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Kernel: Defer creation of Region CoW bitmaps until they're needed

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Instead of allocating and populating a Copy-on-Write bitmap for each
Region up front, wait until we actually clone the Region for sharing
with another process.

In most cases, we never need any CoW bits and we save ourselves a lot
of kmalloc() memory and time.
This commit is contained in:
Andreas Kling 2019-10-01 19:58:41 +02:00
parent 9e7560fae9
commit d481ae95b5
3 changed files with 49 additions and 29 deletions
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22
Kernel/VM/Region.h
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@ -21,10 +21,10 @@ public:
Execute = 4,
};
static NonnullOwnPtr<Region> create_user_accessible(const Range&, const StringView& name, u8 access, bool cow = false);
static NonnullOwnPtr<Region> create_user_accessible(const Range&, NonnullRefPtr<VMObject>, size_t offset_in_vmobject, const StringView& name, u8 access, bool cow = false);
static NonnullOwnPtr<Region> create_user_accessible(const Range&, NonnullRefPtr<Inode>, const StringView& name, u8 access, bool cow = false);
static NonnullOwnPtr<Region> create_kernel_only(const Range&, const StringView& name, u8 access, bool cow = false);
static NonnullOwnPtr<Region> create_user_accessible(const Range&, const StringView& name, u8 access);
static NonnullOwnPtr<Region> create_user_accessible(const Range&, NonnullRefPtr<VMObject>, size_t offset_in_vmobject, const StringView& name, u8 access);
static NonnullOwnPtr<Region> create_user_accessible(const Range&, NonnullRefPtr<Inode>, const StringView& name, u8 access);
static NonnullOwnPtr<Region> create_kernel_only(const Range&, const StringView& name, u8 access);
~Region();
@ -103,8 +103,8 @@ public:
m_page_directory.clear();
}
bool should_cow(size_t page_index) const { return m_cow_map.get(page_index); }
void set_should_cow(size_t page_index, bool cow) { m_cow_map.set(page_index, cow); }
bool should_cow(size_t page_index) const;
void set_should_cow(size_t page_index, bool);
void set_writable(bool b)
{
@ -119,11 +119,13 @@ public:
Region* m_prev { nullptr };
// NOTE: These are public so we can make<> them.
Region(const Range&, const String&, u8 access, bool cow = false);
Region(const Range&, NonnullRefPtr<VMObject>, size_t offset_in_vmo, const String&, u8 access, bool cow = false);
Region(const Range&, RefPtr<Inode>&&, const String&, u8 access, bool cow = false);
Region(const Range&, const String&, u8 access);
Region(const Range&, NonnullRefPtr<VMObject>, size_t offset_in_vmo, const String&, u8 access);
Region(const Range&, RefPtr<Inode>&&, const String&, u8 access);
private:
Bitmap& ensure_cow_map() const;
RefPtr<PageDirectory> m_page_directory;
Range m_range;
size_t m_offset_in_vmo { 0 };
@ -132,5 +134,5 @@ private:
u8 m_access { 0 };
bool m_shared { false };
bool m_user_accessible { false };
Bitmap m_cow_map;
mutable OwnPtr<Bitmap> m_cow_map;
};