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

Vlastimil Babka
2021-05-11T08:33:11Z

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.