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Date: Tue, 30 Aug 2022 15:13:19 +0200 From: Vlastimil Babka <vbabka@...e.cz> To: Christophe Leroy <christophe.leroy@...roup.eu>, Hyeonggon Yoo <42.hyeyoo@...il.com>, Matthew Wilcox <willy@...radead.org> Cc: Dawei Li <set_pte_at@...look.com>, "cl@...ux.com" <cl@...ux.com>, "penberg@...nel.org" <penberg@...nel.org>, "rientjes@...gle.com" <rientjes@...gle.com>, "iamjoonsoo.kim@....com" <iamjoonsoo.kim@....com>, "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, "roman.gushchin@...ux.dev" <roman.gushchin@...ux.dev>, "linux-mm@...ck.org" <linux-mm@...ck.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] mm: simplify size2index conversion of __kmalloc_index 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.
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