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Message-ID: <08ef2c36-2b9c-4b96-9d1d-68cca0f68ba5@redhat.com>
Date: Thu, 16 Nov 2023 14:20:58 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
Ard Biesheuvel <ardb@...nel.org>,
Marc Zyngier <maz@...nel.org>,
Oliver Upton <oliver.upton@...ux.dev>,
James Morse <james.morse@....com>,
Suzuki K Poulose <suzuki.poulose@....com>,
Zenghui Yu <yuzenghui@...wei.com>,
Andrey Ryabinin <ryabinin.a.a@...il.com>,
Alexander Potapenko <glider@...gle.com>,
Andrey Konovalov <andreyknvl@...il.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
Vincenzo Frascino <vincenzo.frascino@....com>,
Andrew Morton <akpm@...ux-foundation.org>,
Anshuman Khandual <anshuman.khandual@....com>,
Matthew Wilcox <willy@...radead.org>,
Yu Zhao <yuzhao@...gle.com>,
Mark Rutland <mark.rutland@....com>,
Kefeng Wang <wangkefeng.wang@...wei.com>,
John Hubbard <jhubbard@...dia.com>, Zi Yan <ziy@...dia.com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 01/14] mm: Batch-copy PTE ranges during fork()
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@....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?
--
Cheers,
David / dhildenb
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