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

David Hildenbrand
2023-11-16T14:13:27Z

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.

Why? in the default case, fork() does not even care about MAP_SHARED
mappings; it does not copy the page tables/ptes. See vma_needs_copy().

Only in corner cases (e.g., uffd-wp, VM_PFNMAP, VM_MIXEDMAP), or in
MAP_PRIVATE mappings, you can even end up in that code.

In MAP_PRIVATE mappings, only anon pages can be R/W, other pages can
never be writable, so it does not matter. In VM_PFNMAP/VM_MIXEDMAP
likely all permissions match either way.

So you might just wrprotect the !anon pages R/O for the child and nobody
should really notice it, write faults will resolve it.

Famous last words :)

--
Cheers,

David / dhildenb