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

[Vlastimil Babka <vbabka@suse.cz>]

On 5/10/21 3:58 PM, Hyeonggon Yoo wrote:
> On Mon, May 10, 2021 at 12:09:55PM +0200, Vlastimil Babka wrote:
>> 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.
>> 
> 
> Thank you so much reviewing and replying to my patch.
> plecase check if I understood well.
> 
> Okay, I'll do that work. then the following patch will:
> 	- remove case when size is more than 32MB
> 	- change "BUG to BUILD_BUG_ON to let compiler know when the size is not supported"
> 	- add comment that there's no need to optimize it
> 
> is it what you mean. right?

Exactly.

> and I have a question. in the lin 751 of mm/slab_common.c,
> thre's struct kmalloc_info_struct kmalloc_info. and it initializes kmalloc info
> up to 64MB, which is currently not supported. should I change it too? in a separate patch?

Yeah that could be also changed, in the same patch.