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David Alan Gilbert" , "Matthew Wilcox (Oracle)" , Vlastimil Babka , Baolin Wang , Miaohe Lin , Yang Shi , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH v2 08/47] hugetlb: add HGM enablement functions Message-ID: References: <20221021163703.3218176-1-jthoughton@google.com> <20221021163703.3218176-9-jthoughton@google.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-ClientProxiedBy: MW4PR04CA0087.namprd04.prod.outlook.com (2603:10b6:303:6b::32) To BY5PR10MB4196.namprd10.prod.outlook.com (2603:10b6:a03:20d::23) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BY5PR10MB4196:EE_|DS0PR10MB7224:EE_ X-MS-Office365-Filtering-Correlation-Id: 3079d202-4263-468f-e28e-08dadec50f6b X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: 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 Gl7fB5oi xllVkzTiLqIQCAtXOlSNM39VuAlic3S5nlVABapRoy5epVN4BXtHKfbYIoQ== 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 12/13/22 10:49, James Houghton wrote: > On Mon, Dec 12, 2022 at 7:14 PM Mike Kravetz wrote: > > > > On 10/21/22 16:36, James Houghton wrote: > > > Currently it is possible for all shared VMAs to use HGM, but it must be > > > enabled first. This is because with HGM, we lose PMD sharing, and page > > > table walks require additional synchronization (we need to take the VMA > > > lock). > > > > Not sure yet, but I expect Peter's series will help with locking for > > hugetlb specific page table walks. > > It should make things a little bit cleaner in this series; I'll rebase > HGM on top of those patches this week (and hopefully get a v1 out > soon). > > I don't think it's possible to implement MADV_COLLAPSE with RCU alone > (as implemented in Peter's series anyway); we still need the VMA lock. As I continue going through the series, I realize that I am not exactly sure what synchronization by the vma lock is required by HGM. As you are aware, it was originally designed to protect against someone doing a pmd_unshare and effectively removing part of the page table. However, since pmd sharing is disabled for vmas with HGM enabled (I think?), then it might be a good idea to explicitly say somewhere the reason for using the lock. -- Mike Kravetz