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

[Jerome Glisse]

On Mon, May 22, 2017 at 01:19:29PM -0700, Dan Williams wrote:
> On Mon, May 22, 2017 at 1:14 PM, Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
> > On Mon, May 22, 2017 at 12:29:53PM -0700, Dan Williams wrote:
> >> On Mon, May 22, 2017 at 9:51 AM, Jérôme 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: Jérôme 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);
> >> > +}
> >> >  #else
> >> >  static inline bool is_zone_device_page(const struct page *page)
> >> >  {
> >> > @@ -805,6 +820,10 @@ static inline bool is_device_private_page(const struct page *page)
> >> >  {
> >> >         return false;
> >> >  }
> >> > +
> >> > +static inline void put_zone_device_private_page(struct page *page)
> >> > +{
> >> > +}
> >> >  #endif
> >> >
> >> >  static inline void get_page(struct page *page)
> >> > @@ -822,6 +841,17 @@ static inline void put_page(struct page *page)
> >> >  {
> >> >         page = compound_head(page);
> >> >
> >> > +       /*
> >> > +        * For private device pages we need to catch refcount transition from
> >> > +        * 2 to 1, when refcount reach one it means the private device page is
> >> > +        * free and we need to inform the device driver through callback. See
> >> > +        * include/linux/memremap.h and HMM for details.
> >> > +        */
> >> > +       if (unlikely(is_device_private_page(page))) {
> >>
> >> Since I presume HMM is a niche use case can we make this a
> >> "static_branch_unlikely(&hmm_key) && is_device_private_page(page))"?
> >> That way non-hmm platforms see minimal overhead.
> >
> > Like i said in the cover letter i am bit anxious about doing for
> 
> I don't think you copied me on the cover letter.
> 
> > an inline function. I don't see any existing case for inline
> > function and static key. Is that suppose to work ?
> >
> > How widespread HMM use will be is hard to guess. Usual chicken
> > and egg plus adoption thing. If GPGPU compte keeps growing and
> > it seems it does then HMM likely gonna be enable and actively
> > use for large chunk of those computer that have GPGPU workload.
> >
> > I will test a static key of that branch and see if it explodes
> > because put_page() is an inline function.
> 
> memcpy_mcsafe() is an existing example of a static inline with a
> static branch. Hasn't seemed to have caused any problems to date.

Ok i will post a new version of 08 with static keys, i shouldn't
need to repost any other patches for that. Andrew that's fine
with you or do you prefer me to repost a full updated patchset ?

Cheers,
Jérôme