Re: [PATCH RFC 4/9] mm/page_alloc: Reuse tail struct pages for compound pagemaps

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

On Tue, Dec 8, 2020 at 9:31 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>
> When PGMAP_COMPOUND is set, all pages are onlined at a given huge page
> alignment and using compound pages to describe them as opposed to a
> struct per 4K.
>

Same s/online/mapped/ comment as other changelogs.

> To minimize struct page overhead and given the usage of compound pages we
> utilize the fact that most tail pages look the same, we online the
> subsection while pointing to the same pages. Thus request VMEMMAP_REUSE
> in add_pages.
>
> With VMEMMAP_REUSE, provided we reuse most tail pages the amount of
> struct pages we need to initialize is a lot smaller that the total
> amount of structs we would normnally online. Thus allow an @init_order
> to be passed to specify how much pages we want to prep upon creating a
> compound page.
>
> Finally when onlining all struct pages in memmap_init_zone_device, make
> sure that we only initialize the unique struct pages i.e. the first 2
> 4K pages from @align which means 128 struct pages out of 32768 for 2M
> @align or 262144 for a 1G @align.
>
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> ---
>  mm/memremap.c   |  4 +++-
>  mm/page_alloc.c | 23 ++++++++++++++++++++---
>  2 files changed, 23 insertions(+), 4 deletions(-)
>
> diff --git a/mm/memremap.c b/mm/memremap.c
> index ecfa74848ac6..3eca07916b9d 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -253,8 +253,10 @@ static int pagemap_range(struct dev_pagemap *pgmap, struct mhp_params *params,
>                         goto err_kasan;
>                 }
>
> -               if (pgmap->flags & PGMAP_COMPOUND)
> +               if (pgmap->flags & PGMAP_COMPOUND) {
>                         params->align = pgmap->align;
> +                       params->flags = MEMHP_REUSE_VMEMMAP;

The "reuse" naming is not my favorite. Yes, page reuse is happening,
but what is more relevant is that the vmemmap is in a given minimum
page size mode. So it's less of a flag and more of enum that selects
between PAGE_SIZE, HPAGE_SIZE, and PUD_PAGE_SIZE (GPAGE_SIZE?).

> +               }
>
>                 error = arch_add_memory(nid, range->start, range_len(range),
>                                         params);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9716ecd58e29..180a7d4e9285 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -691,10 +691,11 @@ void free_compound_page(struct page *page)
>         __free_pages_ok(page, compound_order(page), FPI_NONE);
>  }
>
> -void prep_compound_page(struct page *page, unsigned int order)
> +static void __prep_compound_page(struct page *page, unsigned int order,
> +                                unsigned int init_order)
>  {
>         int i;
> -       int nr_pages = 1 << order;
> +       int nr_pages = 1 << init_order;
>
>         __SetPageHead(page);
>         for (i = 1; i < nr_pages; i++) {
> @@ -711,6 +712,11 @@ void prep_compound_page(struct page *page, unsigned int order)
>                 atomic_set(compound_pincount_ptr(page), 0);
>  }
>
> +void prep_compound_page(struct page *page, unsigned int order)
> +{
> +       __prep_compound_page(page, order, order);
> +}
> +
>  #ifdef CONFIG_DEBUG_PAGEALLOC
>  unsigned int _debug_guardpage_minorder;
>
> @@ -6108,6 +6114,9 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>  }
>
>  #ifdef CONFIG_ZONE_DEVICE
> +
> +#define MEMMAP_COMPOUND_SIZE (2 * (PAGE_SIZE/sizeof(struct page)))
> +
>  void __ref memmap_init_zone_device(struct zone *zone,
>                                    unsigned long start_pfn,
>                                    unsigned long nr_pages,
> @@ -6138,6 +6147,12 @@ void __ref memmap_init_zone_device(struct zone *zone,
>         for (pfn = start_pfn; pfn < end_pfn; pfn++) {
>                 struct page *page = pfn_to_page(pfn);
>
> +               /* Skip already initialized pages. */
> +               if (compound && (pfn % align >= MEMMAP_COMPOUND_SIZE)) {
> +                       pfn = ALIGN(pfn, align) - 1;
> +                       continue;
> +               }
> +
>                 __init_single_page(page, pfn, zone_idx, nid);
>
>                 /*
> @@ -6175,7 +6190,9 @@ void __ref memmap_init_zone_device(struct zone *zone,
>
>         if (compound) {
>                 for (pfn = start_pfn; pfn < end_pfn; pfn += align)
> -                       prep_compound_page(pfn_to_page(pfn), order_base_2(align));
> +                       __prep_compound_page(pfn_to_page(pfn),
> +                                          order_base_2(align),
> +                                          order_base_2(MEMMAP_COMPOUND_SIZE));
>         }

Alex did quite a bit of work to optimize this path, and this
organization appears to undo it. I'd prefer to keep it all in one loop
so a 'struct page' is only initialized once. Otherwise by the time the
above loop finishes and this one starts the 'struct page's are
probably cache cold again.

So I'd break prep_compoud_page into separate head and tail  init and
call them at the right time in one loop.