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

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Date:   Tue, 26 Jan 2021 13:21:46 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Joao Martins <joao.m.martins@...cle.com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...pe.ca>
Subject: Re: [PATCH 2/2] mm/hugetlb: refactor subpage recording

On 1/26/21 11:21 AM, Joao Martins wrote:
> On 1/26/21 6:08 PM, Mike Kravetz wrote:
>> On 1/25/21 12:57 PM, Joao Martins wrote:
>>> 
>>> +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,
>>

Thinking about this a bit more ...

mem_map can be accessed contiguously if we have a virtual memmap.  Correct?
I suspect virtual memmap may be the most common configuration today.  However,
it seems we do need to handle other configurations.

> 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++.

Yes, I believe those would also have the issue.
Cc: John and Jason as they have spent a significant amount of time in gup
code recently.  There may be something that makes that code safe?

> 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.

If my thoughts about virtual memmap are correct, then could we simply have
a !vmemmap version of mem_map_offset (or similar routine) to avoid overhead?

-- 
Mike Kravetz

> 
> 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);
>