Re: [PATCH v1 04/11] mm/memremap: add ZONE_DEVICE support for compound pages

Dan Williams
2021-05-05T18:44:34Z

On Thu, Mar 25, 2021 at 4:10 PM Joao Martins <joao.m.martins@oracle.com> wrote:
>
> Add a new align property for struct dev_pagemap which specifies that a
> pagemap is composed of a set of compound pages of size @align, instead
> of base pages. When these pages are initialised, most 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 | 13 +++++++++++++
> mm/memremap.c | 8 ++++++--
> mm/page_alloc.c | 24 +++++++++++++++++++++++-
> 3 files changed, 42 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index b46f63dcaed3..bb28d82dda5e 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -114,6 +114,7 @@ struct dev_pagemap {
> struct completion done;
> enum memory_type type;
> unsigned int flags;
> + unsigned long align;

I think this wants some kernel-doc above to indicate that non-zero
means "use compound pages with tail-page dedup" and zero / PAGE_SIZE
means "use non-compound base pages". The non-zero value must be
PAGE_SIZE, PMD_PAGE_SIZE or PUD_PAGE_SIZE. Hmm, maybe it should be an
enum:

enum devmap_geometry {
DEVMAP_PTE,
DEVMAP_PMD,
DEVMAP_PUD,
}

...because it's more than just an alignment it's a structural
definition of how the memmap is laid out.

> const struct dev_pagemap_ops *ops;
> void *owner;
> int nr_range;
> @@ -130,6 +131,18 @@ static inline struct vmem_altmap *pgmap_altmap(struct dev_pagemap *pgmap)
> return NULL;
> }
>
> +static inline unsigned long pgmap_align(struct dev_pagemap *pgmap)
> +{
> + if (!pgmap || !pgmap->align)
> + return PAGE_SIZE;
> + return pgmap->align;
> +}
> +
> +static inline unsigned long pgmap_pfn_align(struct dev_pagemap *pgmap)
> +{
> + return PHYS_PFN(pgmap_align(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..d160853670c4 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_align(pgmap) > PAGE_SIZE)
> + percpu_ref_get_many(pgmap->ref, (pfn_end(pgmap, range_id)
> + - pfn_first(pgmap, range_id)) / pgmap_pfn_align(pgmap));
> + 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 58974067bbd4..3a77f9e43f3a 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6285,6 +6285,8 @@ 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 pfn_align = pgmap_pfn_align(pgmap);
> + unsigned int order_align = order_base_2(pfn_align);
> unsigned long zone_idx = zone_idx(zone);
> unsigned long start = jiffies;
> int nid = pgdat->node_id;
> @@ -6302,10 +6304,30 @@ 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 += pfn_align) {

pfn_align is in bytes and pfn is in pages... is there a "pfn_align >>=
PAGE_SHIFT" I missed somewhere?