linux-kernel - 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

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Date:   Fri, 18 Jun 2021 16:05:17 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        linux-next@...r.kernel.org, clang-built-linux@...glegroups.com,
        lkft-triage@...ts.linaro.org, linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Stephen Rothwell <sfr@...b.auug.org.au>
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'

On Fri, Jun 18, 2021 at 10:32:42AM +0530, Aneesh Kumar K.V wrote:
> 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@...aro.org> 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@...omium.org>
> > > > 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.

Thanks for pointing that out. I think what happens with clang-10 and
clang-11 is that move_huge_pud() gets inlined into move_pgt_entry() but
then the compiler does not figure out that the HPAGE_PUD case is dead so
the code sticks around, where as GCC and newer clang versions can figure
that out and eliminate that case.

> Should we fix this ?

Yes, I believe that we should.

> 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)
>  {

That works or we could mirror what has already been done for the
HPAGE_PMD case. No personal preference.

diff --git a/mm/mremap.c b/mm/mremap.c
index 9a7fbec31dc9..5989d3990020 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -460,7 +460,8 @@ static bool move_pgt_entry(enum pgt_entry entry, struct vm_area_struct *vma,
                                      new_entry);
                break;
        case HPAGE_PUD:
-               moved = move_huge_pud(vma, old_addr, new_addr, old_entry,
+               moved = IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
+                       move_huge_pud(vma, old_addr, new_addr, old_entry,
                                      new_entry);
                break;


Cheers,
Nathan