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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id d15-20020a05600c34cf00b0040a4cc876e0sm3832909wmq.40.2023.11.16.06.15.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 16 Nov 2023 06:15:50 -0800 (PST) Message-ID: Date: Thu, 16 Nov 2023 15:15:48 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 01/14] mm: Batch-copy PTE ranges during fork() From: David Hildenbrand 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> <08ef2c36-2b9c-4b96-9d1d-68cca0f68ba5@redhat.com> <2d027a8d-adfb-481d-89ea-c99139e669aa@arm.com> <2618b024-6a95-4bfc-a08d-59d86e9931e5@redhat.com> Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Stat-Signature: 4hyocc7zqh3a1oxuh9gsjw1trcs3udx8 X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 18B05140009 X-Rspam-User: X-HE-Tag: 1700144154-398418 X-HE-Meta: U2FsdGVkX18FiicaWllCDxdxu2wAfpKca7D6wP9F5FfXta8W27YbflxhSuwwLwiT9xzi18IeClbn2TQFrRFlWv4U3R9Rer733T8Ua5a7NfS4dO+ib+cCiLNoSl2h2cWzOnP6FJYwB8ISHOKO0mZLiIuT5P/Rd8RmSSN8JEktfqEgO6JgEQukl9RKfz/iz/MqGgOurBTlcfxb1KexHgsGoSMlTV5ksHgXF5DZ3ATJOve2CEVxABF/SBsctfQrnKVa1q29BnvNYlSGPM67L1D7eBL9z+9Ww/oQKeT2dXTrrcmHUKzv15k9cNSdXTCtSpW6Qc78aEcWC/7+e1uglZ1muYElos+B6fzyQWlZOFv3zPlCtruatYaD2kVNvfWp6zLsEwXyy1LhNFa5NzpqCFjav26lySb9njXVdDghq67B3mNKbxWh9ShI9wNiVQbP588Q+mwQogSPSdD4yLQDfmfHz7eER1U0/4P396p0NQV/Y85UEGJQqL6p1d08y7qAEvRzzP1jBqu12TLILzKVXUCzmoTx0gVJnjkQErx8TKuNzz0F5ALHasakHi9+VzcbJ6nZDrSrNz9Std2kmwBcUOXfoHJSrw67dkAHuGsD2O7t9Xl8EJ1FveAW6ADXLt3x/FLsJsBboQiaaedgFpFV+SJUOh2ViXvjVHs8mUkY56w8lph1tEFvvshxuigklzL/5N9M2CTzXLcJI7/5IHAR71ZJcmAuDxx9r1QGZO43g28S8lmnxAI1+HUu7nBd2pNIp+zpw+pJgMoFwsQ4qYQlnbmBxiHUgFwJIW7FrsFHTmi6SlTL/HebsTwqMz/v/55L8+PYe+YSQRYOqCZhcX5X69SF7Mdodwri9cUb8aRIDR0RFZgaSfA99n+iNSKUHHTkmkmtB85LUaXYunhJmUcUsoayUtzXsfCJDXBEgbRisC3IVK2LgaZhWjhJaoILKge1WoYBAE1wG37Odd0VOnQSYyA d2SVF5vS 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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 15:13, David Hildenbrand wrote: > On 16.11.23 14:49, Ryan Roberts wrote: >> On 16/11/2023 13:20, David Hildenbrand wrote: >>> 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? >> >> Oh I see what you mean - that only works for cow mappings though. If you have a >> shared mapping, you won't be making it read-only at fork. So if we ignore >> pte_write() state when demarking the batches, we will end up with a batch of >> pages with a mix of RO and RW in the parent, but then we set_ptes() for the >> child and those pages will all have the permissions of the first page of the batch. > > I see what you mean. > > After fork(), all anon pages will be R/O in the parent and the child. > Easy. If any PTE is writable, wrprotect all in the parent and the child. > > After fork(), all shared pages can be R/O or R/W in the parent. For > simplicity, I think you can simply set them all R/O in the child. So if > any PTE is writable, wrprotect all in the child. Or better: if any is R/O, set them all R/O. Otherwise just leave them as is. But devil is in the detail. -- Cheers, David / dhildenb