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Shutemov" , Andrea Arcangeli , Matthew Wilcox , Oscar Salvador , Joao Martins , stable@vger.kernel.org References: <20210217184926.33567-1-mike.kravetz@oracle.com> <20210217110252.185c7f5cd5a87c3f7b0c0144@linux-foundation.org> From: Mike Kravetz Message-ID: <1da7b36c-f86a-90e9-cfa2-4ce49bd6f1d1@oracle.com> Date: Wed, 17 Feb 2021 11:38:15 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 In-Reply-To: <20210217110252.185c7f5cd5a87c3f7b0c0144@linux-foundation.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [50.38.35.18] X-ClientProxiedBy: MWHPR13CA0026.namprd13.prod.outlook.com (2603:10b6:300:95::12) To BY5PR10MB4196.namprd10.prod.outlook.com (2603:10b6:a03:20d::23) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.2.112] (50.38.35.18) by MWHPR13CA0026.namprd13.prod.outlook.com (2603:10b6:300:95::12) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3868.12 via Frontend Transport; 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The >> routine update_and_free_page can encounter a gigantic page, yet it assumes >> page structs are contiguous when setting page flags in subpages. >> >> If update_and_free_page encounters non-contiguous page structs, we can >> see “BUG: Bad page state in process …” errors. >> >> Non-contiguous page structs are generally not an issue. However, they can >> exist with a specific kernel configuration and hotplug operations. For >> example: Configure the kernel with CONFIG_SPARSEMEM and >> !CONFIG_SPARSEMEM_VMEMMAP. Then, hotplug add memory for the area where the >> gigantic page will be allocated. >> Zi Yan outlined steps to reproduce here [1]. >> >> [1] https://lore.kernel.org/linux-mm/16F7C58B-4D79-41C5-9B64-A1A1628F4AF2@nvidia.com/ >> >> Fixes: 944d9fec8d7a ("hugetlb: add support for gigantic page allocation at runtime") > > June 2014. That's a long lurk time for a bug. I wonder if some later > commit revealed it. > > I guess it doesn't matter a lot, but some -stable kernel maintainers > might wonder if they really need this fix... I am not sure how common a CONFIG_SPARSEMEM and !CONFIG_SPARSEMEM_VMEMMAP config is. On the more popular architectures, this is not the default. But, you can build a kernel with such options. And, then you need to hotplug memory add and allocate a gigantic page there. It is unlikely to happen, but possible since Zi could force the BUG. The copy_huge_page_from_user bug requires the same non-normal configuration and is just as unlikely to occurr. But, since it can overwrite somewhat random pages I would feel better if it was fixed. -- Mike Kravetz