Re: [PATCH RFC 1/9] memremap: add ZONE_DEVICE support for compound pages

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

On Tue, Dec 8, 2020 at 9:32 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>
> Add a new flag for struct dev_pagemap which designates that a a pagemap
> is described as a set of compound pages or in other words, that how
> pages are grouped together in the page tables are reflected in how we
> describe struct pages. This means that rather than initializing
> individual struct pages, we also initialize these struct pages, as
> compound pages (on x86: 2M or 1G compound 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,
> thus playing the same tricks as hugetlb pages.
>
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> ---
>  include/linux/memremap.h | 2 ++
>  mm/memremap.c            | 8 ++++++--
>  mm/page_alloc.c          | 7 +++++++
>  3 files changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index 79c49e7f5c30..f8f26b2cc3da 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -90,6 +90,7 @@ struct dev_pagemap_ops {
>  };
>
>  #define PGMAP_ALTMAP_VALID     (1 << 0)
> +#define PGMAP_COMPOUND         (1 << 1)

Why is a new flag needed versus just the align attribute? In other
words there should be no need to go back to the old/slow days of
'struct page' per pfn after compound support is added.
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