Re: [PATCH v1 3/3] mm: Batch-zap large anonymous folio PTE mappings

[Zi Yan <ziy@nvidia.com>]

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On 17 Jul 2023, at 11:55, Ryan Roberts wrote:

> On 17/07/2023 16:25, Zi Yan wrote:
>> On 17 Jul 2023, at 10:31, Ryan Roberts wrote:
>>
>>> This allows batching the rmap removal with folio_remove_rmap_range(),
>>> which means we avoid spuriously adding a partially unmapped folio to the
>>> deferrred split queue in the common case, which reduces split queue lock
>>> contention.
>>>
>>> Previously each page was removed from the rmap individually with
>>> page_remove_rmap(). If the first page belonged to a large folio, this
>>> would cause page_remove_rmap() to conclude that the folio was now
>>> partially mapped and add the folio to the deferred split queue. But
>>> subsequent calls would cause the folio to become fully unmapped, meaning
>>> there is no value to adding it to the split queue.
>>>
>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>>> ---
>>>  mm/memory.c | 119 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>  1 file changed, 119 insertions(+)
>>>
>>> diff --git a/mm/memory.c b/mm/memory.c
>>> index 01f39e8144ef..6facb8c8807a 100644
>>> --- a/mm/memory.c
>>> +++ b/mm/memory.c
>>> @@ -1391,6 +1391,95 @@ zap_install_uffd_wp_if_needed(struct vm_area_struct *vma,
>>>  	pte_install_uffd_wp_if_needed(vma, addr, pte, pteval);
>>>  }
>>>
>>> +static inline unsigned long page_addr(struct page *page,
>>> +				struct page *anchor, unsigned long anchor_addr)
>>> +{
>>> +	unsigned long offset;
>>> +	unsigned long addr;
>>> +
>>> +	offset = (page_to_pfn(page) - page_to_pfn(anchor)) << PAGE_SHIFT;
>>> +	addr = anchor_addr + offset;
>>> +
>>> +	if (anchor > page) {
>>> +		if (addr > anchor_addr)
>>> +			return 0;
>>> +	} else {
>>> +		if (addr < anchor_addr)
>>> +			return ULONG_MAX;
>>> +	}
>>> +
>>> +	return addr;
>>> +}
>>> +
>>> +static int calc_anon_folio_map_pgcount(struct folio *folio,
>>> +				       struct page *page, pte_t *pte,
>>> +				       unsigned long addr, unsigned long end)
>>> +{
>>> +	pte_t ptent;
>>> +	int floops;
>>> +	int i;
>>> +	unsigned long pfn;
>>> +
>>> +	end = min(page_addr(&folio->page + folio_nr_pages(folio), page, addr),
>>> +		  end);
>>> +	floops = (end - addr) >> PAGE_SHIFT;
>>> +	pfn = page_to_pfn(page);
>>> +	pfn++;
>>> +	pte++;
>>> +
>>> +	for (i = 1; i < floops; i++) {
>>> +		ptent = ptep_get(pte);
>>> +
>>> +		if (!pte_present(ptent) ||
>>> +		    pte_pfn(ptent) != pfn) {
>>> +			return i;
>>> +		}
>>> +
>>> +		pfn++;
>>> +		pte++;
>>> +	}
>>> +
>>> +	return floops;
>>> +}
>>> +
>>> +static unsigned long zap_anon_pte_range(struct mmu_gather *tlb,
>>> +					struct vm_area_struct *vma,
>>> +					struct page *page, pte_t *pte,
>>> +					unsigned long addr, unsigned long end,
>>> +					bool *full_out)
>>> +{
>>> +	struct folio *folio = page_folio(page);
>>> +	struct mm_struct *mm = tlb->mm;
>>> +	pte_t ptent;
>>> +	int pgcount;
>>> +	int i;
>>> +	bool full;
>>> +
>>> +	pgcount = calc_anon_folio_map_pgcount(folio, page, pte, addr, end);
>>> +
>>> +	for (i = 0; i < pgcount;) {
>>> +		ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
>>> +		tlb_remove_tlb_entry(tlb, pte, addr);
>>> +		full = __tlb_remove_page(tlb, page, 0);
>>> +
>>> +		if (unlikely(page_mapcount(page) < 1))
>>> +			print_bad_pte(vma, addr, ptent, page);
>>> +
>>> +		i++;
>>> +		page++;
>>> +		pte++;
>>> +		addr += PAGE_SIZE;
>>> +
>>> +		if (unlikely(full))
>>> +			break;
>>> +	}
>>> +
>>> +	folio_remove_rmap_range(folio, page - i, i, vma);
>>> +
>>> +	*full_out = full;
>>> +	return i;
>>> +}
>>> +
>>>  static unsigned long zap_pte_range(struct mmu_gather *tlb,
>>>  				struct vm_area_struct *vma, pmd_t *pmd,
>>>  				unsigned long addr, unsigned long end,
>>> @@ -1428,6 +1517,36 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
>>>  			page = vm_normal_page(vma, addr, ptent);
>>>  			if (unlikely(!should_zap_page(details, page)))
>>>  				continue;
>>> +
>>> +			/*
>>> +			 * Batch zap large anonymous folio mappings. This allows
>>> +			 * batching the rmap removal, which means we avoid
>>> +			 * spuriously adding a partially unmapped folio to the
>>> +			 * deferrred split queue in the common case, which
>>> +			 * reduces split queue lock contention. Require the VMA
>>> +			 * to be anonymous to ensure that none of the PTEs in
>>> +			 * the range require zap_install_uffd_wp_if_needed().
>>> +			 */
>>> +			if (page && PageAnon(page) && vma_is_anonymous(vma)) {
>>> +				bool full;
>>> +				int pgcount;
>>> +
>>> +				pgcount = zap_anon_pte_range(tlb, vma,
>>> +						page, pte, addr, end, &full);
>>
>> Are you trying to zap as many ptes as possible if all these ptes are
>> within a folio?
>
> Yes.
>
>> If so, why not calculate end before calling zap_anon_pte_range()?
>> That would make zap_anon_pte_range() simpler.
>
> I'm not sure I follow. That's currently done in calc_anon_folio_map_pgcount(). I
> could move it to here, but I'm not sure that makes things simpler, just puts
> more code in here and less in there?

