Re: [PATCH v3 3/3] mm: Batch-zap large anonymous folio PTE mappings

[Ryan Roberts <ryan.roberts@arm.com>]

On 26/07/2023 17:19, Nathan Chancellor wrote:
> Hi Ryan,
> 
> On Thu, Jul 20, 2023 at 12:29:55PM +0100, 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
>> deferred 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 | 120 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 120 insertions(+)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 01f39e8144ef..189b1cfd823d 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -1391,6 +1391,94 @@ 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_cont_mapped_vaddr(struct page *page,
>> +				struct page *anchor, unsigned long anchor_vaddr)
>> +{
>> +	unsigned long offset;
>> +	unsigned long vaddr;
>> +
>> +	offset = (page_to_pfn(page) - page_to_pfn(anchor)) << PAGE_SHIFT;
>> +	vaddr = anchor_vaddr + offset;
>> +
>> +	if (anchor > page) {
>> +		if (vaddr > anchor_vaddr)
>> +			return 0;
>> +	} else {
>> +		if (vaddr < anchor_vaddr)
>> +			return ULONG_MAX;
>> +	}
>> +
>> +	return vaddr;
>> +}
>> +
>> +static int folio_nr_pages_cont_mapped(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;
>> +	struct page *folio_end;
>> +
>> +	if (!folio_test_large(folio))
>> +		return 1;
>> +
>> +	folio_end = &folio->page + folio_nr_pages(folio);
>> +	end = min(page_cont_mapped_vaddr(folio_end, 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)
>> +			break;
>> +
>> +		pfn++;
>> +		pte++;
>> +	}
>> +
>> +	return i;
>> +}
>> +
>> +static unsigned long try_zap_anon_pte_range(struct mmu_gather *tlb,
>> +					    struct vm_area_struct *vma,
>> +					    struct folio *folio,
>> +					    struct page *page, pte_t *pte,
>> +					    unsigned long addr, int nr_pages,
>> +					    struct zap_details *details)
>> +{
>> +	struct mm_struct *mm = tlb->mm;
>> +	pte_t ptent;
>> +	bool full;
>> +	int i;
>> +
>> +	for (i = 0; i < nr_pages;) {
>> +		ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
>> +		tlb_remove_tlb_entry(tlb, pte, addr);
>> +		zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent);
>> +		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);
>> +
>> +	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 +1516,38 @@ 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.
>> +			 */
>> +			if (page && PageAnon(page)) {
>> +				struct folio *folio = page_folio(page);
>> +				int nr_pages_req, nr_pages;
>> +
>> +				nr_pages_req = folio_nr_pages_cont_mapped(
>> +						folio, page, pte, addr, end);
>> +
>> +				nr_pages = try_zap_anon_pte_range(tlb, vma,
>> +						folio, page, pte, addr,
>> +						nr_pages_req, details);
>> +
>> +				rss[mm_counter(page)] -= nr_pages;
>> +				nr_pages--;
>> +				pte += nr_pages;
>> +				addr += nr_pages << PAGE_SHIFT;
>> +
>> +				if (unlikely(nr_pages < nr_pages_req)) {
>> +					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
>>
> 
> After this change in -next as commit 904d9713b3b0 ("mm: batch-zap large
> anonymous folio PTE mappings"), I see the following splats several times
> when booting Debian's s390x configuration (which I have mirrored at [1])
> in QEMU (bisect log below):

Thanks for the bug report and sorry for the inconvenience. I'm going to need a
little time to figure out a build environment for s390x and get it reproducing.
Hopefully I can come back to you tomorrow with a fix.

Thanks,
Ryan