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[2003:cb:c722:3700:6501:8925:6f9:fcdc]) by smtp.gmail.com with ESMTPSA id i7-20020adffc07000000b0033346408820sm4137550wrr.68.2023.12.04.08.00.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 04 Dec 2023 08:00:06 -0800 (PST) Message-ID: <104de2d6-ecf9-4b0c-a982-5bd8e1aea758@redhat.com> Date: Mon, 4 Dec 2023 17:00:05 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 01/15] mm: Batch-copy PTE ranges during fork() From: David Hildenbrand 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> Autocrypt: addr=david@redhat.com; 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charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 04.12.23 16:47, David Hildenbrand wrote: > 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; > } > With rmap batching from [1] -- rebased+changed on top of that -- we could turn that into an effective (untested): if (page && folio_test_anon(folio)) { + nr = folio_nr_pages_cont_mapped(folio, page, src_pte, addr, end, + pte, enforce_uffd_wp, &nr_dirty, + &nr_writable); /* * 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(folio_try_dup_anon_rmap_pte(folio, page, src_vma))) { + folio_ref_add(folio, nr); + if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page, nr, 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); + 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]++; + rss[MM_ANONPAGES] += nr; } else if (page) { - folio_get(folio); - folio_dup_file_rmap_pte(folio, page); - rss[mm_counter_file(page)]++; + 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); + folio_dup_file_rmap_ptes(folio, page, nr); + rss[mm_counter_file(page)] += nr; } We'll have to test performance, but it could be that we want to specialize more on !folio_test_large(). That code is very performance-sensitive. [1] https://lkml.kernel.org/r/20231204142146.91437-1-david@redhat.com -- Cheers, David / dhildenb _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel