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s=arc-20220608; d=hostedemail.com; t=1700133632; a=rsa-sha256; cv=none; b=0DrwaGmWyVrZFcnfIAktAIE7NCKUt0K2LFsWXfzuuc/8CbCY43RGBIhR/mYg8cphW4c6M3 IxsPeMjhWW2gB/azVGmsyvAOx9wpjmWv3ZFcxeMNTEKE/Ef9UOPXsJWn4qu3CpcTk+foSJ aYCLk16Y89Pi7mLqInXRBtwGWyR8Sfg= ARC-Authentication-Results: i=1; imf17.hostedemail.com; dkim=none; spf=pass (imf17.hostedemail.com: domain of ryan.roberts@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=ryan.roberts@arm.com; dmarc=pass (policy=none) header.from=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 3F0AC1595; Thu, 16 Nov 2023 03:21:17 -0800 (PST) Received: from [10.1.35.163] (XHFQ2J9959.cambridge.arm.com [10.1.35.163]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 49F153F6C4; Thu, 16 Nov 2023 03:20:28 -0800 (PST) Message-ID: <1459f78b-e80c-4f21-bc65-f0ab259d348a@arm.com> Date: Thu, 16 Nov 2023 11:20:27 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 01/14] mm: Batch-copy PTE ranges during fork() Content-Language: en-GB To: David Hildenbrand , 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> From: Ryan Roberts In-Reply-To: <89a9fe07-a5c5-4a99-b588-e6145053c58f@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 50FDF4000D X-Rspam-User: X-Stat-Signature: 4ginrog8c5ixkotcgw3z78mybeczdj6s X-Rspamd-Server: rspam03 X-HE-Tag: 1700133632-569560 X-HE-Meta: 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 C49XMsmT EenIgW+UhFey4vO5gWgmznOuNqOwMiGJML4VgGJ4WEUFvovdAxYHA7aGTll020rUV7oQKRGxNqpz+ShYZAPQL8e+fegjLDdSkAWgU8OYPixISbE7F34ivmXgif3zxrEiAmYAoFzpSN8q23UvfAhaw/ylaIpeUkGhiLEDa+rtHHbwRpsOOr/BoLOEs1LT+Wzj8KlrVTQ3wQazhW0Qgr2C+gCu0Kmv4bQ3B2MOWSMMVFsPP4TBLRj0G7mX9d7uPbUOPg6nJ54Bnu4mMzIqV+yRss9HJZg== 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 16/11/2023 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. The dirty bit has any_dirty special case, because we (deliberately) don't consider access/dirty when determining the batch. Given the batch is all covered by the same folio, and the kernel maintains the access/dirty info per-folio, we don't want to uneccessarily reduce the batch size just because one of the pages in the folio has been written to. >