Re: [HMM 08/15] mm/ZONE_DEVICE: special case put_page() for device private pages

[Jerome Glisse]

On Mon, May 22, 2017 at 01:22:22PM -0700, Hugh Dickins wrote:
> On Mon, 22 May 2017, Jerome Glisse wrote:
> > On Mon, May 22, 2017 at 12:29:53PM -0700, Dan Williams wrote:
> > > On Mon, May 22, 2017 at 9:51 AM, Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
> > > > A ZONE_DEVICE page that reach a refcount of 1 is free ie no longer
> > > > have any user. For device private pages this is important to catch
> > > > and thus we need to special case put_page() for this.
> > > >
> > > > Signed-off-by: Jerome Glisse <jglisse@xxxxxxxxxx>
> > > > Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > > > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > > > Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> > > > ---
> > > >  include/linux/mm.h | 30 ++++++++++++++++++++++++++++++
> > > >  kernel/memremap.c  |  1 -
> > > >  2 files changed, 30 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > > index a825dab..11f7bac 100644
> > > > --- a/include/linux/mm.h
> > > > +++ b/include/linux/mm.h
> > > > @@ -23,6 +23,7 @@
> > > >  #include <linux/page_ext.h>
> > > >  #include <linux/err.h>
> > > >  #include <linux/page_ref.h>
> > > > +#include <linux/memremap.h>
> > > >
> > > >  struct mempolicy;
> > > >  struct anon_vma;
> > > > @@ -795,6 +796,20 @@ static inline bool is_device_private_page(const struct page *page)
> > > >         return ((page_zonenum(page) == ZONE_DEVICE) &&
> > > >                 (page->pgmap->type == MEMORY_DEVICE_PRIVATE));
> > > >  }
> > > > +
> > > > +static inline void put_zone_device_private_page(struct page *page)
> > > > +{
> > > > +       int count = page_ref_dec_return(page);
> > > > +
> > > > +       /*
> > > > +        * If refcount is 1 then page is freed and refcount is stable as nobody
> > > > +        * holds a reference on the page.
> > > > +        */
> > > > +       if (count == 1)
> > > > +               page->pgmap->page_free(page, page->pgmap->data);
> > > > +       else if (!count)
> > > > +               __put_page(page);
> > > > +}
> 
> Is there something else in this patchset that guarantees
> that get_page_unless_zero() is never used on thse pages?
> We have plenty of code that knows that refcount 0 is special:
> having to know that refcount 1 may be special is worrying.
> 
> Hugh

ZONE_DEVICE pages always had this extra refcount since their
inception. All the place that use get_page_unless_zero() should
be unreachable by a ZONE_DEVICE pages (hwpoison, lru, isolate,
ksm, ...). So if that happens it is a bug.

Jérôme