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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id s16-20020adfea90000000b003313439c675sm13765369wrm.66.2023.11.16.06.13.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 16 Nov 2023 06:13:15 -0800 (PST) Message-ID: <2618b024-6a95-4bfc-a08d-59d86e9931e5@redhat.com> Date: Thu, 16 Nov 2023 15:13:14 +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> <08ef2c36-2b9c-4b96-9d1d-68cca0f68ba5@redhat.com> <2d027a8d-adfb-481d-89ea-c99139e669aa@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 620F440003 X-Stat-Signature: y3m41it1u5m5rxu3snj64rh5b9nbeik8 X-Rspam-User: X-HE-Tag: 1700144000-923393 X-HE-Meta: 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 vY3kvotG 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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 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. Why? in the default case, fork() does not even care about MAP_SHARED mappings; it does not copy the page tables/ptes. See vma_needs_copy(). Only in corner cases (e.g., uffd-wp, VM_PFNMAP, VM_MIXEDMAP), or in MAP_PRIVATE mappings, you can even end up in that code. In MAP_PRIVATE mappings, only anon pages can be R/W, other pages can never be writable, so it does not matter. In VM_PFNMAP/VM_MIXEDMAP likely all permissions match either way. So you might just wrprotect the !anon pages R/O for the child and nobody should really notice it, write faults will resolve it. Famous last words :) -- Cheers, David / dhildenb