Re: [v3 PATCH 5/7] mm: thp: kill transparent_hugepage_active()

[Yang Shi <shy828301@gmail.com>]

On Thu, Jun 9, 2022 at 6:03 PM Zach O'Keefe <zokeefe@google.com> wrote:
>
> On Mon, Jun 6, 2022 at 2:44 PM Yang Shi <shy828301@gmail.com> wrote:
> >
> > The transparent_hugepage_active() was introduced to show THP eligibility
> > bit in smaps in proc, smaps is the only user.  But it actually does the
> > similar check as hugepage_vma_check() which is used by khugepaged.  We
> > definitely don't have to maintain two similar checks, so kill
> > transparent_hugepage_active().
>
> I never realized smaps was the only user! Great!
>
> > Also move hugepage_vma_check() to huge_memory.c and huge_mm.h since it
> > is not only for khugepaged anymore.
> >
> > Signed-off-by: Yang Shi <shy828301@gmail.com>
> > ---
> >  fs/proc/task_mmu.c         |  2 +-
> >  include/linux/huge_mm.h    | 16 +++++++-----
> >  include/linux/khugepaged.h |  4 +--
> >  mm/huge_memory.c           | 50 ++++++++++++++++++++++++++++++++-----
> >  mm/khugepaged.c            | 51 +++-----------------------------------
> >  5 files changed, 60 insertions(+), 63 deletions(-)
> >
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index 2dd8c8a66924..fd79566e204c 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -860,7 +860,7 @@ static int show_smap(struct seq_file *m, void *v)
> >         __show_smap(m, &mss, false);
> >
> >         seq_printf(m, "THPeligible:    %d\n",
> > -                  transparent_hugepage_active(vma));
> > +                  hugepage_vma_check(vma, vma->vm_flags, true));
> >
> >         if (arch_pkeys_enabled())
> >                 seq_printf(m, "ProtectionKey:  %8u\n", vma_pkey(vma));
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 79d5919beb83..f561c3e16def 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -209,7 +209,9 @@ static inline bool file_thp_enabled(struct vm_area_struct *vma)
> >                !inode_is_open_for_write(inode) && S_ISREG(inode->i_mode);
> >  }
> >
> > -bool transparent_hugepage_active(struct vm_area_struct *vma);
> > +bool hugepage_vma_check(struct vm_area_struct *vma,
> > +                       unsigned long vm_flags,
> > +                       bool smaps);
> >
> >  #define transparent_hugepage_use_zero_page()                           \
> >         (transparent_hugepage_flags &                                   \
> > @@ -358,11 +360,6 @@ static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
> >         return false;
> >  }
> >
> > -static inline bool transparent_hugepage_active(struct vm_area_struct *vma)
> > -{
> > -       return false;
> > -}
> > -
> >  static inline bool transhuge_vma_size_ok(struct vm_area_struct *vma)
> >  {
> >         return false;
> > @@ -380,6 +377,13 @@ static inline bool transhuge_vma_enabled(struct vm_area_struct *vma,
> >         return false;
> >  }
> >
> > +static inline bool hugepage_vma_check(struct vm_area_struct *vma,
> > +                                      unsigned long vm_flags,
> > +                                      bool smaps)
> > +{
> > +       return false;
> > +}
> > +
> >  static inline void prep_transhuge_page(struct page *page) {}
> >
> >  #define transparent_hugepage_flags 0UL
> > diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
> > index 392d34c3c59a..8a6452e089ca 100644
> > --- a/include/linux/khugepaged.h
> > +++ b/include/linux/khugepaged.h
> > @@ -10,8 +10,6 @@ extern struct attribute_group khugepaged_attr_group;
> >  extern int khugepaged_init(void);
> >  extern void khugepaged_destroy(void);
> >  extern int start_stop_khugepaged(void);
> > -extern bool hugepage_vma_check(struct vm_area_struct *vma,
> > -                              unsigned long vm_flags);
> >  extern void __khugepaged_enter(struct mm_struct *mm);
> >  extern void __khugepaged_exit(struct mm_struct *mm);
> >  extern void khugepaged_enter_vma(struct vm_area_struct *vma,
> > @@ -57,7 +55,7 @@ static inline void khugepaged_enter(struct vm_area_struct *vma,
> >  {
> >         if (!test_bit(MMF_VM_HUGEPAGE, &vma->vm_mm->flags) &&
> >             khugepaged_enabled()) {
> > -               if (hugepage_vma_check(vma, vm_flags))
> > +               if (hugepage_vma_check(vma, vm_flags, false))
> >                         __khugepaged_enter(vma->vm_mm);
> >         }
> >  }
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 36ada544e494..bc8370856e85 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -69,18 +69,56 @@ static atomic_t huge_zero_refcount;
> >  struct page *huge_zero_page __read_mostly;
> >  unsigned long huge_zero_pfn __read_mostly = ~0UL;
> >
> > -bool transparent_hugepage_active(struct vm_area_struct *vma)
> > +bool hugepage_vma_check(struct vm_area_struct *vma,
> > +                       unsigned long vm_flags,
> > +                       bool smaps)
> >  {
> > +       if (!transhuge_vma_enabled(vma, vm_flags))
> > +               return false;
> > +
> > +       if (vm_flags & VM_NO_KHUGEPAGED)
> > +               return false;
> > +
> > +       /* Don't run khugepaged against DAX vma */
> > +       if (vma_is_dax(vma))
> > +               return false;
> > +
> > +       if (vma->vm_file && !IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) -
> > +                               vma->vm_pgoff, HPAGE_PMD_NR))
> > +               return false;
> > +
> >         if (!transhuge_vma_size_ok(vma))
> >                 return false;
> > -       if (vma_is_anonymous(vma))
> > -               return __transparent_hugepage_enabled(vma);
> > -       if (vma_is_shmem(vma))
> > +
> > +       /* Enabled via shmem mount options or sysfs settings. */
> > +       if (shmem_file(vma->vm_file))
> >                 return shmem_huge_enabled(vma);
> > -       if (transhuge_vma_enabled(vma, vma->vm_flags) && file_thp_enabled(vma))
> > +
> > +       if (!khugepaged_enabled())
> > +               return false;
> > +
> > +       /* THP settings require madvise. */
> > +       if (!(vm_flags & VM_HUGEPAGE) && !khugepaged_always())
> > +               return false;
> > +
> > +       /* Only regular file is valid */
> > +       if (file_thp_enabled(vma))
> >                 return true;
> >
> > -       return false;
> > +       if (!vma_is_anonymous(vma))
> > +               return false;
> > +
> > +       if (vma_is_temporary_stack(vma))
> > +               return false;
> > +
> > +       /*
> > +        * THPeligible bit of smaps should show 1 for proper VMAs even
> > +        * though anon_vma is not initialized yet.
> > +        */
> > +       if (!vma->anon_vma)
> > +               return smaps;
> > +
> > +       return true;
> >  }
>
> There are a few cases where the return value for smaps will be
> different from before. I presume this won't be an issue, and that any
> difference resulting from this change is actually a positive
> difference, given it more accurately reflects the thp eligibility of
> the vma? For example, a VM_NO_KHUGEPAGED-marked vma might now show 0
> where it otherwise showed 1.

