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

[Dan Williams]

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, JÃrÃme 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: JÃrÃme 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.