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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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4
Kernel/VM/MemoryManager.cpp
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@ -53,6 +53,8 @@ MemoryManager::MemoryManager()
write_cr3(kernel_page_directory().cr3());
setup_low_identity_mapping();
protect_kernel_image();
m_shared_zero_page = allocate_user_physical_page();
}
MemoryManager::~MemoryManager()
@ -297,7 +299,7 @@ OwnPtr<Region> MemoryManager::allocate_kernel_region(size_t size, const StringVi
region = Region::create_user_accessible(range, name, access, cacheable);
else
region = Region::create_kernel_only(range, name, access, cacheable);
region->set_page_directory(kernel_page_directory());
region->map(kernel_page_directory());
if (should_commit)
region->commit();
return region;