Re: [PATCH v3] mm, slub: change run-time assertion in kmalloc_index() to compile-time
From: Marco Elver <elver@google.com>
Date: 2021-05-13 12:29:39
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On Thu, 13 May 2021 at 14:03, Hyeonggon Yoo [off-list ref] wrote:
On Thu, May 13, 2021 at 12:31:38PM +0200, Marco Elver wrote:
[...]
what about checking size it on top of kmalloc_index? because by definition of KMALLOC_SHIFT_HIGH, it's not always 25. it can be less than 25. for some situations. below is what I suggested beofre. for just reference:
This doesn't solve the problem. We want the compiler to complain whenever kmalloc_index() is used with non-constant in normal code. But it should be possible to use it in allocator tests regardless of size. Either that or export kmalloc_slab(), but I think that's worse. I'll send my patch with an updated comment.
quoted hunk ↗ jump to hunk
--- include/linux/slab.h.orig 2021-05-12 17:56:54.504738768 +0900 +++ include/linux/slab.h 2021-05-13 15:06:25.724565850 +0900@@ -346,9 +346,18 @@ static __always_inline enum kmalloc_cach * 1 = 65 .. 96 bytes * 2 = 129 .. 192 bytes * n = 2^(n-1)+1 .. 2^n + * + * Note: there's no need to optimize kmalloc_index because it's evaluated + * in compile-time. */ static __always_inline unsigned int kmalloc_index(size_t size) { + if (__builtin_constant_p(size)) { + BUILD_BUG_ON_MSG(size > KMALLOC_MAX_CACHE_SIZE , "unexpected size in kmalloc_index()"); + } else if (size > KMALLOC_MAX_CACHE_SIZE) { + BUG(); + } + if (!size) return 0;@@ -382,8 +391,6 @@ static __always_inline unsigned int kmal 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(); /* Will never be reached. Needed because the compiler may complain */ return -1;