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Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.1.2 Subject: Re: [PATCH] mm: simplify size2index conversion of __kmalloc_index Content-Language: en-US To: Christophe Leroy , Hyeonggon Yoo <42.hyeyoo@gmail.com>, Matthew Wilcox Cc: Dawei Li , "cl@linux.com" , "penberg@kernel.org" , "rientjes@google.com" , "iamjoonsoo.kim@lge.com" , "akpm@linux-foundation.org" , "roman.gushchin@linux.dev" , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" References: <1188aeeb-3949-b561-bec0-512ed763d857@suse.cz> From: Vlastimil Babka In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1661865203; a=rsa-sha256; cv=none; b=e2tpmnPn52/K6FH4wWFWGa5lSQUHLnLpNoyBXFoogudEhkeBxGFfw/oEW2u7GZnuHgAwM6 /A9hZqDtVBtai78yHXlRGlwn47K1D2G/J2Hr7714da3nK6C6ohP90PMwlI3/5LJTEXBvXM fvQspH8k7vRmwx0LX1PJpsemD8TcM6w= ARC-Authentication-Results: i=1; imf06.hostedemail.com; dkim=pass header.d=suse.cz header.s=susede2_rsa header.b=vZbDpC6p; 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dkim=pass header.d=suse.cz header.s=susede2_ed25519 header.b=GGYXRXD2; dmarc=none; spf=pass (imf06.hostedemail.com: domain of vbabka@suse.cz designates 195.135.220.28 as permitted sender) smtp.mailfrom=vbabka@suse.cz X-HE-Tag: 1661865201-129972 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 8/30/22 07:51, Christophe Leroy wrote: > > > Le 29/08/2022 à 16:21, Vlastimil Babka a écrit : >> On 8/29/22 05:36, Hyeonggon Yoo wrote: >>> On Mon, Aug 29, 2022 at 04:11:04AM +0100, Matthew Wilcox wrote: >>>> On Sun, Aug 28, 2022 at 11:14:48PM +0800, Dawei Li wrote: >>>>> Current size2index is implemented by one to one hardcode mapping, >>>>> which can be improved by order_base_2(). >>>>> Must be careful to not violate compile-time optimization rule. >>>> >>>> This patch has been NACKed before (when submitted by other people). >>> >>> >>> Hmm right. >>> https://lkml.iu.edu/hypermail/linux/kernel/1606.2/05402.html >>> >>> Christoph Lameter wrote: >>>> On Wed, 22 Jun 2016, Yury Norov wrote: >>>>> There will be no fls() for constant at runtime because ilog2() calculates >>>>> constant values at compile-time as well. From this point of view, >>>>> this patch removes code duplication, as we already have compile-time >>>>> log() calculation in kernel, and should re-use it whenever possible.\ >>> >>>> The reason not to use ilog there was that the constant folding did not >>>> work correctly with one or the other architectures/compilers. If you want >>>> to do this then please verify that all arches reliably do produce a >>>> constant there. >>> >>> Can we re-evaluate this? >> >> Is there a way to turn inability of compile-time calculation to a >> compile-time error? (when size_is_constant=true etc). Then we could try and >> see if anything breaks in -next. >> >> > > The following will generate a build error if the function > constant_check() is not called with a buildtime constant argument. > > static void __always_inline constant_check(unsigned long val) > { > BUILD_BUG_ON(!__builtin_constant_p(val)); > } > > Is that what you are looking for ? Maybe, if we can rely on these two being equivalent: - __kmalloc_index(x) is evaluated compile-time - __builtin_constant_p(__kmalloc_index(x)) is true Logically such equivalency should be expected, and a quick attempt locally with recent gcc seems to work fine, but I guess we'll have to try in -next for a bit and see if anything comes out.