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

Ryan Roberts
2023-11-16T13:49:23Z

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 guess we could special case and do it the way you suggested for cow mappings;
it might be faster, but certainly not cleaner and easier IMHO.

>