Subject: Re: [next] [clang] x86_64-linux-gnu-ld: mm/mremap.o: in function `move_pgt_entry': mremap.c:(.text+0x763): undefined reference to `__compiletime_assert_342'

From: Nathan Chancellor <[email-masked]>
Date: Thu, 17 Jun 2021 11:02:50 -0700

Rebuilt the CC list because most people were added based on the
incorrect bisect result.

On Thu, Jun 17, 2021 at 02:51:49PM +0100, Matthew Wilcox wrote:
> On Thu, Jun 17, 2021 at 06:15:45PM +0530, Naresh Kamboju wrote:
> > On Thu, 17 Jun 2021 at 17:41, Naresh Kamboju <[email-masked]> wrote:
> > > x86_64-linux-gnu-ld: mm/mremap.o: in function `move_pgt_entry':
> > > mremap.c:(.text+0x763): undefined reference to `__compiletime_assert_342'
> > 
> > The git bisect pointed out the first bad commit.
> > 
> > The first bad commit:
> > commit 928cf6adc7d60c96eca760c05c1000cda061604e
> > Author: Stephen Boyd <[email-masked]>
> > Date:   Thu Jun 17 15:21:35 2021 +1000
> >     module: add printk formats to add module build ID to stacktraces
> 
> Your git bisect probably went astray.  There's no way that commit
> caused that regression.

My bisect landed on commit 83f85ac75855 ("mm/mremap: convert huge PUD
move to separate helper"). flush_pud_tlb_range() evaluates to
BUILD_BUG() when CONFIG_TRANSPARENT_HUGEPAGE is unset but this function
is present just based on the value of
CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD.

$ make -skj(nproc) ARCH=x86_64 CC=clang O=build/x86_64 distclean allnoconfig mm/mremap.o

$ llvm-readelf -s build/x86_64/mm/mremap.o &| rg __compiletime_assert
    21: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT   UND __compiletime_assert_337

$ rg TRANSPARENT_ build/x86_64/.config
450:CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE=y
451:CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD=y
562:# CONFIG_TRANSPARENT_HUGEPAGE is not set

Not sure why this does not happen on newer clang versions, presumably
something with inlining decisions? Still seems like a legitimate issue
to me.

Cheers,
Nathan