[PATCH] mm, slub: Fix support for clang 10

[Hyeonggon Yoo]

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@xxxxxxxxxxxxxx/
Link: https://lkml.org/lkml/2021/5/11/872
Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
Suggested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Signed-off-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
---
 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