linux-kernel - Re: [PATCH v2] mm: kmalloc_index: make compiler break when size is not supported

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Message-ID: <cfd5ad12-e589-dcca-52da-6b9db3e26b12@suse.cz>
Date:   Tue, 11 May 2021 10:33:04 +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 v2] mm: kmalloc_index: make compiler break when size is
 not supported

On 5/11/21 5:03 AM, Hyeonggon Yoo wrote:
> On Mon, May 10, 2021 at 04:44:09PM +0100, Matthew Wilcox wrote:
>> On Tue, May 11, 2021 at 12:02:30AM +0900, Hyeonggon Yoo wrote:
>> > @@ -382,8 +385,8 @@ static __always_inline unsigned int kmalloc_index(size_t size)
>> >  	if (size <=  8 * 1024 * 1024) return 23;
>> >  	if (size <=  16 * 1024 * 1024) return 24;
>> >  	if (size <=  32 * 1024 * 1024) return 25;
>> > -	if (size <=  64 * 1024 * 1024) return 26;
>> > -	BUG();
>> > +
>> > +	BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
>> 
>> we're being encouraged to use static_assert() these days.
>> https://en.cppreference.com/w/c/language/_Static_assert
>> 
> 
> can you tell me difference between static_assert and BUILD_BUG_ON?
> it seems that mm subsystem does not make use of it now.

Some difference is explained in include/linux/build_bug.h near static_assert
definition.
But importantly it seems it's not possible to place static_assert(false) in
place of the BUG() because it will trigger despite not being reachable.
BUILD_BUG_ON_MSG(1  "..."); worked as expected for me.