Re: [PATCH 2/2] mm/hugetlb: refactor subpage recording

[Joao Martins]

On 1/26/21 7:21 PM, Joao Martins wrote:
> On 1/26/21 6:08 PM, Mike Kravetz wrote:
>> On 1/25/21 12:57 PM, Joao Martins wrote:
>>> For a given hugepage backing a VA, there's a rather ineficient
>>> loop which is solely responsible for storing subpages in the passed
>>> pages/vmas array. For each subpage we check whether it's within
>>> range or size of @pages and keep incrementing @pfn_offset and a couple
>>> other variables per subpage iteration.
>>>
>>> Simplify this logic and minimize ops per iteration to just
>>> store the output page/vma. Instead of incrementing number of @refs
>>> iteratively, we do it through a precalculation of @refs and having
>>> only a tight loop for storing pinned subpages/vmas.
>>>
>>> pinning consequently improves considerably, bringing us close to
>>> {pin,get}_user_pages_fast:
>>>
>>>  - 16G with 1G huge page size
>>>  gup_test -f /mnt/huge/file -m 16384 -r 10 -L -S -n 512 -w
>>>
>>>  PIN_LONGTERM_BENCHMARK: ~11k us -> ~4400 us
>>>  PIN_FAST_BENCHMARK: ~3700 us
>>>
>>> Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
>>> ---
>>>  mm/hugetlb.c | 49 ++++++++++++++++++++++++++++---------------------
>>>  1 file changed, 28 insertions(+), 21 deletions(-)
>>>
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index 016addc8e413..1f7a95bc7c87 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> @@ -4789,6 +4789,20 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
>>>  	goto out;
>>>  }
>>>  
>>> +static void record_subpages_vmas(struct page *page, struct vm_area_struct *vma,
>>> +				 int refs, struct page **pages,
>>> +				 struct vm_area_struct **vmas)
>>> +{
>>> +	int nr;
>>> +
>>> +	for (nr = 0; nr < refs; nr++) {
>>> +		if (likely(pages))
>>> +			pages[nr] = page++;
>>> +		if (vmas)
>>> +			vmas[nr] = vma;
>>> +	}
>>> +}
>>> +
>>>  long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>>>  			 struct page **pages, struct vm_area_struct **vmas,
>>>  			 unsigned long *position, unsigned long *nr_pages,
>>> @@ -4918,28 +4932,16 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>>>  			continue;
>>>  		}
>>>  
>>> -		refs = 0;
>>> +		refs = min3(pages_per_huge_page(h) - pfn_offset,
>>> +			    (vma->vm_end - vaddr) >> PAGE_SHIFT, remainder);
>>>  
>>> -same_page:
>>> -		if (pages)
>>> -			pages[i] = mem_map_offset(page, pfn_offset);
>>> +		if (pages || vmas)
>>> +			record_subpages_vmas(mem_map_offset(page, pfn_offset),
>>
>> The assumption made here is that mem_map is contiguous for the range of
>> pages in the hugetlb page.  I do not believe you can make this assumption
>> for (gigantic) hugetlb pages which are > MAX_ORDER_NR_PAGES.  For example,
>>
> 
> That would mean get_user_pages_fast() and put_user_pages_fast() are broken for anything
> handling PUDs or above? See record_subpages() in gup_huge_pud() or even gup_huge_pgd().
> It's using the same page++.
> 

Err ... I meant pin_user_pages_fast(), sorry about that.

> This adjustment below probably is what you're trying to suggest.
> 
> Also, nth_page() is slightly more expensive and so the numbers above change from ~4.4k
> usecs to ~7.8k usecs.
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 1f7a95bc7c87..cf66f8c2f92a 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4789,15 +4789,16 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
>         goto out;
>  }
> 
> -static void record_subpages_vmas(struct page *page, struct vm_area_struct *vma,
> +static void record_subpages_vmas(struct page *page, unsigned long pfn_offset,
> +                                struct vm_area_struct *vma,
>                                  int refs, struct page **pages,
>                                  struct vm_area_struct **vmas)
>  {
> -       int nr;
> +       unsigned long nr;
> 
>         for (nr = 0; nr < refs; nr++) {
>                 if (likely(pages))
> -                       pages[nr] = page++;
> +                       pages[nr] = mem_map_offset(page, pfn_offset + nr);
>                 if (vmas)
>                         vmas[nr] = vma;
>         }
> @@ -4936,8 +4937,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct
> *vma,
>                             (vma->vm_end - vaddr) >> PAGE_SHIFT, remainder);
> 
>                 if (pages || vmas)
> -                       record_subpages_vmas(mem_map_offset(page, pfn_offset),
> -                                            vma, refs,
> +                       record_subpages_vmas(page, pfn_offset, vma, refs,
>                                              likely(pages) ? pages + i : NULL,
>                                              vmas ? vmas + i : NULL);
>