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imf27.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=arm.com; spf=pass (imf27.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=1700642163; a=rsa-sha256; cv=none; b=dyV881Zri55SY4PxiSvnoEnPEPnbmumU3tlDISqH4ZJ+sa/r10vXbG26eZDsJmenNiK4da Lm9SoPs/Oac/65rvdgfZmD8VY1xmszeYOcLo70+nLzVef3YxgqYYNRlxPaCWwzDQ+JPR9O gQW2U8VobrFaDiXGD7XR92Xip+8wVJw= 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 BC36B1595; Wed, 22 Nov 2023 00:36:48 -0800 (PST) Received: from [10.57.72.55] (unknown [10.57.72.55]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 857E83F73F; Wed, 22 Nov 2023 00:35:58 -0800 (PST) Message-ID: <11d8391a-96ad-4d91-a761-8675fb0bd509@arm.com> Date: Wed, 22 Nov 2023 08:35:57 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 12/14] arm64/mm: Wire up PTE_CONT for user mappings 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> <87il5umijf.fsf@nvdebian.thelocal> Content-Language: en-GB From: Ryan Roberts In-Reply-To: <87il5umijf.fsf@nvdebian.thelocal> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 0F5CF40016 X-Stat-Signature: qndff96foedqk61jaf5933194rn5p9yr X-HE-Tag: 1700642162-74779 X-HE-Meta: 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 uE460e3B w8vx4yTt3AyRWwu/ykSEOVOhW3kEOTbPgbvbovzPrZn01cWN0F59XWJJbMmdxPSLm60YMSKyA8FkKwI6Fko1cnXPfEI6MPb3cXDv0+PkJWx+Yqy4OKS1/5txNXBm70Iw9xA/SRkNAqTI1RvEM8N6FFsg6ZRecaUZ8tLc3qSukT/CGkgGi63d5VwlVWQaUs0SosJSZdPvvshI6Cwbobnc/GZHgWg02ruO+217SE6oaDOIm86pgqgzeK4Ze8GMVG1iK5IUtsEM2qJVNOCIJEQu/+3/MEN+FN6NRsqkabjxnVyo3i6QvR7RxMoMbhgQY+MGGR00IrK18aehCcknQZ1yGpIrx3MedBCDFXad92aJ4Pl+7TAI= 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 22/11/2023 06:01, Alistair Popple wrote: > > Ryan Roberts writes: > >> 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. That works for me, although sadly I don't have any better ideas > for names atm. Thanks - I'll make this change for v3 and go with contpte_convert(). > > - Alistair > >> Thanks, >> Ryan >> >>> >>> Which IMHO is more immediately understandable. >>> >>> - Alistair >>> >