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dkim=none (message not signed) header.d=none;google.com; dmarc=none action=none header.from=oracle.com; Received: from MWHPR10MB1389.namprd10.prod.outlook.com (2603:10b6:300:21::22) by MWHPR10MB2030.namprd10.prod.outlook.com (2603:10b6:300:10d::13) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3846.27; Fri, 12 Feb 2021 21:27:51 +0000 Received: from MWHPR10MB1389.namprd10.prod.outlook.com ([fe80::897d:a360:92db:3074]) by MWHPR10MB1389.namprd10.prod.outlook.com ([fe80::897d:a360:92db:3074%5]) with mapi id 15.20.3846.035; Fri, 12 Feb 2021 21:27:51 +0000 Subject: Re: [PATCH v5 02/10] hugetlb/userfaultfd: Forbid huge pmd sharing when uffd enabled To: Axel Rasmussen , Peter Xu Cc: Alexander Viro , Alexey Dobriyan , Andrea Arcangeli , Andrew Morton , Anshuman Khandual , Catalin Marinas , Chinwen Chang , Huang Ying , Ingo Molnar , Jann Horn , Jerome Glisse , Lokesh Gidra , "Matthew Wilcox (Oracle)" , Michael Ellerman , =?UTF-8?Q?Michal_Koutn=c3=bd?= , Michel Lespinasse , Mike Rapoport , Nicholas Piggin , Shaohua Li , Shawn Anastasio , Steven Rostedt , Steven Price , Vlastimil Babka , LKML , linux-fsdevel@vger.kernel.org, Linux MM , Adam Ruprecht , Cannon Matthews , "Dr . David Alan Gilbert" , David Rientjes , Mina Almasry , Oliver Upton References: <20210210212200.1097784-1-axelrasmussen@google.com> <20210210212200.1097784-3-axelrasmussen@google.com> <0a991b83-18f8-cd76-46c0-4e0dcd5c87a7@oracle.com> <20210212204028.GC3171@xz-x1> From: Mike Kravetz Message-ID: <4a651313-0e14-35da-0223-8805cd1ac1c7@oracle.com> Date: Fri, 12 Feb 2021 13:27:49 -0800 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: MWHPR1701CA0016.namprd17.prod.outlook.com (2603:10b6:301:14::26) To MWHPR10MB1389.namprd10.prod.outlook.com (2603:10b6:300:21::22) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.2.112] (50.38.35.18) by MWHPR1701CA0016.namprd17.prod.outlook.com (2603:10b6:301:14::26) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3846.27 via Frontend Transport; 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>>> #ifdef CONFIG_USERFAULTFD >>> if (uffd_disable_huge_pmd_share(vma) >>> return false; >>> #endif >>> #ifdef /* XXX */ >>> /* add other checks for things like uffd wp and soft dirty here */ >>> #endif /* XXX */ >>> >>> if (range_in_vma(vma, base, end) >>> return true; >>> return false; >>> >>> Of course, this would require we leave the call to vma_shareable() at the >>> beginning of huge_pmd_share. It also means that we are always making a >>> function call into huge_pmd_share to determine if sharing is possible. >>> That is not any different than today. If we do not want to make that extra >>> function call, then I would suggest putting all that code in want_pmd_share. >>> It just seems that all the vma checks for sharing should be in one place >>> if possible. >> >> I don't worry a lot on that since we've already got huge_pte_alloc() which >> takes care of huge pmd sharing case, so I don't expect e.g. even most hugetlb >> developers to use want_pmd_share() at all, because huge_pte_alloc() will be the >> one that frequently got called. >> >> But yeah we can definitely put the check logic into huge_pmd_share() too. >> Looking at above code it looks still worth a helper like want_pmd_share() or >> with some other name. Then... instead of making this complicated, how about I >> mostly keep this patch but move want_pmd_share() call into huge_pmd_share() >> instead? When looking at this again, all I was suggesting was a single routine to check for the possibility of pmd sharing. That is what the version of want_pmd_share in this patch does. I have some patches for future optimizations that only take i_mmap_rwsem in the fault path if sharing is possible. This is before huge_pte_alloc. want_pmd_share as defined in this patch would work for that. Sorry for the noise. -- Mike Kravetz