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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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51
Kernel/VM/Region.cpp
View file

@ -6,33 +6,30 @@
#include <Kernel/VM/MemoryManager.h>
#include <Kernel/VM/Region.h>
Region::Region(const Range& range, const String& name, u8 access, bool cow)
Region::Region(const Range& range, const String& name, u8 access)
: m_range(range)
, m_vmobject(AnonymousVMObject::create_with_size(size()))
, m_name(name)
, m_access(access)
, m_cow_map(Bitmap::create(m_vmobject->page_count(), cow))
{
MM.register_region(*this);
}
Region::Region(const Range& range, RefPtr<Inode>&& inode, const String& name, u8 access, bool cow)
Region::Region(const Range& range, RefPtr<Inode>&& inode, const String& name, u8 access)
: m_range(range)
, m_vmobject(InodeVMObject::create_with_inode(*inode))
, m_name(name)
, m_access(access)
, m_cow_map(Bitmap::create(m_vmobject->page_count(), cow))
{
MM.register_region(*this);
}
Region::Region(const Range& range, NonnullRefPtr<VMObject> vmo, size_t offset_in_vmo, const String& name, u8 access, bool cow)
Region::Region(const Range& range, NonnullRefPtr<VMObject> vmo, size_t offset_in_vmo, const String& name, u8 access)
: m_range(range)
, m_offset_in_vmo(offset_in_vmo)
, m_vmobject(move(vmo))
, m_name(name)
, m_access(access)
, m_cow_map(Bitmap::create(m_vmobject->page_count(), cow))
{
MM.register_region(*this);
}
@ -77,9 +74,11 @@ NonnullOwnPtr<Region> Region::clone()
vaddr().get());
#endif
// Set up a COW region. The parent (this) region becomes COW as well!
m_cow_map.fill(true);
ensure_cow_map().fill(true);
MM.remap_region(current->process().page_directory(), *this);
return Region::create_user_accessible(m_range, m_vmobject->clone(), m_offset_in_vmo, m_name, m_access, true);
auto clone_region = Region::create_user_accessible(m_range, m_vmobject->clone(), m_offset_in_vmo, m_name, m_access);
clone_region->ensure_cow_map();
return clone_region;
}
int Region::commit()
@ -123,30 +122,50 @@ size_t Region::amount_shared() const
return bytes;
}
NonnullOwnPtr<Region> Region::create_user_accessible(const Range& range, const StringView& name, u8 access, bool cow)
NonnullOwnPtr<Region> Region::create_user_accessible(const Range& range, const StringView& name, u8 access)
{
auto region = make<Region>(range, name, access, cow);
auto region = make<Region>(range, name, access);
region->m_user_accessible = true;
return region;
}
NonnullOwnPtr<Region> Region::create_user_accessible(const Range& range, NonnullRefPtr<VMObject> vmobject, size_t offset_in_vmobject, const StringView& name, u8 access, bool cow)
NonnullOwnPtr<Region> Region::create_user_accessible(const Range& range, NonnullRefPtr<VMObject> vmobject, size_t offset_in_vmobject, const StringView& name, u8 access)
{
auto region = make<Region>(range, move(vmobject), offset_in_vmobject, name, access, cow);
auto region = make<Region>(range, move(vmobject), offset_in_vmobject, name, access);
region->m_user_accessible = true;
return region;
}
NonnullOwnPtr<Region> Region::create_user_accessible(const Range& range, NonnullRefPtr<Inode> inode, const StringView& name, u8 access, bool cow)
NonnullOwnPtr<Region> Region::create_user_accessible(const Range& range, NonnullRefPtr<Inode> inode, const StringView& name, u8 access)
{
auto region = make<Region>(range, move(inode), name, access, cow);
auto region = make<Region>(range, move(inode), name, access);
region->m_user_accessible = true;
return region;
}
NonnullOwnPtr<Region> Region::create_kernel_only(const Range& range, const StringView& name, u8 access, bool cow)
NonnullOwnPtr<Region> Region::create_kernel_only(const Range& range, const StringView& name, u8 access)
{
auto region = make<Region>(range, name, access, cow);
auto region = make<Region>(range, name, access);
region->m_user_accessible = false;
return region;
}
bool Region::should_cow(size_t page_index) const
{
if (m_shared)
return false;
return m_cow_map && m_cow_map->get(page_index);
}
void Region::set_should_cow(size_t page_index, bool cow)
{
ASSERT(!m_shared);
ensure_cow_map().set(page_index, cow);
}
Bitmap& Region::ensure_cow_map() const
{
if (!m_cow_map)
m_cow_map = make<Bitmap>(page_count(), true);
return *m_cow_map;
}