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

[Ryan Roberts]

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@xxxxxxx>
>>>>>>> ---
>>>>>>>      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?


> 
> But devil is in the detail.
>