linux-kernel - Re: [PATCH] mm: simplify size2index conversion of __kmalloc

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Message-ID: <f02b862f-8e8c-0ec9-7b83-c9e26831c242@csgroup.eu>
Date:   Tue, 30 Aug 2022 05:51:06 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Vlastimil Babka <vbabka@...e.cz>,
        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



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 ?