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

[Ryan Roberts]

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?

> 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