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Kernel: Don't bother with page tables for PROT_NONE mappings

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When mapping or unmapping completely inaccessible memory regions,
we don't need to update the page tables at all. This saves a bunch of
time in some situations, most notably during dynamic linking, where we
make a large VM reservation and immediately throw it away. :^)
This commit is contained in:
Andreas Kling 2022-01-15 17:37:17 +01:00
parent 9ddccbc6da
commit c6adefcfc0
2 changed files with 18 additions and 5 deletions
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9
Kernel/Memory/AddressSpace.cpp
View file

@ -191,7 +191,14 @@ ErrorOr<Region*> AddressSpace::allocate_region_with_vmobject(VirtualRange const&
region_name = TRY(KString::try_create(name)); region_name = TRY(KString::try_create(name));
auto region = TRY(Region::try_create_user_accessible(range, move(vmobject), offset_in_vmobject, move(region_name), prot_to_region_access_flags(prot), Region::Cacheable::Yes, shared)); auto region = TRY(Region::try_create_user_accessible(range, move(vmobject), offset_in_vmobject, move(region_name), prot_to_region_access_flags(prot), Region::Cacheable::Yes, shared));
auto* added_region = TRY(add_region(move(region))); auto* added_region = TRY(add_region(move(region)));
TRY(added_region->map(page_directory(), ShouldFlushTLB::No)); if (prot == PROT_NONE) {
// For PROT_NONE mappings, we don't have to set up any page table mappings.
// We do still need to attach the region to the page_directory though.
SpinlockLocker mm_locker(s_mm_lock);
added_region->set_page_directory(page_directory());
} else {
TRY(added_region->map(page_directory(), ShouldFlushTLB::No));
}
return added_region; return added_region;
} }

14
Kernel/Memory/Region.cpp
View file

@ -52,10 +52,16 @@ Region::~Region()
MM.unregister_kernel_region(*this); MM.unregister_kernel_region(*this);
if (m_page_directory) { if (m_page_directory) {
SpinlockLocker page_lock(m_page_directory->get_lock()); SpinlockLocker pd_locker(m_page_directory->get_lock());
SpinlockLocker lock(s_mm_lock); if (!is_readable() && !is_writable() && !is_executable()) {
unmap(ShouldDeallocateVirtualRange::Yes); // If the region is "PROT_NONE", we didn't map it in the first place,
VERIFY(!m_page_directory); // so all we need to do here is deallocate the VM.
m_page_directory->range_allocator().deallocate(range());
} else {
SpinlockLocker mm_locker(s_mm_lock);
unmap_with_locks_held(ShouldDeallocateVirtualRange::Yes, ShouldFlushTLB::Yes, pd_locker, mm_locker);
VERIFY(!m_page_directory);
}
} }
} }