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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id u5-20020a05600c138500b003fee6e170f9sm3657743wmf.45.2023.11.16.05.20.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 16 Nov 2023 05:20:59 -0800 (PST) Message-ID: <08ef2c36-2b9c-4b96-9d1d-68cca0f68ba5@redhat.com> Date: Thu, 16 Nov 2023 14:20:58 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 01/14] mm: Batch-copy PTE ranges during fork() To: Ryan Roberts , Catalin Marinas , Will Deacon , Ard Biesheuvel , Marc Zyngier , Oliver Upton , James Morse , Suzuki K Poulose , Zenghui Yu , Andrey Ryabinin , Alexander Potapenko , Andrey Konovalov , Dmitry Vyukov , Vincenzo Frascino , Andrew Morton , Anshuman Khandual , Matthew Wilcox , Yu Zhao , Mark Rutland , Kefeng Wang , John Hubbard , Zi Yan Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20231115163018.1303287-1-ryan.roberts@arm.com> <20231115163018.1303287-2-ryan.roberts@arm.com> <89a9fe07-a5c5-4a99-b588-e6145053c58f@redhat.com> <1459f78b-e80c-4f21-bc65-f0ab259d348a@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 1FDD21C001B X-Stat-Signature: n4tq73sp1xcqtepgh4z1847sjkco1kq1 X-HE-Tag: 1700140863-828389 X-HE-Meta: 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 HpJ4SFqQ 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 16.11.23 12:20, Ryan Roberts wrote: > On 16/11/2023 11:03, David Hildenbrand wrote: >> On 15.11.23 17:30, Ryan Roberts wrote: >>> Convert copy_pte_range() to copy a set of ptes in a batch. A given batch >>> maps a physically contiguous block of memory, all belonging to the same >>> folio, with the same permissions, and for shared mappings, the same >>> dirty state. This will likely improve performance by a tiny amount due >>> to batching the folio reference count management and calling set_ptes() >>> rather than making individual calls to set_pte_at(). >>> >>> However, the primary motivation for this change is to reduce the number >>> of tlb maintenance operations that the arm64 backend has to perform >>> during fork, as it is about to add transparent support for the >>> "contiguous bit" in its ptes. By write-protecting the parent using the >>> new ptep_set_wrprotects() (note the 's' at the end) function, the >>> backend can avoid having to unfold contig ranges of PTEs, which is >>> expensive, when all ptes in the range are being write-protected. >>> Similarly, by using set_ptes() rather than set_pte_at() to set up ptes >>> in the child, the backend does not need to fold a contiguous range once >>> they are all populated - they can be initially populated as a contiguous >>> range in the first place. >>> >>> This change addresses the core-mm refactoring only, and introduces >>> ptep_set_wrprotects() with a default implementation that calls >>> ptep_set_wrprotect() for each pte in the range. A separate change will >>> implement ptep_set_wrprotects() in the arm64 backend to realize the >>> performance improvement as part of the work to enable contpte mappings. >>> >>> Signed-off-by: Ryan Roberts >>> --- >>>   include/linux/pgtable.h |  13 +++ >>>   mm/memory.c             | 175 +++++++++++++++++++++++++++++++--------- >>>   2 files changed, 150 insertions(+), 38 deletions(-) >>> >>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h >>> index af7639c3b0a3..1c50f8a0fdde 100644 >>> --- a/include/linux/pgtable.h >>> +++ b/include/linux/pgtable.h >>> @@ -622,6 +622,19 @@ static inline void ptep_set_wrprotect(struct mm_struct >>> *mm, unsigned long addres >>>   } >>>   #endif >>>   +#ifndef ptep_set_wrprotects >>> +struct mm_struct; >>> +static inline void ptep_set_wrprotects(struct mm_struct *mm, >>> +                unsigned long address, pte_t *ptep, >>> +                unsigned int nr) >>> +{ >>> +    unsigned int i; >>> + >>> +    for (i = 0; i < nr; i++, address += PAGE_SIZE, ptep++) >>> +        ptep_set_wrprotect(mm, address, ptep); >>> +} >>> +#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 1f18ed4a5497..b7c8228883cf 100644 >>> --- a/mm/memory.c >>> +++ b/mm/memory.c >>> @@ -921,46 +921,129 @@ copy_present_page(struct vm_area_struct *dst_vma, >>> struct vm_area_struct *src_vma >>>           /* Uffd-wp needs to be delivered to dest pte as well */ >>>           pte = pte_mkuffd_wp(pte); >>>       set_pte_at(dst_vma->vm_mm, addr, dst_pte, pte); >>> -    return 0; >>> +    return 1; >>> +} >>> + >>> +static inline unsigned long page_cont_mapped_vaddr(struct page *page, >>> +                struct page *anchor, unsigned long anchor_vaddr) >>> +{ >>> +    unsigned long offset; >>> +    unsigned long vaddr; >>> + >>> +    offset = (page_to_pfn(page) - page_to_pfn(anchor)) << PAGE_SHIFT; >>> +    vaddr = anchor_vaddr + offset; >>> + >>> +    if (anchor > page) { >>> +        if (vaddr > anchor_vaddr) >>> +            return 0; >>> +    } else { >>> +        if (vaddr < anchor_vaddr) >>> +            return ULONG_MAX; >>> +    } >>> + >>> +    return vaddr; >>> +} >>> + >>> +static int folio_nr_pages_cont_mapped(struct folio *folio, >>> +                      struct page *page, pte_t *pte, >>> +                      unsigned long addr, unsigned long end, >>> +                      pte_t ptent, bool *any_dirty) >>> +{ >>> +    int floops; >>> +    int i; >>> +    unsigned long pfn; >>> +    