Yes, returning 1 for VM_NO_KHUGEPAGED vmas is wrong. Actually TBH I
suspect very few people actually use this bit. Anyway I will elaborate
this in the commit log.

>
> >  static bool get_huge_zero_page(void)
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index ca1754d3a827..aa0769e3b0d9 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -437,49 +437,6 @@ static inline int khugepaged_test_exit(struct mm_struct *mm)
> >         return atomic_read(&mm->mm_users) == 0;
> >  }
> >
> > -bool hugepage_vma_check(struct vm_area_struct *vma,
> > -                       unsigned long vm_flags)
> > -{
> > -       if (!transhuge_vma_enabled(vma, vm_flags))
> > -               return false;
> > -
> > -       if (vm_flags & VM_NO_KHUGEPAGED)
> > -               return false;
> > -
> > -       /* Don't run khugepaged against DAX vma */
> > -       if (vma_is_dax(vma))
> > -               return false;
> > -
> > -       if (vma->vm_file && !IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) -
> > -                               vma->vm_pgoff, HPAGE_PMD_NR))
> > -               return false;
> > -
> > -       if (!transhuge_vma_size_ok(vma))
> > -               return false;
> > -
> > -       /* Enabled via shmem mount options or sysfs settings. */
> > -       if (shmem_file(vma->vm_file))
> > -               return shmem_huge_enabled(vma);
> > -
> > -       if (!khugepaged_enabled())
> > -               return false;
> > -
> > -       /* THP settings require madvise. */
> > -       if (!(vm_flags & VM_HUGEPAGE) && !khugepaged_always())
> > -               return false;
> > -
> > -       /* Only regular file is valid */
> > -       if (file_thp_enabled(vma))
> > -               return true;
> > -
> > -       if (!vma->anon_vma || !vma_is_anonymous(vma))
> > -               return false;
> > -       if (vma_is_temporary_stack(vma))
> > -               return false;
> > -
> > -       return true;
> > -}
> > -
> >  void __khugepaged_enter(struct mm_struct *mm)
> >  {
> >         struct mm_slot *mm_slot;
> > @@ -516,7 +473,7 @@ void khugepaged_enter_vma(struct vm_area_struct *vma,
> >  {
> >         if (!test_bit(MMF_VM_HUGEPAGE, &vma->vm_mm->flags) &&
> >             khugepaged_enabled()) {
> > -               if (hugepage_vma_check(vma, vm_flags))
> > +               if (hugepage_vma_check(vma, vm_flags, false))
> >                         __khugepaged_enter(vma->vm_mm);
> >         }
> >  }
> > @@ -961,7 +918,7 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
> >
> >         if (!transhuge_vma_suitable(vma, address))
> >                 return SCAN_ADDRESS_RANGE;
> > -       if (!hugepage_vma_check(vma, vma->vm_flags))
> > +       if (!hugepage_vma_check(vma, vma->vm_flags, false))
> >                 return SCAN_VMA_CHECK;
> >         return 0;
> >  }
> > @@ -1442,7 +1399,7 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
> >          * the valid THP. Add extra VM_HUGEPAGE so hugepage_vma_check()
> >          * will not fail the vma for missing VM_HUGEPAGE
> >          */
> > -       if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE))
> > +       if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE, false))
> >                 return;
> >
> >         /* Keep pmd pgtable for uffd-wp; see comment in retract_page_tables() */
> > @@ -2132,7 +2089,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
> >                         progress++;
> >                         break;
> >                 }
> > -               if (!hugepage_vma_check(vma, vma->vm_flags)) {
> > +               if (!hugepage_vma_check(vma, vma->vm_flags, false)) {
> >  skip:
> >                         progress++;
> >                         continue;
> > --
> > 2.26.3
> >
> >