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?
