> + struct page *page, pte_t *pte,
> + unsigned long addr, unsigned long end,
> + bool *full_out)
> +{
> + struct folio *folio = page_folio(page);
> + struct mm_struct *mm = tlb->mm;
> + pte_t ptent;
> + int pgcount;
> + int i;
> + bool full;
> +
> + pgcount = calc_anon_folio_map_pgcount(folio, page, pte, addr, end);
> +
> + for (i = 0; i < pgcount;) {
> + ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
> + tlb_remove_tlb_entry(tlb, pte, addr);
> + full = __tlb_remove_page(tlb, page, 0);
> +
> + if (unlikely(page_mapcount(page) < 1))
> + print_bad_pte(vma, addr, ptent, page);
> +
> + i++;
> + page++;
> + pte++;
> + addr += PAGE_SIZE;
> +
> + if (unlikely(full))
> + break;
> + }
> +
> + folio_remove_rmap_range(folio, page - i, i, vma);
> +
> + *full_out = full;
> + return i;
> +}
> +
> static unsigned long zap_pte_range(struct mmu_gather *tlb,
> struct vm_area_struct *vma, pmd_t *pmd,
> unsigned long addr, unsigned long end,
> @@ -1428,6 +1517,36 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
> page = vm_normal_page(vma, addr, ptent);
> if (unlikely(!should_zap_page(details, page)))
> continue;
> +
> + /*
> + * Batch zap large anonymous folio mappings. This allows
> + * batching the rmap removal, which means we avoid
> + * spuriously adding a partially unmapped folio to the
> + * deferrred split queue in the common case, which
> + * reduces split queue lock contention. Require the VMA
> + * to be anonymous to ensure that none of the PTEs in
> + * the range require zap_install_uffd_wp_if_needed().
> + */
> + if (page && PageAnon(page) && vma_is_anonymous(vma)) {
> + bool full;
> + int pgcount;
> +
> + pgcount = zap_anon_pte_range(tlb, vma,
> + page, pte, addr, end, &full);
Are you trying to zap as many ptes as possible if all these ptes are
within a folio? If so, why not calculate end before calling zap_anon_pte_range()?
That would make zap_anon_pte_range() simpler. Also check if page is part of
a large folio first to make sure you can batch.
> +
> + rss[mm_counter(page)] -= pgcount;
> + pgcount--;
> + pte += pgcount;
> + addr += pgcount << PAGE_SHIFT;
> +
> + if (unlikely(full)) {
> + force_flush = 1;
> + addr += PAGE_SIZE;
> + break;
> + }
> + continue;
> + }
> +
> ptent = ptep_get_and_clear_full(mm, addr, pte,
> tlb->fullmm);
> tlb_remove_tlb_entry(tlb, pte, addr);
> --
> 2.25.1
--
Best Regards,
Yan, Zi
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