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

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Date:   Mon, 10 May 2021 12:09:55 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: kmalloc_index: remove case when size is more than
 32MB

On 5/9/21 7:33 AM, Hyeonggon Yoo wrote:
> 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.

KMALLOC_MAX_CACHE_SIZE depends on KMALLOC_SHIFT_HIGH
size of kmalloc_caches array depends on KMALLOC_SHIFT_HIGH

So I don't an easy way how it could become reachable while causing the index to
overflow - if someone increased KMALLOC_SHIFT_HIGH from 25 to 26, all should be
fine, AFAICS.

The problem would be if someone increased it to 27, then we might suddenly get a
BUG() in kmalloc_index(). We should probably replace that BUG() with
BUILD_BUG_ON(1) to catch that at compile time. Hopefully no supported compiler
will break because it's not able to do the proper compile-time evaluation - but
if it does, at least we would know.

So I would accept the patch if it also changed BUG() to e.g. BUILD_BUG_ON_MSG(1,
"unexpected size in kmalloc_index()");
and expanded the function's comment that this is always compile-time evaluated
and thus no attempts at "optimizing" the code should be made.