[PATCH v2 01/14] mm: Batch-copy PTE ranges during fork()
David Hildenbrand
david at redhat.com
Thu Nov 16 06:15:48 PST 2023
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 at 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
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