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'

[Aneesh Kumar K.V]

On 6/17/21 11:32 PM, Nathan Chancellor wrote:
> 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 <naresh.kamboju@xxxxxxxxxx> 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 <swboyd@xxxxxxxxxxxx>
> > > 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.

gcc 10 also doesn't give a build error. I guess that is because we evaluate

     if (pud_trans_huge(*old_pud) || pud_devmap(*old_pud)) {

 to if (0) with CONFIG_TRANSPARENT_HUGEPAGE disabled.

switching that to if (1) do results in BUILD_BUG triggering.

Should we fix this ?

modified   mm/mremap.c
@@ -336,7 +336,7 @@ static inline bool move_normal_pud(struct 
vm_area_struct *vma,
 }
 #endif

-#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
+#if defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD) && 
defined(CONFIG_TRANSPARENT_HUGEPAGE)
 static bool move_huge_pud(struct vm_area_struct *vma, unsigned long 
old_addr,
 			  unsigned long new_addr, pud_t *old_pud, pud_t *new_pud)
 {