Re: [RFC PATCH 00/16] 1GB THP support on x86_64

[Zi Yan]

On 2 Sep 2020, at 14:40, Jason Gunthorpe wrote:

> On Wed, Sep 02, 2020 at 02:06:12PM -0400, Zi Yan wrote:
>> From: Zi Yan <ziy@xxxxxxxxxx>
>>
>> Hi all,
>>
>> This patchset adds support for 1GB THP on x86_64. It is on top of
>> v5.9-rc2-mmots-2020-08-25-21-13.
>>
>> 1GB THP is more flexible for reducing translation overhead and increasing the
>> performance of applications with large memory footprint without application
>> changes compared to hugetlb.
>>
>> Design
>> =======
>>
>> 1GB THP implementation looks similar to exiting THP code except some new designs
>> for the additional page table level.
>>
>> 1. Page table deposit and withdraw using a new pagechain data structure:
>>    instead of one PTE page table page, 1GB THP requires 513 page table pages
>>    (one PMD page table page and 512 PTE page table pages) to be deposited
>>    at the page allocaiton time, so that we can split the page later. Currently,
>>    the page table deposit is using ->lru, thus only one page can be deposited.
>>    A new pagechain data structure is added to enable multi-page deposit.
>>
>> 2. Triple mapped 1GB THP : 1GB THP can be mapped by a combination of PUD, PMD,
>>    and PTE entries. Mixing PUD an PTE mapping can be achieved with existing
>>    PageDoubleMap mechanism. To add PMD mapping, PMDPageInPUD and
>>    sub_compound_mapcount are introduced. PMDPageInPUD is the 512-aligned base
>>    page in a 1GB THP and sub_compound_mapcount counts the PMD mapping by using
>>    page[N*512 + 3].compound_mapcount.
>>
>> 3. Using CMA allocaiton for 1GB THP: instead of bump MAX_ORDER, it is more sane
>>    to use something less intrusive. So all 1GB THPs are allocated from reserved
>>    CMA areas shared with hugetlb. At page splitting time, the bitmap for the 1GB
>>    THP is cleared as the resulting pages can be freed via normal page free path.
>>    We can fall back to alloc_contig_pages for 1GB THP if necessary.
>>
>>
>> Patch Organization
>> =======
>>
>> Patch 01 adds the new pagechain data structure.
>>
>> Patch 02 to 13 adds 1GB THP support in variable places.
>>
>> Patch 14 tries to use alloc_contig_pages for 1GB THP allocaiton.
>>
>> Patch 15 moves hugetlb_cma reservation to cma.c and rename it to hugepage_cma.
>>
>> Patch 16 use hugepage_cma reservation for 1GB THP allocation.
>>
>>
>> Any suggestions and comments are welcome.
>>
>>
>> Zi Yan (16):
>>   mm: add pagechain container for storing multiple pages.
>>   mm: thp: 1GB anonymous page implementation.
>>   mm: proc: add 1GB THP kpageflag.
>>   mm: thp: 1GB THP copy on write implementation.
>>   mm: thp: handling 1GB THP reference bit.
>>   mm: thp: add 1GB THP split_huge_pud_page() function.
>>   mm: stats: make smap stats understand PUD THPs.
>>   mm: page_vma_walk: teach it about PMD-mapped PUD THP.
>>   mm: thp: 1GB THP support in try_to_unmap().
>>   mm: thp: split 1GB THPs at page reclaim.
>>   mm: thp: 1GB THP follow_p*d_page() support.
>>   mm: support 1GB THP pagemap support.
>>   mm: thp: add a knob to enable/disable 1GB THPs.
>>   mm: page_alloc: >=MAX_ORDER pages allocation an deallocation.
>>   hugetlb: cma: move cma reserve function to cma.c.
>>   mm: thp: use cma reservation for pud thp allocation.
>
> Surprised this doesn't touch mm/pagewalk.c ?

1GB PUD page support is present for DAX purpose, so the code is there
in mm/pagewalk.c already. I only needed to supply ops->pud_entry when using
the functions in mm/pagewalk.c. :)

—
Best Regards,
Yan Zi

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