Re: [PATCH v7 mm-new 01/10] mm: thp: remove disabled task from khugepaged_mm_slot

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Wed, Sep 10, 2025 at 10:28:30PM -0400, Zi Yan wrote:
> On 10 Sep 2025, at 22:12, Lance Yang wrote:
>
> > Hi Yafang,
> >
> > On 2025/9/11 01:27, kernel test robot wrote:
> >> Hi Yafang,
> >>
> >> kernel test robot noticed the following build errors:
> >>
> >> [auto build test ERROR on akpm-mm/mm-everything]
> >>
> >> url:    https://github.com/intel-lab-lkp/linux/commits/Yafang-Shao/mm-thp-remove-disabled-task-from-khugepaged_mm_slot/20250910-144850
> >> base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
> >> patch link:    https://lore.kernel.org/r/20250910024447.64788-2-laoar.shao%40gmail.com
> >> patch subject: [PATCH v7 mm-new 01/10] mm: thp: remove disabled task from khugepaged_mm_slot
> >> config: x86_64-allnoconfig (https://download.01.org/0day-ci/archive/20250911/202509110109.PSgSHb31-lkp@intel.com/config)
> >> compiler: clang version 20.1.8 (https://github.com/llvm/llvm-project 87f0227cb60147a26a1eeb4fb06e3b505e9c7261)
> >> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250911/202509110109.PSgSHb31-lkp@intel.com/reproduce)
> >>
> >> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> >> the same patch/commit), kindly add following tags
> >> | Reported-by: kernel test robot <lkp@intel.com>
> >> | Closes: https://lore.kernel.org/oe-kbuild-all/202509110109.PSgSHb31-lkp@intel.com/
> >>
> >> All errors (new ones prefixed by >>):
> >>
> >>>> kernel/sys.c:2500:6: error: call to undeclared function 'hugepage_pmd_enabled'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
> >>      2500 |             hugepage_pmd_enabled())
> >>           |             ^
> >>>> kernel/sys.c:2501:3: error: call to undeclared function '__khugepaged_enter'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
> >>      2501 |                 __khugepaged_enter(mm);
> >>           |                 ^
> >>     2 errors generated.
> >
> > Oops, seems like hugepage_pmd_enabled() and __khugepaged_enter() are only
> > available when CONFIG_TRANSPARENT_HUGEPAGE is enabled ;)
> >
> >>
> >>
> >> vim +/hugepage_pmd_enabled +2500 kernel/sys.c
> >>
> >>    2471
> >>    2472	static int prctl_set_thp_disable(bool thp_disable, unsigned long flags,
> >>    2473					 unsigned long arg4, unsigned long arg5)
> >>    2474	{
> >>    2475		struct mm_struct *mm = current->mm;
> >>    2476
> >>    2477		if (arg4 || arg5)
> >>    2478			return -EINVAL;
> >>    2479
> >>    2480		/* Flags are only allowed when disabling. */
> >>    2481		if ((!thp_disable && flags) || (flags & ~PR_THP_DISABLE_EXCEPT_ADVISED))
> >>    2482			return -EINVAL;
> >>    2483		if (mmap_write_lock_killable(current->mm))
> >>    2484			return -EINTR;
> >>    2485		if (thp_disable) {
> >>    2486			if (flags & PR_THP_DISABLE_EXCEPT_ADVISED) {
> >>    2487				mm_flags_clear(MMF_DISABLE_THP_COMPLETELY, mm);
> >>    2488				mm_flags_set(MMF_DISABLE_THP_EXCEPT_ADVISED, mm);
> >>    2489			} else {
> >>    2490				mm_flags_set(MMF_DISABLE_THP_COMPLETELY, mm);
> >>    2491				mm_flags_clear(MMF_DISABLE_THP_EXCEPT_ADVISED, mm);
> >>    2492			}
> >>    2493		} else {
> >>    2494			mm_flags_clear(MMF_DISABLE_THP_COMPLETELY, mm);
> >>    2495			mm_flags_clear(MMF_DISABLE_THP_EXCEPT_ADVISED, mm);
> >>    2496		}
> >>    2497
> >>    2498		if (!mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm) &&
> >>    2499		    !mm_flags_test(MMF_VM_HUGEPAGE, mm) &&
> >>> 2500		    hugepage_pmd_enabled())
> >>> 2501			__khugepaged_enter(mm);
> >>    2502		mmap_write_unlock(current->mm);
> >>    2503		return 0;
> >>    2504	}
> >>    2505
> >
> > So, let's wrap the new logic in an #ifdef CONFIG_TRANSPARENT_HUGEPAGE block.
> >
> > diff --git a/kernel/sys.c b/kernel/sys.c
> > index a1c1e8007f2d..c8600e017933 100644
> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> > @@ -2495,10 +2495,13 @@ static int prctl_set_thp_disable(bool thp_disable, unsigned long flags,
> >                 mm_flags_clear(MMF_DISABLE_THP_EXCEPT_ADVISED, mm);
> >         }
> >
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >         if (!mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm) &&
> >             !mm_flags_test(MMF_VM_HUGEPAGE, mm) &&
> >             hugepage_pmd_enabled())
> >                 __khugepaged_enter(mm);
> > +#endif
> > +
> >         mmap_write_unlock(current->mm);
> >         return 0;
> >  }
>
> Or in the header file,
>
> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> ...
> #else
> bool hugepage_pmd_enabled()
> {
> 	return false;
> }
>
> int __khugepaged_enter(struct mm_struct *mm)
> {
> 	return 0;
> }

It seems we have a convention of just not implementing things here if they're
necessarily used in core code paths (and _with my suggested change_) it's _just_
khugepaged that's invoking them).

Anyway with my suggestion we can fix this entirely with:

#ifdef CONFIG_TRANSPARENT_HUGEPAGE

void khugepaged_enter_mm(struct mm_struct *mm);

#else

void khugepaged_enter_mm(struct mm_struct *mm)
{
}

#endif


Cheers, Lorenzo

> #endif
>
> Best Regards,
> Yan, Zi