Re: [PATCH] mm/swap: Fix release_pages() when releasing devmap pages

[Dan Williams]

On Fri, May 24, 2019 at 8:35 AM Ira Weiny <ira.weiny@xxxxxxxxx> wrote:
>
> On Thu, May 23, 2019 at 08:58:12PM -0700, Dan Williams wrote:
> > On Thu, May 23, 2019 at 3:37 PM <ira.weiny@xxxxxxxxx> wrote:
> > >
> > > From: Ira Weiny <ira.weiny@xxxxxxxxx>
> > >
> > > Device pages can be more than type MEMORY_DEVICE_PUBLIC.
> > >
> > > Handle all device pages within release_pages()
> > >
> > > This was found via code inspection while determining if release_pages()
> > > and the new put_user_pages() could be interchangeable.
> > >
> > > Cc: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> > > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > > Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> > > Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> > > ---
> > >  mm/swap.c | 7 +++----
> > >  1 file changed, 3 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/mm/swap.c b/mm/swap.c
> > > index 3a75722e68a9..d1e8122568d0 100644
> > > --- a/mm/swap.c
> > > +++ b/mm/swap.c
> > > @@ -739,15 +739,14 @@ void release_pages(struct page **pages, int nr)
> > >                 if (is_huge_zero_page(page))
> > >                         continue;
> > >
> > > -               /* Device public page can not be huge page */
> > > -               if (is_device_public_page(page)) {
> > > +               if (is_zone_device_page(page)) {
> > >                         if (locked_pgdat) {
> > >                                 spin_unlock_irqrestore(&locked_pgdat->lru_lock,
> > >                                                        flags);
> > >                                 locked_pgdat = NULL;
> > >                         }
> > > -                       put_devmap_managed_page(page);
> > > -                       continue;
> > > +                       if (put_devmap_managed_page(page))
> >
> > This "shouldn't" fail, and if it does the code that follows might get
>
> I agree it shouldn't based on the check.  However...
>
> > confused by a ZONE_DEVICE page. If anything I would make this a
> > WARN_ON_ONCE(!put_devmap_managed_page(page)), but always continue
> > unconditionally.
>
> I was trying to follow the pattern from put_page()  Where if fails it indicated
> it was not a devmap page and so "regular" processing should continue.

In this case that regular continuation already happened by not taking
the if (is_zone_device_page(page)) branch

>
> Since I'm unsure I'll just ask what does this check do?
>
>         if (!static_branch_unlikely(&devmap_managed_key))
>                 return false;

That attempts to skip the overhead imposed by device-pages, i.e.
->page_free() callback and other extras, if there are no device-page
producers in the system. I.e. use the old simple put_page() path when
there is no hmm or pmem.