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

[Zi Yan <ziy@nvidia.com>]

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On 18 Jul 2023, at 6:19, Ryan Roberts wrote:

> On 17/07/2023 17:15, Zi Yan wrote:
>> 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.
>
> OK I see your point. OK let me pull the page count calculation into here and
> pass it to zap_anon_pte_range(). Then I think we can keep the name as is?

Yes. Thanks.

>
>
>>
>> BTW, page_addr() needs a better name and is easily confused with existing
>> page_address().
>
> Yeah... I'll try to think of something for v2.
>
>>
>>>
>>>> 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


--
Best Regards,
Yan, Zi

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