Re: [PATCH v2 01/14] mm: Batch-copy PTE ranges during fork()

David Hildenbrand
2023-11-16T13:21:08Z

On 16.11.23 12:20, Ryan Roberts wrote:
> On 16/11/2023 11:03, David Hildenbrand wrote:
>> On 15.11.23 17:30, 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, with the same permissions, and for shared mappings, the same
>>> dirty state. 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             | 175 +++++++++++++++++++++++++++++++---------
>>>   2 files changed, 150 insertions(+), 38 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..b7c8228883cf 100644
>>> --- a/mm/memory.c
>>> +++ b/mm/memory.c
>>> @@ -921,46 +921,129 @@ copy_present_page(struct vm_area_struct *dst_vma,
>>> struct vm_area_struct *src_vma
>>>           /* Uffd-wp needs to be delivered to dest pte as well */
>>>           pte = pte_mkuffd_wp(pte);
>>>       set_pte_at(dst_vma->vm_mm, addr, dst_pte, pte);
>>> -    return 0;
>>> +    return 1;
>>> +}
>>> +
>>> +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, bool *any_dirty)
>>> +{
>>> +    int floops;
>>> +    int i;
>>> +    unsigned long pfn;
>>> +    pgprot_t prot;
>>> +    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);
>>> +    prot = pte_pgprot(pte_mkold(pte_mkclean(ptent)));
>>> +
>>> +    *any_dirty = pte_dirty(ptent);
>>> +
>>> +    pfn++;
>>> +    pte++;
>>> +
>>> +    for (i = 1; i < floops; i++) {
>>> +        ptent = ptep_get(pte);
>>> +        ptent = pte_mkold(pte_mkclean(ptent));
>>> +
>>> +        if (!pte_present(ptent) || pte_pfn(ptent) != pfn ||
>>> +            pgprot_val(pte_pgprot(ptent)) != pgprot_val(prot))
>>> +            break;
>>> +
>>> +        if (pte_dirty(ptent))
>>> +            *any_dirty = true;
>>> +
>>> +        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;
>>> +    bool any_dirty = pte_dirty(pte);
>>> +    int i;
>>>         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, &any_dirty);
>>> +
>>> +        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. */
>>> +                    return copy_present_page(
>>> +                        dst_vma, src_vma, dst_pte,
>>> +                        src_pte, addr, rss, prealloc,
>>> +                        page);
>>> +                }
>>> +                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)]++;
>>> +
>>> +        nr = i;
>>> +        folio_ref_add(folio, nr);
>>>       }
>>>         /*
>>> @@ -968,24 +1051,28 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct
>>> vm_area_struct *src_vma,
>>>        * in the parent and the child
>>>        */
>>>       if (is_cow_mapping(vm_flags) && pte_write(pte)) {
>>> -        ptep_set_wrprotect(src_mm, addr, src_pte);
>>> +        ptep_set_wrprotects(src_mm, addr, src_pte, nr);
>>>           pte = pte_wrprotect(pte);
>>
>> You likely want an "any_pte_writable" check here instead, no?
>>
>> Any operations that target a single indiividual PTE while multiple PTEs are
>> adjusted are suspicious :)
>
> The idea is that I've already constrained the batch of pages such that the
> permissions are all the same (see folio_nr_pages_cont_mapped()). So if the first
> pte is writable, then they all are - something has gone badly wrong if some are
> writable and others are not.

I wonder if it would be cleaner and easier to not do that, though.

Simply record if any pte is writable. Afterwards they will *all* be R/O
and you can set the cont bit, correct?

--
Cheers,

David / dhildenb