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

Alistair Popple
2023-11-24T00:09:55Z

Ryan Roberts <ryan.roberts@arm.com> writes:

> On 23/11/2023 04:26, Alistair Popple wrote:
>>
>> Ryan Roberts <ryan.roberts@arm.com> writes:
>>
>>> 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;
>>
>> We should update the function comment to indicate why we return 1 here
>> because it will become non-obvious in future. But perhaps it's better to
>> leave this as is and do the error check/return code calculation in
>> copy_present_ptes().
>
> OK, I'll return 0 for success and fix it up to 1 in copy_present_ptes().
>
>>
>>> +}
>>> +
>>> +static inline unsigned long page_cont_mapped_vaddr(struct page *page,
>>> + struct page *anchor, unsigned long anchor_vaddr)
>>
>> It's likely I'm easily confused but the arguments here don't make much
>> sense to me. Something like this (noting that I've switch the argument
>> order) makes more sense to me at least:
>>
>> static inline unsigned long page_cont_mapped_vaddr(struct page *page,
>> unsigned long page_vaddr, struct page *next_folio_page)
>
> I was originally using page_cont_mapped_vaddr() in more places than here and
> needed a more generic helper than just "what is the virtual address of the end
> of the folio, given a random page within the folio and its virtual address"; (I
> needed "what is the virtual address of a page given a different page and its
> virtual address and assuming the distance between the 2 pages is the same in
> physical and virtual space"). But given I don't need that generality anymore,
> yes, I agree I can simplify this significantly.

Thanks for the explaination, that explains my head scratching.

> I think I can remove the function entirely and replace with this in
> folio_nr_pages_cont_mapped():
>
> /*
> * Loop either to `end` or to end of folio if its contiguously mapped,
> * whichever is smaller.
> */
> floops = (end - addr) >> PAGE_SHIFT;
> floops = min_t(int, floops,
> folio_pfn(folio_next(folio)) - page_to_pfn(page));
>
> where `end` and `addr` are the parameters as passed into the function. What do
> you think?

Will admit by the end of the review I was wondering why we even needed
the extra function so looks good to me (the comment helps too!)

>>
>>> +{
>>> + unsigned long offset;
>>> + unsigned long vaddr;
>>> +
>>> + offset = (page_to_pfn(page) - page_to_pfn(anchor)) << PAGE_SHIFT;
>>
>> Which IMHO makes this much more readable:
>>
>> offset = (page_to_pfn(next_folio_page) - page_to_pfn(page)) << PAGE_SHIFT;
>>
>>> + vaddr = anchor_vaddr + offset;
>>> +
>>> + if (anchor > page) {
>>
>> And also highlights that I think this condition (page > folio_page_end)
>> is impossible to hit. Which is good ...
>>
>>> + if (vaddr > anchor_vaddr)
>>> + return 0;
>>
>> ... because I'm not sure returning 0 is valid as we would end up setting
>> floops = (0 - addr) >> PAGE_SHIFT which doesn't seem like it would end
>> particularly well :-)
>
> This was covering the more general case that I no longer need.
>
>>
>>> + } else {
>>> + if (vaddr < anchor_vaddr)
>>
>> Same here - isn't the vaddr of the next folio always going to be larger
>> than the vaddr for the current page? It seems this function is really
>> just calculating the virtual address of the next folio, or am I deeply
>> confused?
>
> This aims to protect against the corner case, where a page from a folio is
> mremap()ed very high in address space such that the extra pages from the anchor
> page to the end of the folio would actually wrap back to zero. But with the
> approach propsed above, this problem goes away, I think.
>
>>
>>> + 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);
>>
>> I think you can replace this with:
>>
>> folio_end = folio_next(folio)
>
> yep, done - thanks.
>
>>
>> Although given this is only passed to page_cont_mapped_vaddr() perhaps
>> it's better to just pass the folio in and do the calculation there.
>>
>>> + 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);
>>> }
>>> - VM_BUG_ON(page && folio_test_anon(folio) && PageAnonExclusive(page));
>>>
>>> /*
>>> - * If it's a shared mapping, mark it clean in
>>> - * the child
>>> + * If it's a shared mapping, mark it clean in the child. If its a
>>> + * private mapping, mark it dirty in the child if _any_ of the parent
>>> + * mappings in the block were marked dirty. The contiguous block of
>>> + * mappings are all backed by the same folio, so if any are dirty then
>>> + * the whole folio is dirty. This allows us to determine the batch size
>>> + * without having to ever consider the dirty bit. See
>>> + * folio_nr_pages_cont_mapped().
>>> */
>>> - if (vm_flags & VM_SHARED)
>>> - pte = pte_mkclean(pte);
>>> - pte = pte_mkold(pte);
>>> + pte = pte_mkold(pte_mkclean(pte));
>>> + if (!(vm_flags & VM_SHARED) && any_dirty)
>>> + pte = pte_mkdirty(pte);
>>>
>>> if (!userfaultfd_wp(dst_vma))
>>> pte = pte_clear_uffd_wp(pte);
>>>
>>> - set_pte_at(dst_vma->vm_mm, addr, dst_pte, pte);
>>> - return 0;
>>> + set_ptes(dst_vma->vm_mm, addr, dst_pte, pte, nr);
>>> + return nr;
>>> }
>>>
>>> static inline struct folio *page_copy_prealloc(struct mm_struct *src_mm,
>>> @@ -1087,15 +1174,28 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>>> */
>>> WARN_ON_ONCE(ret != -ENOENT);
>>> }
>>> - /* copy_present_pte() will clear `*prealloc' if consumed */
>>> - ret = copy_present_pte(dst_vma, src_vma, dst_pte, src_pte,
>>> - addr, rss, &prealloc);
>>> + /* copy_present_ptes() will clear `*prealloc' if consumed */
>>> + ret = copy_present_ptes(dst_vma, src_vma, dst_pte, src_pte,
>>> + addr, end, rss, &prealloc);
>>> +
>>> /*
>>> * If we need a pre-allocated page for this pte, drop the
>>> * locks, allocate, and try again.
>>> */
>>> if (unlikely(ret == -EAGAIN))
>>> break;
>>> +
>>> + /*
>>> + * Positive return value is the number of ptes copied.
>>> + */
>>> + VM_WARN_ON_ONCE(ret < 1);
>>> + progress += 8 * ret;
>>> + ret--;
>>
>> Took me a second to figure out what was going on here. I think it would
>> be clearer to rename ret to nr_ptes ...
>>
>>> + dst_pte += ret;
>>> + src_pte += ret;
>>> + addr += ret << PAGE_SHIFT;
>>> + ret = 0;
>>> +
>>> if (unlikely(prealloc)) {
>>> /*
>>> * pre-alloc page cannot be reused by next time so as
>>> @@ -1106,7 +1206,6 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>>> folio_put(prealloc);
>>> prealloc = NULL;
>>> }
>>> - progress += 8;
>>> } while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end);
>>
>> ... and do dst_pte += nr_ptes, etc. here instead (noting of course that
>> the continue clauses will need nr_ptes == 1, but perhpas reset that at
>> the start of the loop).
>
> Yes, much cleaner! Implementing for v3...
>
> Thanks for the review!
>
> Thanks,
> Ryan
>
>>
>>> arch_leave_lazy_mmu_mode();
>>