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

David Hildenbrand
2023-11-23T12:12:45Z

On 23.11.23 11:26, Ryan Roberts wrote:
> On 16/11/2023 14:15, David Hildenbrand wrote:
>> On 16.11.23 15:13, David Hildenbrand wrote:
>>> On 16.11.23 14:49, Ryan Roberts wrote:
>>>> On 16/11/2023 13:20, David Hildenbrand wrote:
>>>>> 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?
>>>>
>>>> Oh I see what you mean - that only works for cow mappings though. If you have a
>>>> shared mapping, you won't be making it read-only at fork. So if we ignore
>>>> pte_write() state when demarking the batches, we will end up with a batch of
>>>> pages with a mix of RO and RW in the parent, but then we set_ptes() for the
>>>> child and those pages will all have the permissions of the first page of the
>>>> batch.
>>>
>>> I see what you mean.
>>>
>>> After fork(), all anon pages will be R/O in the parent and the child.
>>> Easy. If any PTE is writable, wrprotect all in the parent and the child.
>>>
>>> After fork(), all shared pages can be R/O or R/W in the parent. For
>>> simplicity, I think you can simply set them all R/O in the child. So if
>>> any PTE is writable, wrprotect all in the child.
>>
>> Or better: if any is R/O, set them all R/O. Otherwise just leave them as is.
>
> I've just come back to this to code it up, and want to clarify this last
> comment; I'm already going to have to collect any_writable for the anon case, so
> I will already have that info for the shared case too. I think you are
> suggesting I *additionally* collect any_readonly, then in the shared case, I
> only apply wrprotect if (any_writable && any_readonly). i.e. only apply
> wrprotect if there is a mix of permissions for the batch, otherwise all the
> permissions are the same (either all RW or all RO) and I can elide the wrprotet.
> Is that what you meant?

Yes. I suspect you might somehow be able to derive "any_readonly = nr -
!any_writable".

Within a VMA, we really should only see:
* writable VMA: some might be R/O, some might be R/W
* VMA applicable to NUMA hinting: some might be PROT_NONE, others R/O or
R/W

One could simply skip batching for now on pte_protnone() and focus on
the "writable" vs. "not-writable".

--
Cheers,

David / dhildenb