From: Nathan Chancellor <nathan@kernel.org>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>,
akpm@linux-foundation.org, iamjoonsoo.kim@lge.com,
rientjes@google.com, penberg@kernel.org, cl@linux.com,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
naresh.kamboju@linaro.org, clang-built-linux@googlegroups.com,
linux-next@vger.kernel.org, ndesaulniers@google.com,
lkft-triage@lists.linaro.org, sfr@canb.auug.org.au,
arnd@arndb.de, Marco Elver <elver@google.com>
Subject: Re: [PATCH v3] mm, slub: change run-time assertion in kmalloc_index() to compile-time
Date: Sat, 15 May 2021 23:34:49 -0700 [thread overview]
Message-ID: <YKC9CeAfw3aBmHTU@archlinux-ax161> (raw)
In-Reply-To: <41c65455-a35b-3ad3-54f9-49ca7105bfa9@suse.cz>
On Sat, May 15, 2021 at 11:24:25PM +0200, Vlastimil Babka wrote:
> On 5/15/21 11:09 PM, Hyeonggon Yoo wrote:
> > Hello Vlastimil, recently kbuild-all test bot reported compile error on
> > clang 10.0.1, with defconfig.
>
> Hm yes, catching some compiler bug was something that was noted to be
> possible to happen.
>
> > Nathan Chancellor wrote:
> >> I think this happens because arch_prepare_optimized_kprobe() calls kzalloc()
> >> with a size of MAX_OPTINSN_SIZE, which is
> >>
> >> #define MAX_OPTINSN_SIZE \
> >> (((unsigned long)optprobe_template_end - \
> >> (unsigned long)optprobe_template_entry) + \
> >> MAX_OPTIMIZED_LENGTH + JMP32_INSN_SIZE)
> >
> >> and the optprobe_template_{end,entry} are not evaluated as constants.
> >>
> >> I am not sure what the solution is. There seem to be a growing list of issues
> >> with LLVM 10 that were fixed in LLVM 11, which might necessitate requiring
> >> LLVM 11 and newer to build the kernel, given this affects a defconfig.
> >> Cheers,
> >> Nathan
> >
> >
> > I think it's because kmalloc compiles successfully when size is constant,
> > and kmalloc_index isn't. so I think compiler seems to be confused.
> >
> > currently if size is non-constant, kmalloc calls dummy function __kmalloc,
> > which always returns NULL.
>
> That's a misunderstanding. __kmalloc() is not a dummy function, you
> probably found only the header declaration.
>
> > so what about changing kmalloc to do compile-time assertion too, and track
> > all callers that are calling kmalloc with non-constant argument.
>
> kmalloc() is expected to be called with both constant and non-constant
> size. __builtin_constant_p() is used to determine which implementation
> to use. One based on kmalloc_index(), other on __kmalloc().
>
> It appears clang 10.0.1 is mistakenly evaluating __builtin_constant_p()
> as true. Probably something to do with LTO, because MAX_OPTINSN_SIZE
> seems it could be a "link-time constant".
This happens with x86_64 defconfig so LTO is not involved.
However, the explanation makes sense, given that the LLVM change I
landed on changes the sparse conditional constant propagation pass,
which I believe can influence how LLVM handles __builtin_constant_p().
> Maybe we could extend Marco Elver's followup patch that uses
> BUILD_BUG_ON vs BUG() depending on size_is_constant parameter. It could
> use BUG() also if the compiler is LLVM < 11 or something. What would be
> the proper code for this condition?
This should work I think:
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 9d316aac0aba..1b653266f2aa 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -413,7 +413,7 @@ 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 (size_is_constant)
+ if ((IS_ENABLED(CONFIG_CC_IS_GCC) || CONFIG_CLANG_VERSION > 110000) && size_is_constant)
BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");
else
BUG();
next prev parent reply other threads:[~2021-05-16 6:34 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20210511173448.GA54466@hyeyoo>
2021-05-15 21:09 ` [PATCH v3] mm, slub: change run-time assertion in kmalloc_index() to compile-time Hyeonggon Yoo
2021-05-15 21:24 ` Vlastimil Babka
2021-05-15 21:56 ` Hyeonggon Yoo
2021-05-16 6:34 ` Nathan Chancellor [this message]
2021-05-18 0:38 ` Hyeonggon Yoo
2021-05-18 0:43 ` Nathan Chancellor
2021-05-18 1:53 ` Hyeonggon Yoo
2021-05-18 9:28 ` Vlastimil Babka
2021-05-18 11:18 ` Hyeonggon Yoo
2021-05-18 11:34 ` Vlastimil Babka
2021-05-19 5:45 ` Hyeonggon Yoo
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