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

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Message-ID: <1188aeeb-3949-b561-bec0-512ed763d857@suse.cz>
Date:   Mon, 29 Aug 2022 16:21:07 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Dawei Li <set_pte_at@...look.com>, cl@...ux.com,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, roman.gushchin@...ux.dev,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: simplify size2index conversion of __kmalloc_index

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.