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imf12.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=arm.com; spf=pass (imf12.hostedemail.com: domain of ryan.roberts@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=ryan.roberts@arm.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1700579701; a=rsa-sha256; cv=none; b=hKY+mmmo0l/x/tVf/RBiqKNNzl8p7mOBs2xJcJ91YWnzO2lMkgAE5lbQodB4VJtGK8YFEu BvmWIfh3a+cJK1XrE6ufjs70XzHQvIrI99RxbskAT+Cvf0dszkqbvqj/ezKMBOlOMV0WqA 9GNNAQ5Amnj1lObfyCGQ3CxJi8FSAKc= 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 7E87DFEC; Tue, 21 Nov 2023 07:15:46 -0800 (PST) Received: from [10.1.26.189] (XHFQ2J9959.cambridge.arm.com [10.1.26.189]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id AA1F03F7A6; Tue, 21 Nov 2023 07:14:56 -0800 (PST) Message-ID: Date: Tue, 21 Nov 2023 15:14:55 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 12/14] arm64/mm: Wire up PTE_CONT for user mappings Content-Language: en-GB To: Alistair Popple Cc: 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 , David Hildenbrand , Kefeng Wang , John Hubbard , Zi Yan , linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20231115163018.1303287-1-ryan.roberts@arm.com> <20231115163018.1303287-13-ryan.roberts@arm.com> <87v89vmjus.fsf@nvdebian.thelocal> From: Ryan Roberts In-Reply-To: <87v89vmjus.fsf@nvdebian.thelocal> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 104FC40028 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: 1t4k8qdahonc9x18cyjjfy41cpwrw6kw X-HE-Tag: 1700579700-667749 X-HE-Meta: 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 oW69Nou1 jCBwJUGCYxpevZ2YI4AFZaoY6oRovdQnN501MCLoXg1xcounqD0wM1GBzlHfA1G1rMaszIZZBNWZ1PQpiRE+KMy/aoDVkxKNMwkBsI/xTFzC/luoOfVm91omTHrm7XKgrksCiLqgYcWXunA5q03mjaOGIiB/o4UPlVG/X5/8WkQIgVmttRv2KtI8n9O1h4xTW3RrOgb/pCHRq/Hhs+0T7TtJfSh2mxIZOi/u6hNaq0iNINJC7tE3UP8Vbi6GvXgE8ysROuSPHHDaWBhPau8oWKPCW23UT42ViGB8LAaNQigjF17O7OGa0EXulIXOdKqyDm1Ai9xYjaCl5mNA2OZaBmKYJkVE0znCW0ZkafgDhPtcNrc8= 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 21/11/2023 11:22, Alistair Popple wrote: > > Ryan Roberts writes: > > [...] > >> +static void contpte_fold(struct mm_struct *mm, unsigned long addr, >> + pte_t *ptep, pte_t pte, bool fold) >> +{ >> + struct vm_area_struct vma = TLB_FLUSH_VMA(mm, 0); >> + unsigned long start_addr; >> + pte_t *start_ptep; >> + int i; >> + >> + start_ptep = ptep = contpte_align_down(ptep); >> + start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE); >> + pte = pfn_pte(ALIGN_DOWN(pte_pfn(pte), CONT_PTES), pte_pgprot(pte)); >> + pte = fold ? pte_mkcont(pte) : pte_mknoncont(pte); >> + >> + for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE) { >> + pte_t ptent = __ptep_get_and_clear(mm, addr, ptep); >> + >> + if (pte_dirty(ptent)) >> + pte = pte_mkdirty(pte); >> + >> + if (pte_young(ptent)) >> + pte = pte_mkyoung(pte); >> + } >> + >> + __flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3); >> + >> + __set_ptes(mm, start_addr, start_ptep, pte, CONT_PTES); >> +} >> + >> +void __contpte_try_fold(struct mm_struct *mm, unsigned long addr, >> + pte_t *ptep, pte_t pte) >> +{ >> + /* >> + * We have already checked that the virtual and pysical addresses are >> + * correctly aligned for a contpte mapping in contpte_try_fold() so the >> + * remaining checks are to ensure that the contpte range is fully >> + * covered by a single folio, and ensure that all the ptes are valid >> + * with contiguous PFNs and matching prots. We ignore the state of the >> + * access and dirty bits for the purpose of deciding if its a contiguous >> + * range; the folding process will generate a single contpte entry which >> + * has