Re: [PATCH v3 04/14] mm/memremap: add ZONE_DEVICE support for compound pages

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

On Wed, Jul 14, 2021 at 12:36 PM Joao Martins <joao.m.martins@oracle.com> wrote:
>
> Add a new align property for struct dev_pagemap which specifies that a

s/align/@geometry/

> pagemap is composed of a set of compound pages of size @align,

s/@align/@geometry/

> instead of
> base pages. When a compound page geometry is requested, all but the first
> page are initialised as tail pages instead of order-0 pages.
>
> For certain ZONE_DEVICE users like device-dax which have a fixed page size,
> this creates an opportunity to optimize GUP and GUP-fast walkers, treating
> it the same way as THP or hugetlb pages.
>
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> ---
>  include/linux/memremap.h | 17 +++++++++++++++++
>  mm/memremap.c            |  8 ++++++--
>  mm/page_alloc.c          | 34 +++++++++++++++++++++++++++++++++-
>  3 files changed, 56 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index 119f130ef8f1..e5ab6d4525c1 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -99,6 +99,10 @@ struct dev_pagemap_ops {
>   * @done: completion for @internal_ref
>   * @type: memory type: see MEMORY_* in memory_hotplug.h
>   * @flags: PGMAP_* flags to specify defailed behavior
> + * @geometry: structural definition of how the vmemmap metadata is populated.
> + *     A zero or PAGE_SIZE defaults to using base pages as the memmap metadata
> + *     representation. A bigger value but also multiple of PAGE_SIZE will set
> + *     up compound struct pages representative of the requested geometry size.
>   * @ops: method table
>   * @owner: an opaque pointer identifying the entity that manages this
>   *     instance.  Used by various helpers to make sure that no
> @@ -114,6 +118,7 @@ struct dev_pagemap {
>         struct completion done;
>         enum memory_type type;
>         unsigned int flags;
> +       unsigned long geometry;
>         const struct dev_pagemap_ops *ops;
>         void *owner;
>         int nr_range;
> @@ -130,6 +135,18 @@ static inline struct vmem_altmap *pgmap_altmap(struct dev_pagemap *pgmap)
>         return NULL;
>  }
>
> +static inline unsigned long pgmap_geometry(struct dev_pagemap *pgmap)
> +{
> +       if (!pgmap || !pgmap->geometry)
> +               return PAGE_SIZE;
> +       return pgmap->geometry;
> +}
> +
> +static inline unsigned long pgmap_pfn_geometry(struct dev_pagemap *pgmap)
> +{
> +       return PHYS_PFN(pgmap_geometry(pgmap));
> +}

Are both needed? Maybe just have ->geometry natively be in nr_pages
units directly, because pgmap_pfn_geometry() makes it confusing
whether it's a geometry of the pfn or the geometry of the pgmap.

> +
>  #ifdef CONFIG_ZONE_DEVICE
>  bool pfn_zone_device_reserved(unsigned long pfn);
>  void *memremap_pages(struct dev_pagemap *pgmap, int nid);
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 805d761740c4..ffcb924eb6a5 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -318,8 +318,12 @@ static int pagemap_range(struct dev_pagemap *pgmap, struct mhp_params *params,
>         memmap_init_zone_device(&NODE_DATA(nid)->node_zones[ZONE_DEVICE],
>                                 PHYS_PFN(range->start),
>                                 PHYS_PFN(range_len(range)), pgmap);
> -       percpu_ref_get_many(pgmap->ref, pfn_end(pgmap, range_id)
> -                       - pfn_first(pgmap, range_id));
> +       if (pgmap_geometry(pgmap) > PAGE_SIZE)

This would become

if (pgmap_geometry(pgmap) > 1)

> +               percpu_ref_get_many(pgmap->ref, (pfn_end(pgmap, range_id)
> +                       - pfn_first(pgmap, range_id)) / pgmap_pfn_geometry(pgmap));

...and this would be pgmap_geometry()

> +       else
> +               percpu_ref_get_many(pgmap->ref, pfn_end(pgmap, range_id)
> +                               - pfn_first(pgmap, range_id));
>         return 0;
>
>  err_add_memory:
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 79f3b38afeca..188cb5f8c308 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6597,6 +6597,31 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
>         }
>  }
>
> +static void __ref memmap_init_compound(struct page *page, unsigned long pfn,

I'd feel better if @page was renamed @head... more below:

> +                                       unsigned long zone_idx, int nid,
> +                                       struct dev_pagemap *pgmap,
> +                                       unsigned long nr_pages)
> +{
> +       unsigned int order_align = order_base_2(nr_pages);
> +       unsigned long i;
> +
> +       __SetPageHead(page);
> +
> +       for (i = 1; i < nr_pages; i++) {

The switch of loop styles is jarring. I.e. the switch from
memmap_init_zone_device() that is using pfn, end_pfn, and a local
'struct page *' variable to this helper using pfn + i and a mix of
helpers (__init_zone_device_page,  prep_compound_tail) that have
different expectations of head page + tail_idx and current page.

I.e. this reads more obviously correct to me, but maybe I'm just in
the wrong headspace:

        for (pfn = head_pfn + 1; pfn < end_pfn; pfn++) {
                struct page *page = pfn_to_page(pfn);

                __init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
                prep_compound_tail(head, pfn - head_pfn);

> +               __init_zone_device_page(page + i, pfn + i, zone_idx,
> +                                       nid, pgmap);
> +               prep_compound_tail(page, i);
> +
> +               /*
> +                * The first and second tail pages need to
> +                * initialized first, hence the head page is
> +                * prepared last.

I'd change this comment to say why rather than restate what can be
gleaned from the code. It's actually not clear to me why this order is
necessary.

> +                */
> +               if (i == 2)
> +                       prep_compound_head(page, order_align);
> +       }
> +}
> +
>  void __ref memmap_init_zone_device(struct zone *zone,
>                                    unsigned long start_pfn,
>                                    unsigned long nr_pages,
> @@ -6605,6 +6630,7 @@ void __ref memmap_init_zone_device(struct zone *zone,
>         unsigned long pfn, end_pfn = start_pfn + nr_pages;
>         struct pglist_data *pgdat = zone->zone_pgdat;
>         struct vmem_altmap *altmap = pgmap_altmap(pgmap);
> +       unsigned int pfns_per_compound = pgmap_pfn_geometry(pgmap);
>         unsigned long zone_idx = zone_idx(zone);
>         unsigned long start = jiffies;
>         int nid = pgdat->node_id;
> @@ -6622,10 +6648,16 @@ void __ref memmap_init_zone_device(struct zone *zone,
>                 nr_pages = end_pfn - start_pfn;
>         }
>
> -       for (pfn = start_pfn; pfn < end_pfn; pfn++) {
> +       for (pfn = start_pfn; pfn < end_pfn; pfn += pfns_per_compound) {
>                 struct page *page = pfn_to_page(pfn);
>
>                 __init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
> +
> +               if (pfns_per_compound == 1)
> +                       continue;
> +
> +               memmap_init_compound(page, pfn, zone_idx, nid, pgmap,
> +                                    pfns_per_compound);

I otherwise don't see anything broken with this patch, so feel free to include:

Reviewed-by: Dan Williams <dan.j.williams@intel.com>

...on the resend with the fixups.