Re: [PATCHv4 03/39] mm: implement zero_huge_user_segment and friends

[Kirill A. Shutemov]

Hillf Danton wrote:
> On Sun, May 12, 2013 at 9:23 AM, Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> >
> > Let's add helpers to clear huge page segment(s). They provide the same
> > functionallity as zero_user_segment and zero_user, but for huge pages.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > ---
> >  include/linux/mm.h |    7 +++++++
> >  mm/memory.c        |   36 ++++++++++++++++++++++++++++++++++++
> >  2 files changed, 43 insertions(+)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index c05d7cf..5e156fb 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1797,6 +1797,13 @@ extern void dump_page(struct page *page);
> >  extern void clear_huge_page(struct page *page,
> >                             unsigned long addr,
> >                             unsigned int pages_per_huge_page);
> > +extern void zero_huge_user_segment(struct page *page,
> > +               unsigned start, unsigned end);
> > +static inline void zero_huge_user(struct page *page,
> > +               unsigned start, unsigned len)
> > +{
> > +       zero_huge_user_segment(page, start, start + len);
> > +}
> >  extern void copy_user_huge_page(struct page *dst, struct page *src,
> >                                 unsigned long addr, struct vm_area_struct *vma,
> >                                 unsigned int pages_per_huge_page);
> > diff --git a/mm/memory.c b/mm/memory.c
> > index f7a1fba..f02a8be 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4266,6 +4266,42 @@ void clear_huge_page(struct page *page,
> >         }
> >  }
> >
> > +void zero_huge_user_segment(struct page *page, unsigned start, unsigned end)
> > +{
> > +       int i;
> > +       unsigned start_idx, end_idx;
> > +       unsigned start_off, end_off;
> > +
> > +       BUG_ON(end < start);
> > +
> > +       might_sleep();
> > +
> > +       if (start == end)
> > +               return;
> > +
> > +       start_idx = start >> PAGE_SHIFT;
> > +       start_off = start & ~PAGE_MASK;
> > +       end_idx = (end - 1) >> PAGE_SHIFT;
> > +       end_off = ((end - 1) & ~PAGE_MASK) + 1;
> > +
> > +       /*
> > +        * if start and end are on the same small page we can call
> > +        * zero_user_segment() once and save one kmap_atomic().
> > +        */
> > +       if (start_idx == end_idx)
> > +               return zero_user_segment(page + start_idx, start_off, end_off);
> > +
> > +       /* zero the first (possibly partial) page */
> > +       zero_user_segment(page + start_idx, start_off, PAGE_SIZE);
> > +       for (i = start_idx + 1; i < end_idx; i++) {
> > +               cond_resched();
> > +               clear_highpage(page + i);
> > +               flush_dcache_page(page + i);
> 
> Can we use the function again?
> 	zero_user_segment(page + i, 0, PAGE_SIZE);

No. zero_user_segment() is memset()-based. clear_highpage() is higly
optimized for page clearing on many architectures.

-- 
 Kirill A. Shutemov
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