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[2003:cb:c722:3700:6501:8925:6f9:fcdc]) by smtp.gmail.com with ESMTPSA id iv7-20020a05600c548700b00405959469afsm15440924wmb.3.2023.12.04.07.47.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 04 Dec 2023 07:47:19 -0800 (PST) Message-ID: Date: Mon, 4 Dec 2023 16:47:17 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 01/15] mm: Batch-copy PTE ranges during fork() To: Ryan Roberts , Catalin Marinas , Will Deacon , Ard Biesheuvel , Marc Zyngier , Oliver Upton , James Morse , Suzuki K Poulose , Zenghui Yu , Andrey Ryabinin , Alexander Potapenko , Andrey Konovalov , Dmitry Vyukov , Vincenzo Frascino , Andrew Morton , Anshuman Khandual , Matthew Wilcox , Yu Zhao , Mark Rutland , Kefeng Wang , John Hubbard , Zi Yan , Barry Song <21cnbao@gmail.com>, Alistair Popple , Yang Shi Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20231204105440.61448-1-ryan.roberts@arm.com> <20231204105440.61448-2-ryan.roberts@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 663F7C001F X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: tsr8p8hfkm4n8xhs8fxqftczuwhjsndr X-HE-Tag: 1701704843-351131 X-HE-Meta: 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 ZgcnjIwy 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 04.12.23 11:54, 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. 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 > --- > include/linux/pgtable.h | 13 +++ > mm/memory.c | 195 ++++++++++++++++++++++++++++++---------- > 2 files changed, 162 insertions(+), 46 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..8a87a488950c 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -924,68 +924,162 @@ copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma > return 0; > } > > +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 enforce_uffd_wp, > + int *dirty_nr, int *writable_nr) > +{ > + int floops; > + int i; > + unsigned long pfn; > + bool prot_none; > + bool uffd_wp; > + > + if (!folio_test_large(folio)) > + return 1; > + > + /* > + * 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)); > + > + pfn = page_to_pfn(page); > + prot_none = pte_protnone(ptent); > + uffd_wp = pte_uffd_wp(ptent); > + > + *dirty_nr = !!pte_dirty(ptent); > + *writable_nr = !!pte_write(ptent); > + > + pfn++; > + pte++; > + > + for (i = 1; i < floops; i++) { > + ptent = ptep_get(pte); > + > + if (!pte_present(ptent) || pte_pfn(ptent) != pfn || > + prot_none != pte_protnone(ptent) || > + (enforce_uffd_wp && uffd_wp != pte_uffd_wp(ptent))) > + break; > + > + if (pte_dirty(ptent)) > + (*dirty_nr)++; > + if (pte_write(ptent)) > + (*writable_nr)++; > + > + 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 = false; > + bool enforce_uffd_wp = userfaultfd_wp(dst_vma); > + int nr_dirty = !!pte_dirty(pte); > + int nr_writable = !!pte_write(pte); > + int i, ret; > > 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, enforce_uffd_wp, &nr_dirty, > + &nr_writable); > + folio_ref_add(folio, nr); > + > + 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. */ > + folio_ref_sub(folio, nr); > + ret = copy_present_page( > + dst_vma, src_vma, dst_pte, > + src_pte, addr, rss, prealloc, > + page); > + return ret == 0 ? 1 : ret; > + } > + 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)]++; > - } This likely looks a lot neater if you keep the existing structure. For example, you can simply have on the !anon path } else if (page) { folio = page_folio(page); nr = folio_nr_pages_cont_mapped ... folio_ref_add(folio, nr); for (i = 0; i < nr; i++, page++) page_dup_file_rmap(page, false); rss[mm_counter_file(&folio->page)] += nr; } -- Cheers, David / dhildenb