a single access and dirty bit. Those 2 bits are the logical OR of >> + * their respective bits in the constituent pte entries. In order to >> + * ensure the contpte range is covered by a single folio, we must >> + * recover the folio from the pfn, but special mappings don't have a >> + * folio backing them. Fortunately contpte_try_fold() already checked >> + * that the pte is not special - we never try to fold special mappings. >> + * Note we can't use vm_normal_page() for this since we don't have the >> + * vma. >> + */ >> + >> + struct page *page = pte_page(pte); >> + struct folio *folio = page_folio(page); >> + unsigned long folio_saddr = addr - (page - &folio->page) * PAGE_SIZE; >> + unsigned long folio_eaddr = folio_saddr + folio_nr_pages(folio) * PAGE_SIZE; >> + unsigned long cont_saddr = ALIGN_DOWN(addr, CONT_PTE_SIZE); >> + unsigned long cont_eaddr = cont_saddr + CONT_PTE_SIZE; >> + unsigned long pfn; >> + pgprot_t prot; >> + pte_t subpte; >> + pte_t *orig_ptep; >> + int i; >> + >> + if (folio_saddr > cont_saddr || folio_eaddr < cont_eaddr) >> + return; >> + >> + pfn = pte_pfn(pte) - ((addr - cont_saddr) >> PAGE_SHIFT); >> + prot = pte_pgprot(pte_mkold(pte_mkclean(pte))); >> + orig_ptep = ptep; >> + ptep = contpte_align_down(ptep); >> + >> + for (i = 0; i < CONT_PTES; i++, ptep++, pfn++) { >> + subpte = __ptep_get(ptep); >> + subpte = pte_mkold(pte_mkclean(subpte)); >> + >> + if (!pte_valid(subpte) || >> + pte_pfn(subpte) != pfn || >> + pgprot_val(pte_pgprot(subpte)) != pgprot_val(prot)) >> + return; >> + } >> + >> + contpte_fold(mm, addr, orig_ptep, pte, true); >> +} >> +EXPORT_SYMBOL(__contpte_try_fold); >> + >> +void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr, >> + pte_t *ptep, pte_t pte) >> +{ >> + /* >> + * We have already checked that the ptes are contiguous in >> + * contpte_try_unfold(), so we can unfold unconditionally here. >> + */ >> + >> + contpte_fold(mm, addr, ptep, pte, false); > > I'm still working my way through the series but Thanks for taking the time to review! > calling a fold during an > unfold stood out as it seemed wrong. Obviously further reading revealed > the boolean flag that changes the functions meaning but I think it would > be better to refactor that. Yes that sounds reasonable. > > We could easily rename contpte_fold() to eg. set_cont_ptes() and factor > the pte calculation loop into a separate helper > (eg. calculate_contpte_dirty_young() or some hopefully better name) > called further up the stack. That has an added benefit of providing a > spot to add the nice comment for young/dirty rules you provided in the > patch description ;-) > > In other words we'd have something like: > > void __contpte_try_unfold() { > pte = calculate_contpte_dirty_young(mm, addr, ptep, pte); > pte = pte_mknoncont(pte); > set_cont_ptes(mm, addr, ptep, pte); > } My concern with this approach is that calculate_contpte_dirty_young() has side effects; it has to clear each PTE as it loops through it prevent a race between our reading access/dirty and another thread causing access/dirty to be set. So its not just a "calculation", its the teardown portion of the process too. I guess its a taste thing, so happy for it to be argued the other way, but I would prefer to keep it all together in one function. How about renaming contpte_fold() to contpte_convert() or contpte_repaint() (other suggestions welcome), and extracting the pte_mkcont()/pte_mknoncont() part (so we can remove the bool param): void __contpte_try_unfold() { pte = pte_mknoncont(pte); contpte_convert(mm, addr, ptep, pte); } Thanks, Ryan > > Which IMHO is more immediately understandable. > > - Alistair >