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

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Message-ID: <90e28835-cc36-5432-bb3b-4142fbbb2c21@oracle.com>
Date:   Tue, 26 Jan 2021 23:20:15 +0000
From:   Joao Martins <joao.m.martins@...cle.com>
To:     Mike Kravetz <mike.kravetz@...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 9:21 PM, Mike Kravetz wrote:
> 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?

Right.

> I suspect virtual memmap may be the most common configuration today.  However,
> it seems we do need to handle other configurations.
> 

At the moment mem_map_offset(page, n) in turn does this for >= MAX_ORDER:

	pfn_to_page(page_to_pfn(page) + n)


For CONFIG_SPARSE_VMEMMAP or FLATMEM will resolve into something:

	vmemmap + ((page - vmemmap) + n)

It isn't really different than incrementing the @page.

I can only think that CONFIG_SPARSEMEM and CONFIG_DISCONTIGMEM as the offending
cases which respectively look into section info or pgdat.

[CONFIG_DISCONTIGMEM doesnt isn't auto selected by any arch at the moment.]

>> That would mean get_user_pages_fast() and pin_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?
> 
Maybe -- Looking back, gup-fast has always relied on that page pointer arithmetic, even
before its refactors around __record_subpages() and what not.

>> 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?
> In that case, we could ifdef out on SPARSEMEM || DISCONTIGMEM for mem_map_offset() either
internally or within the helper I added for follow_hugetlb_page().