Re: [PATCH 2/3] mm: Introduce subsection_dev_map

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

On Wed, Nov 13, 2019 at 11:53 AM David Hildenbrand <david@redhat.com> wrote:
>
>
>
> > Am 13.11.2019 um 20:06 schrieb Dan Williams <dan.j.williams@intel.com>:
> >
> > On Wed, Nov 13, 2019 at 10:51 AM David Hildenbrand <david@redhat.com> wrote:
> >>
> >>> On 08.11.19 20:13, Dan Williams wrote:
> >>> On Thu, Nov 7, 2019 at 4:15 PM Toshiki Fukasawa
> >>> <t-fukasawa@vx.jp.nec.com> wrote:
> >>>>
> >>>> Currently, there is no way to identify pfn on ZONE_DEVICE.
> >>>> Identifying pfn on system memory can be done by using a
> >>>> section-level flag. On the other hand, identifying pfn on
> >>>> ZONE_DEVICE requires a subsection-level flag since ZONE_DEVICE
> >>>> can be created in units of subsections.
> >>>>
> >>>> This patch introduces a new bitmap subsection_dev_map so that
> >>>> we can identify pfn on ZONE_DEVICE.
> >>>>
> >>>> Also, subsection_dev_map is used to prove that struct pages
> >>>> included in the subsection have been initialized since it is
> >>>> set after memmap_init_zone_device(). We can avoid accessing
> >>>> pages currently being initialized by checking subsection_dev_map.
> >>>>
> >>>> Signed-off-by: Toshiki Fukasawa <t-fukasawa@vx.jp.nec.com>
> >>>> ---
> >>>>  include/linux/mmzone.h | 19 +++++++++++++++++++
> >>>>  mm/memremap.c          |  2 ++
> >>>>  mm/sparse.c            | 32 ++++++++++++++++++++++++++++++++
> >>>>  3 files changed, 53 insertions(+)
> >>>>
> >>>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> >>>> index bda2028..11376c4 100644
> >>>> --- a/include/linux/mmzone.h
> >>>> +++ b/include/linux/mmzone.h
> >>>> @@ -1174,11 +1174,17 @@ static inline unsigned long section_nr_to_pfn(unsigned long sec)
> >>>>
> >>>>  struct mem_section_usage {
> >>>>         DECLARE_BITMAP(subsection_map, SUBSECTIONS_PER_SECTION);
> >>>> +#ifdef CONFIG_ZONE_DEVICE
> >>>> +       DECLARE_BITMAP(subsection_dev_map, SUBSECTIONS_PER_SECTION);
> >>>> +#endif
> >>>
> >>> Hi Toshiki,
> >>>
> >>> There is currently an effort to remove the PageReserved() flag as some
> >>> code is using that to detect ZONE_DEVICE. In reviewing those patches
> >>> we realized that what many code paths want is to detect online memory.
> >>> So instead of a subsection_dev_map add a subsection_online_map. That
> >>> way pfn_to_online_page() can reliably avoid ZONE_DEVICE ranges. I
> >>> otherwise question the use case for pfn_walkers to return pages for
> >>> ZONE_DEVICE pages, I think the skip behavior when pfn_to_online_page()
> >>> == false is the right behavior.
> >>
> >> To be more precise, I recommended an subsection_active_map, to indicate
> >> which memmaps were fully initialized and can safely be touched (e.g., to
> >> read the zone/nid). This map would also be set when the devmem memmaps
> >> were initialized (race between adding memory/growing the section and
> >> initializing the memmap).
> >>
> >> See
> >>
> >> https://lkml.org/lkml/2019/10/10/87
> >>
> >> and
> >>
> >> https://www.spinics.net/lists/linux-driver-devel/msg130012.html
> >
> > I'm still struggling to understand the motivation of distinguishing
> > "active" as something distinct from "online". As long as the "online"
> > granularity is improved from sections down to subsections then most
> > code paths are good to go. The others can use get_devpagemap() to
> > check for ZONE_DEVICE in a race free manner as they currently do.
>
> I thought we wanted to unify access if we don’t really care about the zone as in most pfn walkers - E.g., for zone shrinking.

Agree, when the zone does not matter, which is most cases, then
pfn_online() and pfn_valid() are sufficient.

> Anyhow, a subsection online map would be a good start, we can reuse that later for ZONE_DEVICE as well.

Cool, good to go with me sending a patch to introduce pfn_online() and
a corresponding subsection_map for the same?