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Date:   Mon, 13 Jun 2022 08:06:27 -0700
From:   Zach O'Keefe <zokeefe@google.com>
To:     Yang Shi <shy828301@gmail.com>
Cc:     Vlastimil Babka <vbabka@suse.cz>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [v3 PATCH 5/7] mm: thp: kill transparent_hugepage_active()
Message-ID: <YqdSc3cFTefHkolc@google.com>
References: <20220606214414.736109-1-shy828301@gmail.com>
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On 10 Jun 10:02, Yang Shi wrote:
> 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;

I know we just introduced transhuge_vma_size_ok(), but is there a way to
consolidate the above two checks into a single transhuge_vma_suitable(), the
same way it used to be done in transparent_hugepage_active()? I.e.

transhuge_vma_suitable(vma, vma->vm_end - HPAGE_PMD_SIZE).

Which checks if the vma can hold an aligned hugepage, as well as centralizes
the (what I think to be) complicated file mapping check.

> > > -       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
> > >
> > >