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Date:   Thu, 19 May 2022 15:38:24 +0800
From:   Muchun Song <songmuchun@bytedance.com>
To:     Yang Shi <shy828301@gmail.com>
Cc:     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: [v2 PATCH] mm: pvmw: check possible huge PMD map by
 transhuge_vma_suitable()
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On Wed, May 18, 2022 at 11:45:14AM -0700, Yang Shi wrote:
> On Tue, May 17, 2022 at 10:31 PM Muchun Song <songmuchun@bytedance.com> wrote:
> >
> > On Fri, May 13, 2022 at 12:17:05PM -0700, Yang Shi wrote:
> > > IIUC PVMW checks if the vma is possibly huge PMD mapped by
> > > transparent_hugepage_active() and "pvmw->nr_pages >= HPAGE_PMD_NR".
> > >
> > > Actually pvmw->nr_pages is returned by compound_nr() or
> > > folio_nr_pages(), so the page should be THP as long as "pvmw->nr_pages
> > > >= HPAGE_PMD_NR".  And it is guaranteed THP is allocated for valid VMA
> > > in the first place.  But it may be not PMD mapped if the VMA is file
> > > VMA and it is not properly aligned.  The transhuge_vma_suitable()
> > > is used to do such check, so replace transparent_hugepage_active() to
> > > it, which is too heavy and overkilling.
> > >
> > > Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> > > Cc: Muchun Song <songmuchun@bytedance.com>
> > > Signed-off-by: Yang Shi <shy828301@gmail.com>
> > > ---
> > > v2: * Fixed build error for !CONFIG_TRANSPARENT_HUGEPAGE
> > >     * Removed fixes tag per Willy
> > >
> > >  include/linux/huge_mm.h | 8 ++++++--
> > >  mm/page_vma_mapped.c    | 2 +-
> > >  2 files changed, 7 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > > index fbf36bb1be22..c2826b1f4069 100644
> > > --- a/include/linux/huge_mm.h
> > > +++ b/include/linux/huge_mm.h
> > > @@ -117,8 +117,10 @@ extern struct kobj_attribute shmem_enabled_attr;
> > >  extern unsigned long transparent_hugepage_flags;
> > >
> > >  static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
> > > -             unsigned long haddr)
> > > +             unsigned long addr)
> > >  {
> > > +     unsigned long haddr;
> > > +
> > >       /* Don't have to check pgoff for anonymous vma */
> > >       if (!vma_is_anonymous(vma)) {
> > >               if (!IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
> > > @@ -126,6 +128,8 @@ static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
> > >                       return false;
> > >       }
> > >
> > > +     haddr = addr & HPAGE_PMD_MASK;
> > > +
> > >       if (haddr < vma->vm_start || haddr + HPAGE_PMD_SIZE > vma->vm_end)
> > >               return false;
> > >       return true;
> > > @@ -328,7 +332,7 @@ static inline bool transparent_hugepage_active(struct vm_area_struct *vma)
> > >  }
> > >
> > >  static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
> > > -             unsigned long haddr)
> > > +             unsigned long addr)
> > >  {
> > >       return false;
> > >  }
> > > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> > > index c10f839fc410..e971a467fcdf 100644
> > > --- a/mm/page_vma_mapped.c
> > > +++ b/mm/page_vma_mapped.c
> > > @@ -243,7 +243,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> > >                        * cleared *pmd but not decremented compound_mapcount().
> > >                        */
> > >                       if ((pvmw->flags & PVMW_SYNC) &&
> > > -                         transparent_hugepage_active(vma) &&
> > > +                         transhuge_vma_suitable(vma, pvmw->address) &&
> >
> > How about the following diff? Then we do not need to change
> > transhuge_vma_suitable().  All the users of transhuge_vma_suitable()
> > are already do the alignment by themselves.
> 
> Thanks for the suggestion. But TBH I don't think this is a better way.
> I did think about this before proposing v2, but I don't prefer to
> pollute the code with IS_ENABLED(CONFIG_xxx) since the definition of
> transhuge_vma_suitable() is already protected by #ifdef. Rounding the
> address in transhuge_vma_suitable() seems neater and more readable to
> me IMHO.
> 
> Some callers of transhuge_vma_suitable() do round the address before
> calling it, but the rounded address is used by other codes in the
> callers too.
>

All right.

Reviewed-by: Muchun Song <songmuchun@bytedance.com>

Thanks.