Re: [PATCH v7 5/5] mm/page_alloc: reuse tail struct pages for compound devmaps

[Muchun Song <songmuchun@bytedance.com>]

On Fri, Mar 4, 2022 at 5:33 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>
> Currently memmap_init_zone_device() ends up initializing 32768 pages
> when it only needs to initialize 128 given tail page reuse. That
> number is worse with 1GB compound pages, 262144 instead of 128. Update
> memmap_init_zone_device() to skip redundant initialization, detailed
> below.
>
> When a pgmap @vmemmap_shift is set, all pages are mapped at a given
> huge page alignment and use compound pages to describe them as opposed
> to a struct per 4K.
>
> With @vmemmap_shift > 0 and when struct pages are stored in ram
> (!altmap) most tail pages are reused. Consequently, the amount of
> unique struct pages is a lot smaller than the total amount of struct
> pages being mapped.
>
> The altmap path is left alone since it does not support memory savings
> based on compound pages devmap.
>
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>

Reviewed-by: Muchun Song <songmuchun@bytedance.com>

But a nit below.

> ---
>  mm/page_alloc.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e0c1e6bb09dd..e9282d043cca 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6653,6 +6653,21 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
>         }
>  }
>
> +/*
> + * With compound page geometry and when struct pages are stored in ram most
> + * tail pages are reused. Consequently, the amount of unique struct pages to
> + * initialize is a lot smaller that the total amount of struct pages being
> + * mapped. This is a paired / mild layering violation with explicit knowledge
> + * of how the sparse_vmemmap internals handle compound pages in the lack
> + * of an altmap. See vmemmap_populate_compound_pages().
> + */
> +static inline unsigned long compound_nr_pages(struct vmem_altmap *altmap,
> +                                             unsigned long nr_pages)
> +{
> +       return is_power_of_2(sizeof(struct page)) &&
> +               !altmap ? 2 * (PAGE_SIZE/sizeof(struct page)) : nr_pages;

It is better to add spaces around that '/'.