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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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5
Kernel/VM/MemoryManager.cpp
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@@ -307,7 +307,6 @@ bool MemoryManager::zero_page(Region& region, unsigned page_index_in_region)
#ifdef PAGE_FAULT_DEBUG
dbgprintf(" >> ZERO P%p\n", physical_page->paddr().get());
#endif
region.set_should_cow(page_index_in_region, false);
vmo.physical_pages()[page_index_in_region] = move(physical_page);
remap_region_page(region, page_index_in_region);
return true;
@@ -468,9 +467,9 @@ OwnPtr<Region> MemoryManager::allocate_kernel_region(size_t size, const StringVi
ASSERT(range.is_valid());
OwnPtr<Region> region;
if (user_accessible)
region = Region::create_user_accessible(range, name, PROT_READ | PROT_WRITE | PROT_EXEC, false);
region = Region::create_user_accessible(range, name, PROT_READ | PROT_WRITE | PROT_EXEC);
else
region = Region::create_kernel_only(range, name, PROT_READ | PROT_WRITE | PROT_EXEC, false);
region = Region::create_kernel_only(range, name, PROT_READ | PROT_WRITE | PROT_EXEC);
MM.map_region_at_address(*m_kernel_page_directory, *region, range.base());
// FIXME: It would be cool if these could zero-fill on demand instead.
if (should_commit)