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

David Hildenbrand
2023-11-16T14:15:58Z

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.

But devil is in the detail.

--
Cheers,

David / dhildenb