pgprot_t prot; >>> +    struct page *folio_end; >>> + >>> +    if (!folio_test_large(folio)) >>> +        return 1; >>> + >>> +    folio_end = &folio->page + folio_nr_pages(folio); >>> +    end = min(page_cont_mapped_vaddr(folio_end, page, addr), end); >>> +    floops = (end - addr) >> PAGE_SHIFT; >>> +    pfn = page_to_pfn(page); >>> +    prot = pte_pgprot(pte_mkold(pte_mkclean(ptent))); >>> + >>> +    *any_dirty = pte_dirty(ptent); >>> + >>> +    pfn++; >>> +    pte++; >>> + >>> +    for (i = 1; i < floops; i++) { >>> +        ptent = ptep_get(pte); >>> +        ptent = pte_mkold(pte_mkclean(ptent)); >>> + >>> +        if (!pte_present(ptent) || pte_pfn(ptent) != pfn || >>> +            pgprot_val(pte_pgprot(ptent)) != pgprot_val(prot)) >>> +            break; >>> + >>> +        if (pte_dirty(ptent)) >>> +            *any_dirty = true; >>> + >>> +        pfn++; >>> +        pte++; >>> +    } >>> + >>> +    return i; >>>   } >>>     /* >>> - * Copy one pte.  Returns 0 if succeeded, or -EAGAIN if one preallocated page >>> - * is required to copy this pte. >>> + * Copy set of contiguous ptes.  Returns number of ptes copied if succeeded >>> + * (always gte 1), or -EAGAIN if one preallocated page is required to copy the >>> + * first pte. >>>    */ >>>   static inline int >>> -copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, >>> -         pte_t *dst_pte, pte_t *src_pte, unsigned long addr, int *rss, >>> -         struct folio **prealloc) >>> +copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct >>> *src_vma, >>> +          pte_t *dst_pte, pte_t *src_pte, >>> +          unsigned long addr, unsigned long end, >>> +          int *rss, struct folio **prealloc) >>>   { >>>       struct mm_struct *src_mm = src_vma->vm_mm; >>>       unsigned long vm_flags = src_vma->vm_flags; >>>       pte_t pte = ptep_get(src_pte); >>>       struct page *page; >>>       struct folio *folio; >>> +    int nr = 1; >>> +    bool anon; >>> +    bool any_dirty = pte_dirty(pte); >>> +    int i; >>>         page = vm_normal_page(src_vma, addr, pte); >>> -    if (page) >>> +    if (page) { >>>           folio = page_folio(page); >>> -    if (page && folio_test_anon(folio)) { >>> -        /* >>> -         * If this page may have been pinned by the parent process, >>> -         * copy the page immediately for the child so that we'll always >>> -         * guarantee the pinned page won't be randomly replaced in the >>> -         * future. >>> -         */ >>> -        folio_get(folio); >>> -        if (unlikely(page_try_dup_anon_rmap(page, false, 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); >>> +        anon = folio_test_anon(folio); >>> +        nr = folio_nr_pages_cont_mapped(folio, page, src_pte, addr, >>> +                        end, pte, &any_dirty); >>> + >>> +        for (i = 0; i < nr; i++, page++) { >>> +            if (anon) { >>> +                /* >>> +                 * If this page may have been pinned by the >>> +                 * parent process, copy the page immediately for >>> +                 * the child so that we'll always guarantee the >>> +                 * pinned page won't be randomly replaced in the >>> +                 * future. >>> +                 */ >>> +                if (unlikely(page_try_dup_anon_rmap( >>> +                        page, false, src_vma))) { >>> +                    if (i != 0) >>> +                        break; >>> +                    /* Page may be pinned, we have to copy. */ >>> +                    return copy_present_page( >>> +                        dst_vma, src_vma, dst_pte, >>> +                        src_pte, addr, rss, prealloc, >>> +                        page); >>> +                } >>> +                rss[MM_ANONPAGES]++; >>> +                VM_BUG_ON(PageAnonExclusive(page)); >>> +            } else { >>> +                page_dup_file_rmap(page, false); >>> +                rss[mm_counter_file(page)]++; >>> +            } >>>           } >>> -        rss[MM_ANONPAGES]++; >>> -    } else if (page) { >>> -        folio_get(folio); >>> -        page_dup_file_rmap(page, false); >>> -        rss[mm_counter_file(page)]++; >>> + >>> +        nr = i; >>> +        folio_ref_add(folio, nr); >>>       } >>>         /* >>> @@ -968,24 +1051,28 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct >>> vm_area_struct *src_vma, >>>        * in the parent and the child >>>        */ >>>       if (is_cow_mapping(vm_flags) && pte_write(pte)) { >>> -        ptep_set_wrprotect(src_mm, addr, src_pte); >>> +        ptep_set_wrprotects(src_mm, addr, src_pte, nr); >>>           pte = pte_wrprotect(pte); >> >> You likely want an "any_pte_writable" check here instead, no? >> >> Any operations that target a single indiividual PTE while multiple PTEs are >> adjusted are suspicious :) > > The idea is that I've already constrained the batch of pages such that the > permissions are all the same (see folio_nr_pages_cont_mapped()). So if the first > pte is writable, then they all are - something has gone badly wrong if some are > writable and others are not. I wonder if it would be cleaner and easier to not do that, though. Simply record if any pte is writable. Afterwards they will *all* be R/O and you can set the cont bit, correct? -- Cheers, David / dhildenb