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

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Message-ID: <Yww0NV54DXTzPG+R@hyeyoo>
Date:   Mon, 29 Aug 2022 12:36:21 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     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, vbabka@...e.cz,
        roman.gushchin@...ux.dev, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: simplify size2index conversion of __kmalloc_index

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?

-- 
Thanks,
Hyeonggon