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Received: from viro by zeniv.linux.org.uk with local (Exim 4.99.4 #2 (Red Hat Linux)) id 1wcBn2-00000001S2J-26im; Wed, 24 Jun 2026 00:48:28 +0000 Date: Wed, 24 Jun 2026 01:48:28 +0100 From: Al Viro To: "Vlastimil Babka (SUSE)" Cc: linux-mm@kvack.org, Vlastimil Babka , Harry Yoo , linux-fsdevel@vger.kernel.org, Linus Torvalds , Christian Brauner , Jan Kara , Mateusz Guzik , linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH v3 00/10] kmem_cache instances with static storage duration Message-ID: <20260624004828.GI2636677@ZenIV> References: <20260611171425.1671254-1-viro@zeniv.linux.org.uk> <20260613050951.855141-1-viro@zeniv.linux.org.uk> <151b1c96-0ed5-44ce-b9d5-86a48d8ad592@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <151b1c96-0ed5-44ce-b9d5-86a48d8ad592@kernel.org> X-Rspamd-Queue-Id: B17B740007 X-Stat-Signature: jmiddm7rfu3mqfmu3gdqotherz38d1c8 X-Rspam-User: X-Rspamd-Server: rspam03 X-HE-Tag: 1782262112-387713 X-HE-Meta: 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 8KfCleH6 1kJ9jeCmxDpLNrDNbmHObAcPOGy1Lh5eFPNESrkr8BoFhnQ1Nr+mG8zrkfIXSOE437KzR8hSvRPFw1dY8jGVdhQzvn2RKZJ95ncPsc5W18imoy8nXpg+2oaHZzFvstYd1RJwmvQP6UXtjVQdIUCGIuOA3curYSIKXStQ63N+kkx7HUMHkng/Hkt2W2vpcp9M2nyZ9K4hpv21R4ojWOd8Lsuxo+rY0sZcDmcEA/u5P89/fFAXhk9zYYl8wN5ikxLWviWNPREZFY7npxjxBqEPr4ESj67fqvXxYl2rUkFckzaK+maqe/ODc1phIN3faY72GbjWWS8W/yDcw3O3qGKnW1VAZ0ogeLFA6d5NWVX9Zf0wZhmNvx+cMxzl7g0phh6uZXem9W8LULWZk+f3NZ9k38auRt7xcq7dbcHYjHvaO34ITPrLUiwa+UWfCTGlYY/SOwBt6qGLqizpZnneKj1nh22vAJ4G4VI3kVKPCce0OKHfFprI= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Jun 23, 2026 at 10:09:41AM +0200, Vlastimil Babka (SUSE) wrote: > But the argument for doing the static duration support is that it should be > faster, not just "not slower"? So is runtime_const equivalent or for some > fundamental reason it's slower than plain &? Yes, on any 64bit RISC. And if nothing else, arm64 has enough users to care about. runtime_const for _shifts_ tends to be nice and easy; for pointers... not really. Compiler does *not* build the address of global variable in a sequence of shifts and bitwise operations when it needs to pass it to a function. runtime_const_ptr() must be able to handle an arbitrary address; it can't avoid doing the general "build a 64bit value in register", which tends to be nasty on RISC. If you want real ugliness, take a look at riscv - AFAICS, they wanted to avoid a long dependency chain, so they load chunks of constant into 4 registers, then shift and combine those. arm64 doesn't bother; IIRC, the hardware does recognize this sequence, but I wouldn't swear on that. > Or is the advantage that static caches can support modules and runtime_const > can't? Probably, but I'm not ready to give examples at the moment. There definitely used to be __getname() users in modules, but that stopped using names_cache.