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

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

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.


>         /* See declaration of similar field in struct zone */
>         unsigned long pageblock_flags[0];
>  };
>
>  void subsection_map_init(unsigned long pfn, unsigned long nr_pages);
> +#ifdef CONFIG_ZONE_DEVICE
> +void subsections_mark_device(unsigned long start_pfn, unsigned long size);
> +#endif
>
>  struct page;
>  struct page_ext;
> @@ -1367,6 +1373,19 @@ static inline int pfn_present(unsigned long pfn)
>         return present_section(__nr_to_section(pfn_to_section_nr(pfn)));
>  }
>
> +static inline int pfn_zone_device(unsigned long pfn)
> +{
> +#ifdef CONFIG_ZONE_DEVICE
> +       if (pfn_valid(pfn)) {
> +               struct mem_section *ms = __pfn_to_section(pfn);
> +               int idx = subsection_map_index(pfn);
> +
> +               return test_bit(idx, ms->usage->subsection_dev_map);
> +       }
> +#endif
> +       return 0;
> +}
> +
>  /*
>   * These are _only_ used during initialisation, therefore they
>   * can use __initdata ...  They could have names to indicate
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 03ccbdf..8a97fd4 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -303,6 +303,8 @@ void *memremap_pages(struct dev_pagemap *pgmap, int nid)
>         memmap_init_zone_device(&NODE_DATA(nid)->node_zones[ZONE_DEVICE],
>                                 PHYS_PFN(res->start),
>                                 PHYS_PFN(resource_size(res)), pgmap);
> +       subsections_mark_device(PHYS_PFN(res->start),
> +                               PHYS_PFN(resource_size(res)));
>         percpu_ref_get_many(pgmap->ref, pfn_end(pgmap) - pfn_first(pgmap));
>         return __va(res->start);
>
> diff --git a/mm/sparse.c b/mm/sparse.c
> index f6891c1..a3fc9e0a 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -603,6 +603,31 @@ void __init sparse_init(void)
>         vmemmap_populate_print_last();
>  }
>
> +#ifdef CONFIG_ZONE_DEVICE
> +void subsections_mark_device(unsigned long start_pfn, unsigned long size)
> +{
> +       struct mem_section *ms;
> +       unsigned long *dev_map;
> +       unsigned long sec, start_sec, end_sec, pfns;
> +
> +       start_sec = pfn_to_section_nr(start_pfn);
> +       end_sec = pfn_to_section_nr(start_pfn + size - 1);
> +       for (sec = start_sec; sec <= end_sec;
> +            sec++, start_pfn += pfns, size -= pfns) {
> +               pfns = min(size, PAGES_PER_SECTION
> +                       - (start_pfn & ~PAGE_SECTION_MASK));
> +               if (WARN_ON(!valid_section_nr(sec)))
> +                       continue;
> +               ms = __pfn_to_section(start_pfn);
> +               if (!ms->usage)
> +                       continue;
> +
> +               dev_map = &ms->usage->subsection_dev_map[0];
> +               subsection_mask_set(dev_map, start_pfn, pfns);
> +       }
> +}
> +#endif
> +
>  #ifdef CONFIG_MEMORY_HOTPLUG
>
>  /* Mark all memory sections within the pfn range as online */
> @@ -782,7 +807,14 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>                 memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
>                 ms->section_mem_map = sparse_encode_mem_map(NULL, section_nr);
>         }
> +#ifdef CONFIG_ZONE_DEVICE
> +       /* deactivation of a partial section on ZONE_DEVICE */
> +       if (ms->usage) {
> +               unsigned long *dev_map = &ms->usage->subsection_dev_map[0];
>
> +               bitmap_andnot(dev_map, dev_map, map, SUBSECTIONS_PER_SECTION);
> +       }
> +#endif
>         if (section_is_early && memmap)
>                 free_map_bootmem(memmap);
>         else
> --
> 1.8.3.1
>