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

Ryan Roberts
2023-11-23T12:28:40Z

On 23/11/2023 12:12, David Hildenbrand wrote:
> 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".

Yep, nice.

>
> 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".

I'm not sure we can simply "skip" batching on pte_protnone() since we will need
to terminate the batch if we spot it. But if we have to look for it anyway, we
might as well just terminate the batch when the value of pte_protnone()
*changes*. I'm also proposing to take this approach for pte_uffd_wp() which also
needs to be carefully preserved per-pte.

>