Re: [PATCH v3 01/15] mm: Batch-copy PTE ranges during fork()

[David Hildenbrand <david@redhat.com>]

On 04.12.23 16:47, David Hildenbrand wrote:
> On 04.12.23 11:54, Ryan Roberts wrote:
>> Convert copy_pte_range() to copy a set of ptes in a batch. A given batch
>> maps a physically contiguous block of memory, all belonging to the same
>> folio. This will likely improve performance by a tiny amount due to
>> batching the folio reference count management and calling set_ptes()
>> rather than making individual calls to set_pte_at().
>>
>> However, the primary motivation for this change is to reduce the number
>> of tlb maintenance operations that the arm64 backend has to perform
>> during fork, as it is about to add transparent support for the
>> "contiguous bit" in its ptes. By write-protecting the parent using the
>> new ptep_set_wrprotects() (note the 's' at the end) function, the
>> backend can avoid having to unfold contig ranges of PTEs, which is
>> expensive, when all ptes in the range are being write-protected.
>> Similarly, by using set_ptes() rather than set_pte_at() to set up ptes
>> in the child, the backend does not need to fold a contiguous range once
>> they are all populated - they can be initially populated as a contiguous
>> range in the first place.
>>
>> This change addresses the core-mm refactoring only, and introduces
>> ptep_set_wrprotects() with a default implementation that calls
>> ptep_set_wrprotect() for each pte in the range. A separate change will
>> implement ptep_set_wrprotects() in the arm64 backend to realize the
>> performance improvement as part of the work to enable contpte mappings.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>>    include/linux/pgtable.h |  13 +++
>>    mm/memory.c             | 195 ++++++++++++++++++++++++++++++----------
>>    2 files changed, 162 insertions(+), 46 deletions(-)
>>
>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>> index af7639c3b0a3..1c50f8a0fdde 100644
>> --- a/include/linux/pgtable.h
>> +++ b/include/linux/pgtable.h
>> @@ -622,6 +622,19 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addres
>>    }
>>    #endif
>>    
>> +#ifndef ptep_set_wrprotects
>> +struct mm_struct;
>> +static inline void ptep_set_wrprotects(struct mm_struct *mm,
>> +				unsigned long address, pte_t *ptep,
>> +				unsigned int nr)
>> +{
>> +	unsigned int i;
>> +
>> +	for (i = 0; i < nr; i++, address += PAGE_SIZE, ptep++)
>> +		ptep_set_wrprotect(mm, address, ptep);
>> +}
>> +#endif
>> +
>>    /*
>>     * On some architectures hardware does not set page access bit when accessing
>>     * memory page, it is responsibility of software setting this bit. It brings
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 1f18ed4a5497..8a87a488950c 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -924,68 +924,162 @@ copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
>>    	return 0;
>>    }
>>    
>> +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, bool enforce_uffd_wp,
>> +				      int *dirty_nr, int *writable_nr)
>> +{
>> +	int floops;
>> +	int i;
>> +	unsigned long pfn;
>> +	bool prot_none;
>> +	bool uffd_wp;
>> +
>> +	if (!folio_test_large(folio))
>> +		return 1;
>> +
>> +	/*
>> +	 * Loop either to `end` or to end of folio if its contiguously mapped,
>> +	 * whichever is smaller.
>> +	 */
>> +	floops = (end - addr) >> PAGE_SHIFT;
>> +	floops = min_t(int, floops,
>> +		       folio_pfn(folio_next(folio)) - page_to_pfn(page));
>> +
>> +	pfn = page_to_pfn(page);
>> +	prot_none = pte_protnone(ptent);
>> +	uffd_wp = pte_uffd_wp(ptent);
>> +
>> +	*dirty_nr = !!pte_dirty(ptent);
>> +	*writable_nr = !!pte_write(ptent);
>> +
>> +	pfn++;
>> +	pte++;
>> +
>> +	for (i = 1; i < floops; i++) {
>> +		ptent = ptep_get(pte);
>> +
>> +		if (!pte_present(ptent) || pte_pfn(ptent) != pfn ||
>> +		    prot_none != pte_protnone(ptent) ||
>> +		    (enforce_uffd_wp && uffd_wp != pte_uffd_wp(ptent)))
>> +			break;
>> +
>> +		if (pte_dirty(ptent))
>> +			(*dirty_nr)++;
>> +		if (pte_write(ptent))
>> +			(*writable_nr)++;
>> +
>> +		pfn++;
>> +		pte++;
>> +	}
>> +
>> +	return i;
>> +}
>> +
>>    /*
>> - * Copy one pte.  Returns 0 if succeeded, or -EAGAIN if one preallocated page
>> - * is required to copy this pte.
>> + * Copy set of contiguous ptes.  Returns number of ptes copied if succeeded
>> + * (always gte 1), or -EAGAIN if one preallocated page is required to copy the
>> + * first pte.
>>     */
>>    static inline int
>> -copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>> -		 pte_t *dst_pte, pte_t *src_pte, unsigned long addr, int *rss,
>> -		 struct folio **prealloc)
>> +copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>> +		  pte_t *dst_pte, pte_t *src_pte,
>> +		  unsigned long addr, unsigned long end,
>> +		  int *rss, struct folio **prealloc)
>>    {
>>    	struct mm_struct *src_mm = src_vma->vm_mm;
>>    	unsigned long vm_flags = src_vma->vm_flags;
>>    	pte_t pte = ptep_get(src_pte);
>>    	struct page *page;
>>    	struct folio *folio;
>> +	int nr = 1;
>> +	bool anon = false;
>> +	bool enforce_uffd_wp = userfaultfd_wp(dst_vma);
>> +	int nr_dirty = !!pte_dirty(pte);
>> +	int nr_writable = !!pte_write(pte);
>> +	int i, ret;
>>    
>>    	page = vm_normal_page(src_vma, addr, pte);
>> -	if (page)
>> +	if (page) {
>>    		folio = page_folio(page);
>> -	if (page && folio_test_anon(folio)) {
>> -		/*
>> -		 * If this page may have been pinned by the parent process,
>> -		 * copy the page immediately for the child so that we'll always
>> -		 * guarantee the pinned page won't be randomly replaced in the
>> -		 * future.
>> -		 */
>> -		folio_get(folio);
>> -		if (unlikely(page_try_dup_anon_rmap(page, false, src_vma))) {
>> -			/* Page may be pinned, we have to copy. */
>> -			folio_put(folio);
>> -			return copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
>> -						 addr, rss, prealloc, page);
>> +		anon = folio_test_anon(folio);
>> +		nr = folio_nr_pages_cont_mapped(folio, page, src_pte, addr, end,
>> +						pte, enforce_uffd_wp, &nr_dirty,
>> +						&nr_writable);
>> +		folio_ref_add(folio, nr);
>> +
>> +		for (i = 0; i < nr; i++, page++) {
>> +			if (anon) {
>> +				/*
>> +				 * If this page may have been pinned by the
>> +				 * parent process, copy the page immediately for
>> +				 * the child so that we'll always guarantee the
>> +				 * pinned page won't be randomly replaced in the
>> +				 * future.
>> +				 */
>> +				if (unlikely(page_try_dup_anon_rmap(
>> +						page, false, src_vma))) {
>> +					if (i != 0)
>> +						break;
>> +					/* Page may be pinned, we have to copy. */
>> +					folio_ref_sub(folio, nr);
>> +					ret = copy_present_page(
>> +						dst_vma, src_vma, dst_pte,
>> +						src_pte, addr, rss, prealloc,
>> +						page);
>> +					return ret == 0 ? 1 : ret;
>> +				}
>> +				rss[MM_ANONPAGES]++;
>> +				VM_BUG_ON(PageAnonExclusive(page));
>> +			} else {
>> +				page_dup_file_rmap(page, false);
>> +				rss[mm_counter_file(page)]++;
>> +			}
>>    		}
>> -		rss[MM_ANONPAGES]++;
>> -	} else if (page) {
>> -		folio_get(folio);
>> -		page_dup_file_rmap(page, false);
>> -		rss[mm_counter_file(page)]++;
>> -	}
> 
> This likely looks a lot neater if you keep the existing structure.
> 
> For example, you can simply have on the !anon path
> 
> } else if (page) {
> 	folio = page_folio(page);
> 	nr = folio_nr_pages_cont_mapped ...
> 	folio_ref_add(folio, nr);
> 	for (i = 0; i < nr; i++, page++)
> 		page_dup_file_rmap(page, false);
> 	rss[mm_counter_file(&folio->page)] += nr;
> }
> 