Otherwise your zap_anon_pte_range() is really zap_anon_pte_in_folio_range() or
some other more descriptive name. When I first look at the name, I thought
PTEs will be zapped until the end. But that is not the case when I look at the
code. And future users can easily be confused too and use it in a wrong way.

BTW, page_addr() needs a better name and is easily confused with existing
page_address().

>
>> Also check if page is part of
>> a large folio first to make sure you can batch.
>
> Yeah that's fair. I'd be inclined to put that in zap_anon_pte_range() to short
> circuit calc_anon_folio_map_pgcount(). But ultimately zap_anon_pte_range() would
> still zap the single pte.
>
>
>>
>>> +
>>> +				rss[mm_counter(page)] -= pgcount;
>>> +				pgcount--;
>>> +				pte += pgcount;
>>> +				addr += pgcount << PAGE_SHIFT;
>>> +
>>> +				if (unlikely(full)) {
>>> +					force_flush = 1;
>>> +					addr += PAGE_SIZE;
>>> +					break;
>>> +				}
>>> +				continue;
>>> +			}
>>> +
>>>  			ptent = ptep_get_and_clear_full(mm, addr, pte,
>>>  							tlb->fullmm);
>>>  			tlb_remove_tlb_entry(tlb, pte, addr);
>>> -- 
>>> 2.25.1
>>
>>
>> --
>> Best Regards,
>> Yan, Zi


--
Best Regards,
Yan, Zi

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