Re: [v3 PATCH 2/7] mm: thp: introduce transhuge_vma_size_ok() helper

[Zach O'Keefe]

On Thu, Jun 9, 2022 at 5:08 PM Yang Shi <shy828301@xxxxxxxxx> wrote:
>
> On Thu, Jun 9, 2022 at 3:21 PM Zach O'Keefe <zokeefe@xxxxxxxxxx> wrote:
> >
> > On Mon, Jun 6, 2022 at 2:44 PM Yang Shi <shy828301@xxxxxxxxx> wrote:
> > >
> > > There are couple of places that check whether the vma size is ok for
> > > THP or not, they are open coded and duplicate, introduce
> > > transhuge_vma_size_ok() helper to do the job.
> > >
> > > Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> > > ---
> > >  include/linux/huge_mm.h | 17 +++++++++++++++++
> > >  mm/huge_memory.c        |  5 +----
> > >  mm/khugepaged.c         | 12 ++++++------
> > >  3 files changed, 24 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > > index 648cb3ce7099..a8f61db47f2a 100644
> > > --- a/include/linux/huge_mm.h
> > > +++ b/include/linux/huge_mm.h
> > > @@ -116,6 +116,18 @@ extern struct kobj_attribute shmem_enabled_attr;
> > >
> > >  extern unsigned long transparent_hugepage_flags;
> > >
> > > +/*
> > > + * The vma size has to be large enough to hold an aligned HPAGE_PMD_SIZE area.
> > > + */
> > > +static inline bool transhuge_vma_size_ok(struct vm_area_struct *vma)
> > > +{
> > > +       if (round_up(vma->vm_start, HPAGE_PMD_SIZE) <
> > > +           (vma->vm_end & HPAGE_PMD_MASK))
> > > +               return true;
> > > +
> > > +       return false;
> > > +}
> >
> > First time coming across round_up() - thanks for that - but for
> > symmetry, maybe also use round_down() for the end? No strong opinion -
> > just a suggestion given I've just discovered it.
>
> Yeah, round_down is fine too.
>
> >
> > >  static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
> > >                 unsigned long addr)
> > >  {
> > > @@ -345,6 +357,11 @@ 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;
> > > +}
> > > +
> > >  static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
> > >                 unsigned long addr)
> > >  {
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index 48182c8fe151..36ada544e494 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -71,10 +71,7 @@ unsigned long huge_zero_pfn __read_mostly = ~0UL;
> > >
> > >  bool transparent_hugepage_active(struct vm_area_struct *vma)
> > >  {
> > > -       /* The addr is used to check if the vma size fits */
> > > -       unsigned long addr = (vma->vm_end & HPAGE_PMD_MASK) - HPAGE_PMD_SIZE;
> > > -
> > > -       if (!transhuge_vma_suitable(vma, addr))
> > > +       if (!transhuge_vma_size_ok(vma))
> > >                 return false;
> > >         if (vma_is_anonymous(vma))
> > >                 return __transparent_hugepage_enabled(vma);
> >
> > Do we need a check for vma->vm_pgoff alignment here, after
> > !vma_is_anonymous(), and now that we don't call
> > transhuge_vma_suitable()?
>
> Actually I was thinking about this too. But the THPeligible bit shown
> by smaps is a little bit ambiguous for file vma. The document says:
> "THPeligible" indicates whether the mapping is eligible for allocating
> THP pages - 1 if true, 0 otherwise.
>
> Even though it doesn't fulfill the alignment, it is still possible to
> get THP allocated, but just can't be PMD mapped. So the old behavior
> of THPeligible for file vma seems problematic, or at least doesn't
> match the document.

I think the term "THP" is used ambiguously. Often, but not always, in
the code, folks will go out of their way to specify "hugepage-sized"
page vs "pmd-mapped hugepage" - but at least from my experience,
external documentation doesn't. Given that THP as a concept doesn't
make much sense without the possibility of pmd-mapping, I think
"THPeligible here means "pmd mappable". For example, AnonHugePages in
smaps means  pmd-mapped anon hugepages.

That all said - the following patches will delete
transparent_hugepage_active() anyways.

> I should elaborate this in the commit log.
>
> >
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 84b9cf4b9be9..d0f8020164fc 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -454,6 +454,9 @@ bool hugepage_vma_check(struct vm_area_struct *vma,
> > >                                 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);
> > > @@ -512,9 +515,7 @@ void khugepaged_enter_vma(struct vm_area_struct *vma,
> > >                           unsigned long vm_flags)
> > >  {
> > >         if (!test_bit(MMF_VM_HUGEPAGE, &vma->vm_mm->flags) &&
> > > -           khugepaged_enabled() &&
> > > -           (((vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK) <
> > > -            (vma->vm_end & HPAGE_PMD_MASK))) {
> > > +           khugepaged_enabled()) {
> > >                 if (hugepage_vma_check(vma, vm_flags))
> > >                         __khugepaged_enter(vma->vm_mm);
> > >         }
> > > @@ -2142,10 +2143,9 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
> > >                         progress++;
> > >                         continue;
> > >                 }
> > > -               hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
> > > +
> > > +               hstart = round_up(vma->vm_start, HPAGE_PMD_SIZE);
> > >                 hend = vma->vm_end & HPAGE_PMD_MASK;
> > > -               if (hstart >= hend)
> > > -                       goto skip;
> > >                 if (khugepaged_scan.address > hend)
> > >                         goto skip;
> > >                 if (khugepaged_scan.address < hstart)
> >
> > Likewise, could do round_down() here (just a suggestion)
>
> Fine to me.
>
> >
> > > --
> > > 2.26.3
> > >
> > >