On 6/12/21 2:19 AM, Hyeonggon Yoo wrote: > On Sat, Jun 12, 2021, 8:59 AM Vlastimil Babka <vbabka@xxxxxxx > <mailto:vbabka@xxxxxxx>> wrote: > > > > But little bit worried :( > > The code is getting too complicated... > > How do you think? > > Yeah, I expected that problems like this could occur as we're poking at some > rare corner cases of compiler implementations here. But if that leads to fixes > in compilers, good for everyone I'd say. > > So I would try this, even if it becomes complicated. > > > Okay, I see the code is okay, but one thing to suggest: > The problem already existed in kmalloc_node before the patch And we found it > by adding BUILD_BUG_ON in kmalloc_index. > > So I suggest adding the condition in kmalloc_node. How about this? > If we add it to kmalloc_node() it means with CONFIG_PROFILE_ALL_BRANCHES we will not be using the compile-time optimized kmalloc_index anywhere. But we get the error only with one file, so in all the other cases the gcc optimizer seems to work fine even with this config enabled. Placing the condition to kmalloc_index() thus limits it only to cases where it doesn't work. As you indicated you would be away starting this week, let me just sent the fix right now so we stop getting the -next reports: ----8<---- >From 56c164dc5c0485c2e133190c0f56e4069844ad62 Mon Sep 17 00:00:00 2001 From: Vlastimil Babka <vbabka@xxxxxxx> Date: Mon, 14 Jun 2021 11:16:03 +0200 Subject: [PATCH] mm-slub-change-run-time-assertion-in-kmalloc_index-to-compile-time-fix-3 Kernel test robot reports a false-positive compile-time assert while compiling kernel/bpf/local_storage.c In file included from <command-line>: In function 'kmalloc_index', inlined from 'kmalloc_node' at include/linux/slab.h:572:20, inlined from 'bpf_map_kmalloc_node.isra.0.part.0' at include/linux/bpf.h:1319:9: >> >> include/linux/compiler_types.h:328:38: error: call to '__compiletime_assert_183' declared with attribute error: unexpected size in kmalloc_index() 328 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__) | ^ include/linux/compiler_types.h:309:4: note: in definition of macro '__compiletime_assert' 309 | prefix ## suffix(); \ | ^~~~~~ include/linux/compiler_types.h:328:2: note: in expansion of macro '_compiletime_assert' 328 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__) | ^~~~~~~~~~~~~~~~~~~ include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert' 39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg) | ^~~~~~~~~~~~~~~~~~ include/linux/slab.h:389:2: note: in expansion of macro 'BUILD_BUG_ON_MSG' 389 | BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()"); | ^~~~~~~~~~~~~~~~ This only happens with CONFIG_PROFILE_ALL_BRANCHES enabled. Informal conversation with gcc developers suggest that this config pushes the optimizer to a corner case where in kmalloc_node() the 'size' is considered a builtin-constant and thus kmalloc_index() is chosen, but later in kmalloc_index() the notion of compile-time constant is lost and thus the compiletime assert becomes reachable. While there's plan to submit a proper gcc bug report with reduced testcase, for now add CONFIG_PROFILE_ALL_BRANCHES to the list of situations where we keep using the runtime BUG_ON() instead of compile-time assert. This is a 3rd fix for mmotm patch mm-slub-change-run-time-assertion-in-kmalloc_index-to-compile-time.patch [1] https://lore.kernel.org/linux-mm/202106051442.G1VJubTz-lkp@xxxxxxxxx/ Reported-by: kernel test robot <lkp@xxxxxxxxx> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> --- include/linux/slab.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/include/linux/slab.h b/include/linux/slab.h index 8d8dd8571261..083f3ce550bc 100644 --- a/include/linux/slab.h +++ b/include/linux/slab.h @@ -413,7 +413,8 @@ static __always_inline unsigned int __kmalloc_index(size_t size, if (size <= 16 * 1024 * 1024) return 24; if (size <= 32 * 1024 * 1024) return 25; - if ((IS_ENABLED(CONFIG_CC_IS_GCC) || CONFIG_CLANG_VERSION >= 110000) && size_is_constant) + if ((IS_ENABLED(CONFIG_CC_IS_GCC) || CONFIG_CLANG_VERSION >= 110000) + && !IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES) && size_is_constant) BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()"); else BUG(); -- 2.32.0
