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.
