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https://github.com/SerenityOS/serenity
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After a fork(), page faults on anonymous mmaps can cause a redundant page fault to occur. This happens because VMObjects for anonymous mmaps are initially filled with references to the lazy_committed_page or shared_zero_page. If there is a fork, VMObject::try_clone() is called and all pages of the VMObject are marked as cow (via the m_cow_map). Page faults on a zero/lazy page are handled by handle_zero_fault(). handle_zero_fault() does not update m_cow_map, so if the page was marked cow before the fault, it will still be marked cow after the fault. This causes a second (redundant) page fault when the CPU retries the write. This commit removes the redundant page fault by not marking zero/lazy pages as cow in m_cow_map.
95 lines
3.7 KiB
C++
95 lines
3.7 KiB
C++
/*
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* Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#pragma once
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#include <Kernel/Memory/AllocationStrategy.h>
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#include <Kernel/Memory/MemoryManager.h>
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#include <Kernel/Memory/PageFaultResponse.h>
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#include <Kernel/Memory/PhysicalAddress.h>
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#include <Kernel/Memory/VMObject.h>
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namespace Kernel::Memory {
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class AnonymousVMObject final : public VMObject {
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public:
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virtual ~AnonymousVMObject() override;
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static ErrorOr<NonnullLockRefPtr<AnonymousVMObject>> try_create_with_size(size_t, AllocationStrategy);
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static ErrorOr<NonnullLockRefPtr<AnonymousVMObject>> try_create_for_physical_range(PhysicalAddress paddr, size_t size);
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static ErrorOr<NonnullLockRefPtr<AnonymousVMObject>> try_create_with_physical_pages(Span<NonnullRefPtr<PhysicalRAMPage>>);
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static ErrorOr<NonnullLockRefPtr<AnonymousVMObject>> try_create_purgeable_with_size(size_t, AllocationStrategy);
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static ErrorOr<NonnullLockRefPtr<AnonymousVMObject>> try_create_physically_contiguous_with_size(size_t);
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virtual ErrorOr<NonnullLockRefPtr<VMObject>> try_clone() override;
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[[nodiscard]] NonnullRefPtr<PhysicalRAMPage> allocate_committed_page(Badge<Region>);
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PageFaultResponse handle_cow_fault(size_t, VirtualAddress);
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size_t cow_pages() const;
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bool should_cow(size_t page_index, bool) const;
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ErrorOr<void> set_should_cow(size_t page_index, bool);
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bool is_purgeable() const { return m_purgeable; }
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bool is_volatile() const { return m_volatile; }
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ErrorOr<void> set_volatile(bool is_volatile, bool& was_purged);
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size_t purge();
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private:
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class SharedCommittedCowPages;
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static ErrorOr<NonnullLockRefPtr<AnonymousVMObject>> try_create_with_shared_cow(AnonymousVMObject const&, NonnullLockRefPtr<SharedCommittedCowPages>, FixedArray<RefPtr<PhysicalRAMPage>>&&);
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explicit AnonymousVMObject(FixedArray<RefPtr<PhysicalRAMPage>>&&, AllocationStrategy, Optional<CommittedPhysicalPageSet>);
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explicit AnonymousVMObject(PhysicalAddress, FixedArray<RefPtr<PhysicalRAMPage>>&&);
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explicit AnonymousVMObject(FixedArray<RefPtr<PhysicalRAMPage>>&&);
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explicit AnonymousVMObject(LockWeakPtr<AnonymousVMObject>, NonnullLockRefPtr<SharedCommittedCowPages>, FixedArray<RefPtr<PhysicalRAMPage>>&&);
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virtual StringView class_name() const override { return "AnonymousVMObject"sv; }
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AnonymousVMObject& operator=(AnonymousVMObject const&) = delete;
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AnonymousVMObject& operator=(AnonymousVMObject&&) = delete;
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AnonymousVMObject(AnonymousVMObject&&) = delete;
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virtual bool is_anonymous() const override { return true; }
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ErrorOr<void> ensure_cow_map();
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ErrorOr<void> ensure_or_reset_cow_map();
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void reset_cow_map();
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Optional<CommittedPhysicalPageSet> m_unused_committed_pages;
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Bitmap m_cow_map;
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// AnonymousVMObject shares committed COW pages with cloned children (happens on fork)
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class SharedCommittedCowPages final : public AtomicRefCounted<SharedCommittedCowPages> {
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AK_MAKE_NONCOPYABLE(SharedCommittedCowPages);
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public:
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SharedCommittedCowPages() = delete;
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explicit SharedCommittedCowPages(CommittedPhysicalPageSet&&);
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~SharedCommittedCowPages();
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[[nodiscard]] bool is_empty() const { return m_committed_pages.is_empty(); }
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[[nodiscard]] NonnullRefPtr<PhysicalRAMPage> take_one();
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void uncommit_one();
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private:
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Spinlock<LockRank::None> m_lock {};
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CommittedPhysicalPageSet m_committed_pages;
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};
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LockWeakPtr<AnonymousVMObject> m_cow_parent;
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LockRefPtr<SharedCommittedCowPages> m_shared_committed_cow_pages;
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bool m_purgeable { false };
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bool m_volatile { false };
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bool m_was_purged { false };
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};
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}
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