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Mon, 20 Mar 2023 13:39:03 +0000 Received: from CO1PR11MB4820.namprd11.prod.outlook.com ([fe80::8073:f55d:5f64:7c6]) by CO1PR11MB4820.namprd11.prod.outlook.com ([fe80::8073:f55d:5f64:7c6%8]) with mapi id 15.20.6178.037; Mon, 20 Mar 2023 13:39:03 +0000 Message-ID: <483fd440-df7b-fab3-b138-f3789f2dc078@intel.com> Date: Mon, 20 Mar 2023 21:38:55 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Firefox/102.0 Thunderbird/102.9.0 Subject: Re: [PATCH v4 35/36] mm: Convert do_set_pte() to set_pte_range() Content-Language: en-US To: Matthew Wilcox CC: Ryan Roberts , , , , References: <20230315051444.3229621-1-willy@infradead.org> <20230315051444.3229621-36-willy@infradead.org> <6dd5cdf8-400e-8378-22be-994f0ada5cc2@arm.com> <2fa5a911-8432-2fce-c6e1-de4e592219d8@arm.com> From: "Yin, Fengwei" In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SG2P153CA0052.APCP153.PROD.OUTLOOK.COM (2603:1096:4:c6::21) To CO1PR11MB4820.namprd11.prod.outlook.com (2603:10b6:303:6f::8) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CO1PR11MB4820:EE_|CO1PR11MB5090:EE_ X-MS-Office365-Filtering-Correlation-Id: 4512ea2f-0145-4bd0-8271-08db29487834 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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Previously this >>>>>> would be evaluated per page, now its evaluated once for the whole range. The >>>>>> intention below is that directly faulted pages are mapped young and prefaulted >>>>>> pages are mapped old. But now a whole range will be mapped the same. >>>>> >>>>> Yes. You are right here. >>>>> >>>>> Look at the prefault and cpu_has_hw_af for ARM64, it looks like we >>>>> can avoid to handle vmf->address == addr specially. It's OK to >>>>> drop prefault and change the logic here a little bit to: >>>>> if (arch_wants_old_prefaulted_pte()) >>>>> entry = pte_mkold(entry); >>>>> else >>>>> entry = pte_sw_mkyong(entry); >>>>> >>>>> It's not necessary to use pte_sw_mkyong for vmf->address == addr >>>>> because HW will set the ACCESS bit in page table entry. >>>>> >>>>> Add Will Deacon in case I missed something here. Thanks. >>>> >>>> I'll defer to Will's response, but not all arm HW supports HW access flag >>>> management. In that case it's done by SW, so I would imagine that by setting >>>> this to old initially, we will get a second fault to set the access bit, which >>>> will slow things down. I wonder if you will need to split this into (up to) 3 >>>> calls to set_ptes()? >>> >>> I don't think we should do that. The limited information I have from >>> various microarchitectures is that the PTEs must differ only in their >>> PFN bits in order to use larger TLB entries. That includes the Accessed >>> bit (or equivalent). So we should mkyoung all the PTEs in the same >>> folio, at least initially. >>> >>> That said, we should still do this conditionally. We'll prefault some >>> other folios too. So I think this should be: >>> >>> bool prefault = (addr > vmf->address) || ((addr + nr) < vmf->address); >>> >> According to commit 46bdb4277f98e70d0c91f4289897ade533fe9e80, if hardware access >> flag is supported on ARM64, there is benefit if prefault PTEs is set as "old". >> If we change prefault like above, the PTEs is set as "yong" which loose benefit >> on ARM64 with hardware access flag. >> >> ITOH, if from "old" to "yong" is cheap, why not leave all PTEs of folio as "old" >> and let hardware to update it to "yong"? > > Because we're tracking the entire folio as a single entity. So we're > better off avoiding the extra pagefaults to update the accessed bit, > which won't actually give us any information (vmscan needs to know "were > any of the accessed bits set", not "how many of them were set"). > > Anyway, hopefully Ryan can test this and let us know if it fixes the > regression he sees. Thanks a lot to Ryan for helping to test the debug patch I made. Ryan confirmed that the following change could fix the kernel build regression: diff --git a/mm/filemap.c b/mm/filemap.c index db86e459dde6..343d6ff36b2c 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -3557,7 +3557,7 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf, ret |= filemap_map_folio_range(vmf, folio, xas.xa_index - folio->index, addr, nr_pages); - xas.xa_index += nr_pages; + xas.xa_index += folio_test_large(folio) ? nr_pages : 0; folio_unlock(folio); folio_put(folio); I will make upstream-able change as "xas.xa_index += nr_pages - 1;" Ryan and I also identify some other changes needed. I am not sure how to integrate those changes to this series. Maybe an add-on patch after this series? Thanks. Regards Yin, Fengwei