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

[Yafang Shao <laoar.shao@gmail.com>]

On Thu, Sep 11, 2025 at 9:43 PM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> On Wed, Sep 10, 2025 at 10:44:38AM +0800, Yafang Shao wrote:
> > Since a task with MMF_DISABLE_THP_COMPLETELY cannot use THP, remove it from
> > the khugepaged_mm_slot to stop khugepaged from processing it.
> >
> > After this change, the following semantic relationship always holds:
> >
> >   MMF_VM_HUGEPAGE is set     == task is in khugepaged mm_slot
> >   MMF_VM_HUGEPAGE is not set == task is not in khugepaged mm_slot
> >
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > Cc: Lance Yang <ioworker0@gmail.com>
>
> (Obviously on basis of fixing issue bot reported).
>
> > ---
> >  include/linux/khugepaged.h |  1 +
> >  kernel/sys.c               |  6 ++++++
> >  mm/khugepaged.c            | 19 +++++++++----------
> >  3 files changed, 16 insertions(+), 10 deletions(-)
> >
> > diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
> > index eb1946a70cff..6cb9107f1006 100644
> > --- a/include/linux/khugepaged.h
> > +++ b/include/linux/khugepaged.h
> > @@ -19,6 +19,7 @@ extern void khugepaged_min_free_kbytes_update(void);
> >  extern bool current_is_khugepaged(void);
> >  extern int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
> >                                  bool install_pmd);
> > +bool hugepage_pmd_enabled(void);
>
> Need to provide a !CONFIG_TRANSPARENT_HUGEPAGE version, or to not invoke
> this in a context where CONFIG_TRANSPARENT_HUGEPAGE is specified.
>
> >
> >  static inline void khugepaged_fork(struct mm_struct *mm, struct mm_struct *oldmm)
> >  {
> > diff --git a/kernel/sys.c b/kernel/sys.c
> > index a46d9b75880b..a1c1e8007f2d 100644
> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> > @@ -8,6 +8,7 @@
> >  #include <linux/export.h>
> >  #include <linux/mm.h>
> >  #include <linux/mm_inline.h>
> > +#include <linux/khugepaged.h>
> >  #include <linux/utsname.h>
> >  #include <linux/mman.h>
> >  #include <linux/reboot.h>
> > @@ -2493,6 +2494,11 @@ static int prctl_set_thp_disable(bool thp_disable, unsigned long flags,
> >               mm_flags_clear(MMF_DISABLE_THP_COMPLETELY, mm);
> >               mm_flags_clear(MMF_DISABLE_THP_EXCEPT_ADVISED, mm);
> >       }
> > +
> > +     if (!mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm) &&
> > +         !mm_flags_test(MMF_VM_HUGEPAGE, mm) &&
> > +         hugepage_pmd_enabled())
> > +             __khugepaged_enter(mm);
>
> Let's refactor this so it's not open-coded.
>
> We can have:
>
> void khugepaged_enter_mm(struct mm_struct *mm)
> {
>         if (mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm))
>                 return;
>         if (mm_flags_test(MMF_VM_HUGEPAGE, mm))
>                 return;
>         if (!hugepage_pmd_enabled())
>                 return;
>
>         __khugepaged_enter(mm);
> }
>
> void khugepaged_enter_vma(struct vm_area_struct *vma,
>                           vm_flags_t vm_flags)
> {
>         if (!thp_vma_allowable_order(vma, vm_flags, TVA_KHUGEPAGED, PMD_ORDER))
>                 return;
>
>         khugepaged_enter_mm(vma->vm_mm);
> }
>
> Then just invoke khugepaged_enter_mm() here.

That's better.

