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Message-ID: <e0c127f9e80146c19fab9f987bb2f588@AcuMS.aculab.com> Date: Fri, 7 Oct 2022 09:14:54 +0000 From: David Laight <David.Laight@...LAB.COM> To: 'Christophe Leroy' <christophe.leroy@...roup.eu>, "Jason A. Donenfeld" <Jason@...c4.com> CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "patches@...ts.linux.dev" <patches@...ts.linux.dev>, Andreas Noever <andreas.noever@...il.com>, Andrew Morton <akpm@...ux-foundation.org>, "Andy Shevchenko" <andriy.shevchenko@...ux.intel.com>, Borislav Petkov <bp@...en8.de>, Catalin Marinas <catalin.marinas@....com>, Christoph Böhmwalder <christoph.boehmwalder@...bit.com>, Christoph Hellwig <hch@....de>, Daniel Borkmann <daniel@...earbox.net>, "Dave Airlie" <airlied@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>, "David S . Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Florian Westphal <fw@...len.de>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "H . Peter Anvin" <hpa@...or.com>, "Heiko Carstens" <hca@...ux.ibm.com>, Helge Deller <deller@....de>, Herbert Xu <herbert@...dor.apana.org.au>, Huacai Chen <chenhuacai@...nel.org>, "Hugh Dickins" <hughd@...gle.com>, Jakub Kicinski <kuba@...nel.org>, "James E . J . Bottomley" <jejb@...ux.ibm.com>, Jan Kara <jack@...e.com>, Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>, Johannes Berg <johannes@...solutions.net>, Jonathan Corbet <corbet@....net>, "Jozsef Kadlecsik" <kadlec@...filter.org>, KP Singh <kpsingh@...nel.org>, Kees Cook <keescook@...omium.org>, Marco Elver <elver@...gle.com>, "Mauro Carvalho Chehab" <mchehab@...nel.org>, Michael Ellerman <mpe@...erman.id.au>, "Pablo Neira Ayuso" <pablo@...filter.org>, Paolo Abeni <pabeni@...hat.com>, "Peter Zijlstra" <peterz@...radead.org>, Richard Weinberger <richard@....at>, "Russell King" <linux@...linux.org.uk>, Theodore Ts'o <tytso@....edu>, "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>, Thomas Gleixner <tglx@...utronix.de>, Thomas Graf <tgraf@...g.ch>, Ulf Hansson <ulf.hansson@...aro.org>, Vignesh Raghavendra <vigneshr@...com>, WANG Xuerui <kernel@...0n.name>, Will Deacon <will@...nel.org>, Yury Norov <yury.norov@...il.com>, "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>, "kasan-dev@...glegroups.com" <kasan-dev@...glegroups.com>, "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>, "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>, "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>, "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>, "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>, "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>, "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>, "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>, "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>, "linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>, "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>, "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>, "linux-um@...ts.infradead.org" <linux-um@...ts.infradead.org>, "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>, "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>, "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>, "loongarch@...ts.linux.dev" <loongarch@...ts.linux.dev>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>, "x86@...nel.org" <x86@...nel.org>, Toke Høiland-Jørgensen <toke@...e.dk>, Chuck Lever <chuck.lever@...cle.com>, Jan Kara <jack@...e.cz> Subject: RE: [PATCH v3 3/5] treewide: use get_random_u32() when possible From: Christophe Leroy > Sent: 06 October 2022 18:43 ... > But taking into account that sp must remain 16 bytes aligned, would it > be better to do something like ? > > sp -= prandom_u32_max(PAGE_SIZE >> 4) << 4; That makes me think... If prandom_u32_max() is passed a (constant) power of 2 it doesn't need to do the multiply, it can just do a shift right. Doesn't it also always get a 32bit random value? So actually get_random_u32() & PAGE_MASK & ~0xf is faster! When PAGE_SIZE is 4k, PAGE_SIZE >> 4 is 256 so it could use: get_ramdom_u8() << 4 You also seem to have removed prandom_u32() in favour of get_random_u32() but have added more prandom_xxxx() functions. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
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