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Tue, 17 Aug 2021 01:04:54 +0000 Received: from BY5PR10MB4196.namprd10.prod.outlook.com ([fe80::8d2a:558d:ab4a:9c2a]) by BY5PR10MB4196.namprd10.prod.outlook.com ([fe80::8d2a:558d:ab4a:9c2a%7]) with mapi id 15.20.4415.023; Tue, 17 Aug 2021 01:04:54 +0000 Subject: Re: [PATCH 5/8] hugetlb: document the demote sysfs interfaces To: Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, David Hildenbrand , Michal Hocko , Oscar Salvador , Zi Yan , Muchun Song , Naoya Horiguchi , David Rientjes References: <20210816224953.157796-1-mike.kravetz@oracle.com> <20210816224953.157796-6-mike.kravetz@oracle.com> <20210816162829.a228c4629b411900702b7f5f@linux-foundation.org> From: Mike Kravetz Message-ID: Date: Mon, 16 Aug 2021 18:04:52 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 In-Reply-To: <20210816162829.a228c4629b411900702b7f5f@linux-foundation.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW2PR16CA0020.namprd16.prod.outlook.com (2603:10b6:907::33) To BY5PR10MB4196.namprd10.prod.outlook.com (2603:10b6:a03:20d::23) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.2.123] (50.38.35.18) by MW2PR16CA0020.namprd16.prod.outlook.com (2603:10b6:907::33) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4415.15 via Frontend Transport; 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dkim=pass header.d=oracle.com header.s=corp-2021-07-09 header.b=XhYkqT9l; dkim=pass header.d=oracle.com header.s=corp-2020-01-29 header.b=x3hD1kCc; dkim=pass header.d=oracle.onmicrosoft.com header.s=selector2-oracle-onmicrosoft-com header.b=qr+Jm0d5; spf=none (imf10.hostedemail.com: domain of mike.kravetz@oracle.com has no SPF policy when checking 205.220.177.32) smtp.mailfrom=mike.kravetz@oracle.com; dmarc=pass (policy=none) header.from=oracle.com X-Rspamd-Server: rspam01 X-Stat-Signature: 1667j7tcpah3r6j197p4876kd9m9u35x X-HE-Tag: 1629162299-551881 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: On 8/16/21 4:28 PM, Andrew Morton wrote: > On Mon, 16 Aug 2021 15:49:50 -0700 Mike Kravetz wrote: > >> Describe demote and demote_size interfaces. >> >> Signed-off-by: Mike Kravetz >> --- >> Documentation/admin-guide/mm/hugetlbpage.rst | 29 ++++++++++++++++++-- >> 1 file changed, 27 insertions(+), 2 deletions(-) >> >> diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst >> index 8abaeb144e44..902059a0257b 100644 >> --- a/Documentation/admin-guide/mm/hugetlbpage.rst >> +++ b/Documentation/admin-guide/mm/hugetlbpage.rst >> @@ -234,8 +234,12 @@ will exist, of the form:: >> >> hugepages-${size}kB >> >> -Inside each of these directories, the same set of files will exist:: >> +Inside each of these directories, the set of files contained in ``/proc`` >> +will exist. In addition, two additional interfaces for demoting huge >> +pages will exist:: >> >> + demote >> + demote_size >> nr_hugepages >> nr_hugepages_mempolicy >> nr_overcommit_hugepages >> @@ -243,7 +247,28 @@ Inside each of these directories, the same set of files will exist:: >> resv_hugepages >> surplus_hugepages >> >> -which function as described above for the default huge page-sized case. >> +The demote interfaces provide the ability to split a huge page into >> +smaller huge pages. For example, the x86 architecture supports both >> +1GB and 2MB huge pages sizes. A 1GB huge page can be split into 512 >> +2MB huge pages. The demote interfaces are: >> + >> +demote_size >> + is the size of demoted pages. When a page is demoted a corresponding >> + number of huge pages of demote_size will be created. For huge pages >> + of the smallest supported size (2MB on x86), demote_size will be the >> + system page size (PAGE_SIZE). If demote_size is the system page size >> + then demoting a page will simply free the huge page. demote_size is >> + a read only interface. >> + >> +demote >> + is used to demote a number of huge pages. A user with root privileges >> + can write to this file. It may not be possible to demote the >> + requested number of huge pages. To determine how many pages were >> + actually demoted, compare the value of nr_hugepages before and after >> + writing to the demote interface. demote is a write only interface. >> + >> +The interfaces which are the same as in ``/proc`` function as described >> +above for the default huge page-sized case. > > Are these new demote interfaces duplicated in /proc? > Documentation/admin-guide/mm/hugetlbpage.rst says "The ``/proc`` > interfaces discussed above have been retained for backwards > compatibility.", so new interfaces need not appear in /proc? > The new demote interfaces are only in sysfs, they are not /proc. -- Mike Kravetz