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

[Joao Martins <joao.m.martins@oracle.com>]

On 5/5/21 11:20 PM, Dan Williams wrote:
> On Wed, May 5, 2021 at 12:50 PM Joao Martins <joao.m.martins@oracle.com> wrote:
>> 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()?
> 
> I was thinking in dev_pagemap because this value is more than just a
> plain alignment its restructuring the layout and construction of the
> memmap(), but when I say it that way it seems much less like a vanilla
> align value compared to a construction / geometry mode setting.
> 
/me nods

>>
>>> 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)?
> 
> I think it is ok for dax/nvdimm to continue to maintain their align
> value because it should be ok to have 4MB align if the device really
> wanted. However, when it goes to map that alignment with
> memremap_pages() it can pick a mode. For example, it's already the
> case that dax->align == 1GB is mapped with DEVMAP_PTE today, so
> they're already separate concepts that can stay separate.
> 
Gotcha.

>>
>> 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.
>>>

Somehow I missed this other part of your response.

>>>>         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));
>> +}
> 
> Ah yup, my eyes glazed over that. I think this is another place that
> benefits from a more specific name than "align". "pfns_per_compound"
> "compound_pfns"?
> 
We are still describing a page, just not a base page. So perhaps @pfns_per_hpage ?

I am fine with @pfns_per_compound or @compound_pfns as well.
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