linux-kernel - Re: [PATCH] mm, slub: Fix support for clang 10

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Date:   Tue, 18 May 2021 13:30:05 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Nathan Chancellor <nathan@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>
Subject: Re: [PATCH] mm, slub: Fix support for clang 10

On Tue, May 18, 2021 at 11:12 AM Hyeonggon Yoo <42.hyeyoo@...il.com> wrote:
>
> Previously in 'commit ff3daafe3fd3 ("mm, slub: change run-time assertion
> in kmalloc_index() to compile-time")', changed kmalloc_index's run-time
> assertion to compile-time assertion.
>
> But clang 10 has a bug misevaluating __builtin_constant_p() as true,
> making it unable to compile. This bug was fixed in clang 11.
>
> To support clang 10, introduce a macro to do run-time assertion if clang
> version is less than 11, even if the size is constant. Might revert this
> commit later if we choose not to support clang 10.
>
> Fixes: ff3daafe3fd3 ("mm, slub: change run-time assertion in kmalloc_index() to compile-time")
> Link: https://lore.kernel.org/r/CA+G9fYvYxqVhUTkertjZjcrUq8LWPnO7qC==Wum3gYCwWF9D6Q@mail.gmail.com/
> Link: https://lkml.org/lkml/2021/5/11/872
> Reported-by: Naresh Kamboju <naresh.kamboju@...aro.org>
> Suggested-by: Nathan Chancellor <nathan@...nel.org>
> Signed-off-by: Hyeonggon Yoo <42.hyeyoo@...il.com>

Thanks for the patch; hopefully this isn't too much burden in order to
help us support clang-10 a little bit longer.  We will rip it out when
dropping clang-10 someday; grepping for CLANG_VERSION should turn this
up easily.

Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

> ---
>  include/linux/slab.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 9d316aac0aba..8d8dd8571261 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();
> --
> 2.25.1
>


-- 
Thanks,
~Nick Desaulniers