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

Joao Martins
2021-05-05T19:50:36Z

On 5/5/21 7:44 PM, Dan Williams wrote:
> 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".

Got it. Are you thinking a kernel_doc on top of the variable above, or preferably on top
of pgmap_align()?

> 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,
> }
>
I suppose a converter between devmap_geometry and page_size would be needed too? And maybe
the whole dax/nvdimm align values change meanwhile (as a followup improvement)?

Although to be fair we only ever care about compound page size in this series (and
similarly dax/nvdimm @align properties).

> ...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?
>
@pfn_align is in pages too. It's pgmap_align() which is in bytes:

+static inline unsigned long pgmap_pfn_align(struct dev_pagemap *pgmap)
+{
+ return PHYS_PFN(pgmap_align(pgmap));
+}