>
>
> >       mmap_write_unlock(current->mm);
> >       return 0;
> >  }
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 4ec324a4c1fe..88ac482fb3a0 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -413,7 +413,7 @@ static inline int hpage_collapse_test_exit_or_disable(struct mm_struct *mm)
> >               mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm);
> >  }
> >
> > -static bool hugepage_pmd_enabled(void)
> > +bool hugepage_pmd_enabled(void)
> >  {
> >       /*
> >        * We cover the anon, shmem and the file-backed case here; file-backed
> > @@ -445,6 +445,7 @@ void __khugepaged_enter(struct mm_struct *mm)
> >
> >       /* __khugepaged_exit() must not run from under us */
> >       VM_BUG_ON_MM(hpage_collapse_test_exit(mm), mm);
> > +     WARN_ON_ONCE(mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm));
>
> Not sure why this needs to be a naked WARN_ON_ONCE()? Seems that'd be a
> programmatic eror, so VM_WARN_ON_ONCE() more appropriate?

ack

>
> Can also change the VM_BUG_ON_MM() to VM_WARN_ON_ONCE_MM() while we're here.

ack

>
> >       if (unlikely(mm_flags_test_and_set(MMF_VM_HUGEPAGE, mm)))
> >               return;
> >
> > @@ -472,7 +473,8 @@ void __khugepaged_enter(struct mm_struct *mm)
> >  void khugepaged_enter_vma(struct vm_area_struct *vma,
> >                         vm_flags_t vm_flags)
> >  {
> > -     if (!mm_flags_test(MMF_VM_HUGEPAGE, vma->vm_mm) &&
> > +     if (!mm_flags_test(MMF_DISABLE_THP_COMPLETELY, vma->vm_mm) &&
> > +         !mm_flags_test(MMF_VM_HUGEPAGE, vma->vm_mm) &&
> >           hugepage_pmd_enabled()) {
> >               if (thp_vma_allowable_order(vma, vm_flags, TVA_KHUGEPAGED, PMD_ORDER))
> >                       __khugepaged_enter(vma->vm_mm);
>
> See above, we can refactor this.

ack

>
> > @@ -1451,16 +1453,13 @@ static void collect_mm_slot(struct khugepaged_mm_slot *mm_slot)
> >
> >       lockdep_assert_held(&khugepaged_mm_lock);
> >
> > -     if (hpage_collapse_test_exit(mm)) {
> > +     if (hpage_collapse_test_exit_or_disable(mm)) {
> >               /* free mm_slot */
> >               hash_del(&slot->hash);
> >               list_del(&slot->mm_node);
> >
> > -             /*
> > -              * Not strictly needed because the mm exited already.
> > -              *
> > -              * mm_flags_clear(MMF_VM_HUGEPAGE, mm);
> > -              */
> > +             /* If the mm is disabled, this flag must be cleared. */
> > +             mm_flags_clear(MMF_VM_HUGEPAGE, mm);
> >
> >               /* khugepaged_mm_lock actually not necessary for the below */
> >               mm_slot_free(mm_slot_cache, mm_slot);
> > @@ -2507,9 +2506,9 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> >       VM_BUG_ON(khugepaged_scan.mm_slot != mm_slot);
> >       /*
> >        * Release the current mm_slot if this mm is about to die, or
> > -      * if we scanned all vmas of this mm.
> > +      * if we scanned all vmas of this mm, or if this mm is disabled.
> >        */
> > -     if (hpage_collapse_test_exit(mm) || !vma) {
> > +     if (hpage_collapse_test_exit_or_disable(mm) || !vma) {
> >               /*
> >                * Make sure that if mm_users is reaching zero while
> >                * khugepaged runs here, khugepaged_exit will find
>
> Seems reasonable, but makes me wonder if we actually always want to invoke
> hpage_collapse_test_exit_or_disable()?
>
> I guess the VM_BUG_ON() (though it should be a VM_WARN_ON_ONCE()) in
> __khugepaged_enter() is a legit use, but the only other case is
> retract_page_tables().
>
> I wonder if we should change this also? Seems reasonable to.

Right, we can change it also. Thanks for your suggestion.


-- 
Regards
Yafang