Subject: Re: Build failure in -next

From: Nathan Chancellor <[email-masked]>
Date: Wed, 23 Jun 2021 16:41:53 -0700

Hi Paul,

On 6/23/2021 3:30 PM, Paul E. McKenney wrote:
> Hello, Aneesh!
> 
> Yesterday evening's next-20210622 testing gave me the following
> kernel-build error:
> 
> ld: mm/mremap.o: in function `move_huge_pud':
> /home/git/linux-next/mm/mremap.c:372: undefined reference to `__compiletime_assert_395'
> 
> Bisection landed on this commit:
> 
> 257121c5aabe ("mm/mremap: convert huge PUD move to separate helper")
> 
> I have no idea how this commit relates to that error message, but
> reverting this commit on top of next-20210622 really does get rid of
> the problem.

This was reported by Naresh Kamboju last week:

https://lore.kernel.org/r/CA+G9fYsWHE5Vu9T3FV-vtHHbVFJWEF=[email-masked]/

It seems to be related to the fact that older versions of clang cannot 
figure out that the HPAGE_PUD case in move_pgt_entry() is never called 
when CONFIG_TRANSPARENT_HUGE is unset so it is not eliminated, which is 
problematic because flush_pud_tlb_range() evaluates to BUILD_BUG() in 
that case, which is called within move_huge_pud().

> The following reproducer provokes this error:
> 
> tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --torture lock --configs LOCK07 --build-only --kconfig "CONFIG_DEBUG_LOCK_ALLOC=y CONFIG_PROVE_LOCKING=y" --kmake-arg "CC=clang-11"
> 
> Run the above command in the top-level directory of your -next source
> tree, and using this compiler:
> 
> $ clang-11 -v
> Ubuntu clang version 11.1.0-++20210428103817+1fdec59bffc1-1~exp1~20210428204431.166
> Target: x86_64-pc-linux-gnu
> 
> Thoughts?

Aneesh had a patch in that thread above that would resolve the issue, it 
just needs to be sent to Andrew for a fixup patch.

Cheers,
Nathan