linux-kernel - Re: [PATCH] mm: kmalloc_index: remove case when size is more than 32MB

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Date:   Sun, 9 May 2021 14:33:38 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, vbabka@...e.cz, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: kmalloc_index: remove case when size is more than 32MB

On Sun, May 09, 2021 at 12:19:40AM +0100, Matthew Wilcox wrote:
> On Sun, May 09, 2021 at 07:13:28AM +0900, Hyeonggon Yoo wrote:
> > the return value of kmalloc_index is used as index of kmalloc_caches,
>
> it doesn't matter.  every few weeks somebody posts a patch to "optimise"
> kmalloc_index, failing to appreciate that it's only ever run at compile
> time because it's all under __builtin_constant_p().

Oh thanks, I didn't know about __builtin_constant_p.

But I was not optimizing kmalloc_index. isn't it confusing that
kmalloc_caches alllows maximum size of 32MB, and kmalloc_index allows
maximum size of 64MB?

and even if the code I removed is never reached because 64MB is always
bigger than KMALLOC_MAX_CACHE_SIZE, it will cause an error if reached.