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Kernel: Use a shared physical page for zero-filled pages until written

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This patch adds a globally shared zero-filled PhysicalPage that will
be mapped into every slot of every zero-filled AnonymousVMObject until
that page is written to, achieving CoW-like zero-filled pages.

Initial testing show that this doesn't actually achieve any sharing yet
but it seems like a good design regardless, since it may reduce the
number of page faults taken by programs.

If you look at the refcount of MM.shared_zero_page() it will have quite
a high refcount, but that's just because everything maps it everywhere.
If you want to see the "real" refcount, you can build with the
MAP_SHARED_ZERO_PAGE_LAZILY flag, and we'll defer mapping of the shared
zero page until the first NP read fault.

I've left this behavior behind a flag for future testing of this code.
This commit is contained in:
Andreas Kling 2020-02-15 13:12:02 +01:00
parent a4d857e3c5
commit c624d3875e
5 changed files with 41 additions and 8 deletions
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9
Kernel/VM/MemoryManager.h
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@ -132,6 +132,8 @@ public:
void dump_kernel_regions();
PhysicalPage& shared_zero_page() { return *m_shared_zero_page; }
private:
MemoryManager();
~MemoryManager();
@ -172,6 +174,8 @@ private:
RefPtr<PageDirectory> m_kernel_page_directory;
RefPtr<PhysicalPage> m_low_page_table;
RefPtr<PhysicalPage> m_shared_zero_page;
unsigned m_user_physical_pages { 0 };
unsigned m_user_physical_pages_used { 0 };
unsigned m_super_physical_pages { 0 };
@ -223,3 +227,8 @@ inline bool is_user_range(VirtualAddress vaddr, size_t size)
return false;
return is_user_address(vaddr) && is_user_address(vaddr.offset(size));
}
inline bool PhysicalPage::is_shared_zero_page() const
{
return this == &MM.shared_zero_page();
}