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

David Hildenbrand
2023-12-04T16:00:15Z

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