With rmap batching from [1] -- rebased+changed on top of that -- we could turn
that into an effective (untested):

         if (page && folio_test_anon(folio)) {
+               nr = folio_nr_pages_cont_mapped(folio, page, src_pte, addr, end,
+                                               pte, enforce_uffd_wp, &nr_dirty,
+                                               &nr_writable);
                 /*
                  * If this page may have been pinned by the parent process,
                  * copy the page immediately for the child so that we'll always
                  * guarantee the pinned page won't be randomly replaced in the
                  * future.
                  */
-               folio_get(folio);
-               if (unlikely(folio_try_dup_anon_rmap_pte(folio, page, src_vma))) {
+               folio_ref_add(folio, nr);
+               if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page, nr, src_vma))) {
                         /* Page may be pinned, we have to copy. */
-                       folio_put(folio);
-                       return copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
-                                                addr, rss, prealloc, page);
+                       folio_ref_sub(folio, nr);
+                       ret = copy_present_page(dst_vma, src_vma, dst_pte,
+                                               src_pte, addr, rss, prealloc,
+                                               page);
+                       return ret == 0 ? 1 : ret;
                 }
-               rss[MM_ANONPAGES]++;
+               rss[MM_ANONPAGES] += nr;
         } else if (page) {
-               folio_get(folio);
-               folio_dup_file_rmap_pte(folio, page);
-               rss[mm_counter_file(page)]++;
+               nr = folio_nr_pages_cont_mapped(folio, page, src_pte, addr, end,
+                                               pte, enforce_uffd_wp, &nr_dirty,
+                                               &nr_writable);
+               folio_ref_add(folio, nr);
+               folio_dup_file_rmap_ptes(folio, page, nr);
+               rss[mm_counter_file(page)] += nr;
         }


We'll have to test performance, but it could be that we want to specialize
more on !folio_test_large(). That code is very performance-sensitive.


[1] https://lkml.kernel.org/r/20231204142146.91437-1-david@redhat.com

-- 
Cheers,

David / dhildenb