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

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Date:   Tue, 26 Jan 2021 17:58:53 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Jason Gunthorpe <jgg@...pe.ca>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     Joao Martins <joao.m.martins@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH 2/2] mm/hugetlb: refactor subpage recording

On 1/26/21 4:07 PM, Jason Gunthorpe wrote:
> On Tue, Jan 26, 2021 at 01:21:46PM -0800, 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?
>> 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?
> 
> I'm looking at Matt's folio patches and see:
> 
> +static inline struct folio *next_folio(struct folio *folio)
> +{
> +       return folio + folio_nr_pages(folio);
> +}
> 
> And checking page_trans_huge_mapcount():
> 
> 	for (i = 0; i < thp_nr_pages(page); i++) {
> 		mapcount = atomic_read(&page[i]._mapcount) + 1;
> 
> And we have the same logic in hmm_vma_walk_pud():
> 
> 	if (pud_huge(pud) && pud_devmap(pud)) {
> 		pfn = pud_pfn(pud) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
> 		for (i = 0; i < npages; ++i, ++pfn)
> 			hmm_pfns[i] = pfn | cpu_flags;
> 
> So, if page[n] does not access the tail pages of a compound we have
> many more people who are surprised by this than just GUP.
> 
> Where are these special rules for hugetlb compound tails documented?
> Why does it need to be like this? 

The commit where this was first addressed/pointed out is 69d177c2fc70
"hugetlbfs: handle pages higher order than MAX_ORDER" from back in 2008.
I only know about this because I stumbled upon it a few times in the
hugetlb code.  Although, it does not appear to be hugetlb specific.

As pointed out by Joao, you can also see the differences in pfn_to_page
for CONFIG_SPARSE_VMEMMAP and CONFIG_SPARSEMEM.  The only time we might
have issues is with CONFIG_SPARSEMEM.  I would bet CONFIG_SPARSE_VMEMMAP
is far more common.

Cc: Dave as he has sparsemem history.
-- 
Mike Kravetz

> Isn't it saner to forbid a compound and its tails from being
> non-linear in the page array? That limits when compounds can be
> created, but seems more likely to happen than a full mm audit to find
> all the places that assume linearity.