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Fri, 6 Sep 2024 08:14:20 +0000 Received: from SJ0PR10MB5613.namprd10.prod.outlook.com ([fe80::4239:cf6f:9caa:940e]) by SJ0PR10MB5613.namprd10.prod.outlook.com ([fe80::4239:cf6f:9caa:940e%5]) with mapi id 15.20.7939.016; Fri, 6 Sep 2024 08:14:20 +0000 Date: Fri, 6 Sep 2024 09:14:17 +0100 From: Lorenzo Stoakes To: Arnd Bergmann Cc: Charlie Jenkins , Richard Henderson , Ivan Kokshaysky , Matt Turner , Vineet Gupta , Russell King , guoren , Huacai Chen , WANG Xuerui , Thomas Bogendoerfer , "James E . J . Bottomley" , Helge Deller , Michael Ellerman , Nicholas Piggin , Christophe Leroy , Naveen N Rao , Alexander Gordeev , Gerald Schaefer , Heiko Carstens , Vasily Gorbik , Christian Borntraeger , Sven Schnelle , Yoshinori Sato , Rich Felker , John Paul Adrian Glaubitz , "David S . Miller" , Andreas Larsson , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Andy Lutomirski , Peter Zijlstra , Muchun Song , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , shuah , Christoph Hellwig , Michal Hocko , "Kirill A. Shutemov" , Chris Torek , Linux-Arch , linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org, linux-snps-arc@lists.infradead.org, linux-arm-kernel@lists.infradead.org, "linux-csky@vger.kernel.org" , loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-abi-devel@lists.sourceforge.net Subject: Re: [PATCH RFC v3 1/2] mm: Add personality flag to limit address to 47 bits Message-ID: <58f39d58-579e-4dd3-8084-baebf86f1ae0@lucifer.local> References: <20240905-patches-below_hint_mmap-v3-0-3cd5564efbbb@rivosinc.com> <20240905-patches-below_hint_mmap-v3-1-3cd5564efbbb@rivosinc.com> <9fc4746b-8e9d-4a75-b966-e0906187e6b7@app.fastmail.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9fc4746b-8e9d-4a75-b966-e0906187e6b7@app.fastmail.com> X-ClientProxiedBy: LO0P265CA0015.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:355::6) To SJ0PR10MB5613.namprd10.prod.outlook.com (2603:10b6:a03:3d0::5) Precedence: bulk X-Mailing-List: linux-parisc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ0PR10MB5613:EE_|SJ0PR10MB5647:EE_ X-MS-Office365-Filtering-Correlation-Id: 9f683e53-a5ea-4b1d-bd88-08dcce4be8f7 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|7416014|366016; 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This > > personality can be set to cause all allocations to be below the 47-bit > > boundary. Using MAP_FIXED with mmap() will bypass this restriction. > > > > Signed-off-by: Charlie Jenkins > > I think having an architecture-independent mechanism to limit the size > of the 64-bit address space is useful in general, and we've discussed > the same thing for arm64 in the past, though we have not actually > reached an agreement on the ABI previously. The thread on the original proposals attests to this being rather a fraught topic, and I think the weight of opinion was more so in favour of opt-in rather than opt-out. > > > @@ -22,6 +22,7 @@ enum { > > WHOLE_SECONDS = 0x2000000, > > STICKY_TIMEOUTS = 0x4000000, > > ADDR_LIMIT_3GB = 0x8000000, > > + ADDR_LIMIT_47BIT = 0x10000000, > > }; > > I'm a bit worried about having this done specifically in the > personality flag bits, as they are rather limited. We obviously > don't want to add many more such flags when there could be > a way to just set the default limit. Since I'm the one who suggested it, I feel I should offer some kind of vague defence here :) We shouldn't let perfect be the enemy of the good. This is a relatively straightforward means of achieving the aim (assuming your concern about arch_get_mmap_end() below isn't a blocker) which has the least impact on existing code. Of course we can end up in absurdities where we start doing ADDR_LIMIT_xxBIT... but again - it's simple, shouldn't represent an egregious maintenance burden and is entirely opt-in so has things going for it. > > It's also unclear to me how we want this flag to interact with > the existing logic in arch_get_mmap_end(), which attempts to > limit the default mapping to a 47-bit address space already. How does ADDR_LIMIT_3GB presently interact with that? > > For some reason, it appears that the arch_get_mmap_end() > logic on RISC-V defaults to the maximum address > space for the 'addr==0' case which is inconsistentn with > the other architectures, so we should probably fix that > part first, possibly moving more of that logic into a > shared implementation. > > Arnd