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

Hyeonggon Yoo
2021-05-10T14:37:41Z

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?

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?