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Fri, 14 May 2021 17:56:39 +0000 Subject: Re: [PATCH] mm, hugetlb: fix resv_huge_pages underflow on UFFDIO_COPY To: Peter Xu Cc: Mina Almasry , Axel Rasmussen , Linux-MM , Andrew Morton , open list References: <20210513234309.366727-1-almasrymina@google.com> <09dc0712-48e8-8ba2-f170-4c2febcfff83@oracle.com> From: Mike Kravetz Message-ID: Date: Fri, 14 May 2021 10:56:36 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [50.38.35.18] X-ClientProxiedBy: CO1PR15CA0101.namprd15.prod.outlook.com (2603:10b6:101:21::21) 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 CO1PR15CA0101.namprd15.prod.outlook.com (2603:10b6:101:21::21) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4108.25 via Frontend Transport; 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dkim=pass header.d=oracle.com header.s=corp-2020-01-29 header.b=MbBBafb0; dkim=pass header.d=oracle.onmicrosoft.com header.s=selector2-oracle-onmicrosoft-com header.b=t4xFtfBU; dmarc=pass (policy=none) header.from=oracle.com; spf=none (imf03.hostedemail.com: domain of mike.kravetz@oracle.com has no SPF policy when checking 156.151.31.86) smtp.mailfrom=mike.kravetz@oracle.com X-Rspamd-Server: rspam03 X-Stat-Signature: 4qub5r5uwnguy4p86nhfiigndzc6fgwr X-HE-Tag: 1621015004-158768 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 5/14/21 5:31 AM, Peter Xu wrote: > Hi, Mike, > > On Thu, May 13, 2021 at 09:02:15PM -0700, Mike Kravetz wrote: > > [...] > >> I am also concerned with the semantics of this approach and what happens >> when a fault races with the userfaultfd copy. Previously I asked Peter >> if we could/should use a page found in the cache for the copy. His >> answer was as follows: >> >> AFAICT that's the expected behavior, and it need to be like that so as to avoid >> silent data corruption (if the page cache existed, it means the page is not >> "missing" at all, then it does not suite for a UFFDIO_COPY as it's only used >> for uffd page missing case). > > I didn't follow the rest discussion in depth yet... but just to mention that > the above answer was for the question whether we can "update the page in the > page cache", rather than "use a page found in the page cache". > > I think reuse the page should be fine, however it'll definitely break existing > user interface (as it'll expect -EEXIST for now - we have kselftest covers > that), meanwhile I don't see why the -EEXIST bothers a lot: it still tells the > user that this page was filled in already. Normally it was filled in by > another UFFDIO_COPY (as we could have multiple uffd service threads) along with > a valid pte, then this userspace thread can simply skip this message as it > means the event has been handled by some other servicing thread. > > (This also reminded me that there won't be a chance of UFFDIO_COPY race on page > no page fault at least, since no page fault will always go into the uffd > missing handling rather than filling in the page cache for a VM_UFFD_MISSING > vma; while mmap read lock should guarantee VM_UFFD_MISSING be persistent) Perhaps I am missing something. Since this is a shared mapping, can we not have a 'regular' mapping to the same range that is uffd registered? And, that regular mappings could fault and race with the uffd copy code? -- Mike Kravetz