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

[Ryan Roberts]

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@xxxxxxx>
>>>> ---
>>>>  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?


> 
> 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