Re: [PATCH v3] mm/swap: Fix release_pages() when releasing devmap pages

[Dan Williams <dan.j.williams@intel.com>]

On Tue, Jun 4, 2019 at 1:17 PM John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 6/4/19 1:11 PM, Dan Williams wrote:
> > On Tue, Jun 4, 2019 at 12:48 PM John Hubbard <jhubbard@nvidia.com> wrote:
> >>
> >> On 6/4/19 9:48 AM, ira.weiny@intel.com wrote:
> >>> From: Ira Weiny <ira.weiny@intel.com>
> >>>
> ...
> >>> diff --git a/mm/swap.c b/mm/swap.c
> >>> index 7ede3eddc12a..6d153ce4cb8c 100644
> >>> --- a/mm/swap.c
> >>> +++ b/mm/swap.c
> >>> @@ -740,15 +740,20 @@ 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;
> >>> +                     /*
> >>> +                      * Not all zone-device-pages require special
> >>> +                      * processing.  Those pages return 'false' from
> >>> +                      * put_devmap_managed_page() expecting a call to
> >>> +                      * put_page_testzero()
> >>> +                      */
> >>
> >> Just a documentation tweak: how about:
> >>
> >>                         /*
> >>                          * ZONE_DEVICE pages that return 'false' from
> >>                          * put_devmap_managed_page() do not require special
> >>                          * processing, and instead, expect a call to
> >>                          * put_page_testzero().
> >>                          */
> >
> > Looks better to me, but maybe just go ahead and list those
> > expectations explicitly. Something like:
> >
> >                         /*
> >                          * put_devmap_managed_page() only handles
> >                          * ZONE_DEVICE (struct dev_pagemap managed)
> >                          * pages when the hosting dev_pagemap has the
> >                          * ->free() or ->fault() callback handlers
> >                          *  implemented as indicated by
> >                          *  dev_pagemap.type. Otherwise the expectation
> >                          *  is to fall back to a plain decrement /
> >                          *  put_page_testzero().
> >                          */
>
> I like it--but not here, because it's too much internal detail in a
> call site that doesn't use that level of detail. The call site looks
> at the return value, only.
>
> Let's instead put that blurb above (or in) the put_devmap_managed_page()
> routine itself. And leave the blurb that I wrote where it is. And then I
> think everything will have an appropriate level of detail in the right places.

Ok.  Ideally there wouldn't be any commentary needed at the call site
and the put_page() could be handled internal to
put_devmap_managed_page(), but I did not see a way to do that without
breaking the compile out / static branch optimization when there are
no active ZONE_DEVICE users.