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

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

On 2/20/21 1:24 AM, Dan Williams wrote:
> 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.
> 
Ack, I suppose I could just use pgmap @align attribute as you mentioned.

	Joao
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