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Kernel: Implement zone-based buddy allocator for physical memory

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The previous allocator was very naive and kept the state of all pages
in one big bitmap. When allocating, we had to scan through the bitmap
until we found an unset bit.

This patch introduces a new binary buddy allocator that manages the
physical memory pages.

Each PhysicalRegion is divided into zones (PhysicalZone) of 16MB each.
Any extra pages at the end of physical RAM that don't fit into a 16MB
zone are turned into 15 or fewer 1MB zones.

Each zone starts out with one full-sized block, which is then
recursively subdivided into halves upon allocation, until a block of
the request size can be returned.

There are more opportunities for improvement here: the way zone objects
are allocated and stored is non-optimal. Same goes for the allocation
of buddy block state bitmaps.
This commit is contained in:
Andreas Kling 2021-07-12 22:52:17 +02:00
parent be83b3aff4
commit ba87571366
9 changed files with 411 additions and 145 deletions
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181
Kernel/VM/PhysicalRegion.cpp
View file

@ -8,10 +8,28 @@
#include <AK/RefPtr.h>
#include <Kernel/Assertions.h>
#include <Kernel/Random.h>
#include <Kernel/VM/MemoryManager.h>
#include <Kernel/VM/PhysicalRegion.h>
#include <Kernel/VM/PhysicalZone.h>
namespace Kernel {
static constexpr u32 next_power_of_two(u32 value)
{
value--;
value |= value >> 1;
value |= value >> 2;
value |= value >> 4;
value |= value >> 8;
value |= value >> 16;
value++;
return value;
}
PhysicalRegion::~PhysicalRegion()
{
}
PhysicalRegion::PhysicalRegion(PhysicalAddress lower, PhysicalAddress upper)
: m_lower(lower)
, m_upper(upper)
@ -26,17 +44,34 @@ void PhysicalRegion::expand(PhysicalAddress lower, PhysicalAddress upper)
m_upper = upper;
}
void PhysicalRegion::initialize_zones()
{
size_t remaining_pages = m_pages;
auto base_address = m_lower;
auto make_zones = [&](size_t zone_size) {
while (remaining_pages >= zone_size) {
m_zones.append(make<PhysicalZone>(base_address, zone_size));
dmesgln(" * Zone {:016x}-{:016x} ({} bytes)", base_address.get(), base_address.get() + zone_size * PAGE_SIZE - 1, zone_size * PAGE_SIZE);
base_address = base_address.offset(zone_size * PAGE_SIZE);
remaining_pages -= zone_size;
}
};
make_zones(4096);
make_zones(256);
}
unsigned PhysicalRegion::finalize_capacity()
{
VERIFY(!m_pages);
m_pages = (m_upper.get() - m_lower.get()) / PAGE_SIZE;
m_bitmap.grow(m_pages, false);
return size();
}
PhysicalRegion PhysicalRegion::take_pages_from_beginning(unsigned page_count)
OwnPtr<PhysicalRegion> PhysicalRegion::try_take_pages_from_beginning(unsigned page_count)
{
VERIFY(m_used == 0);
VERIFY(page_count > 0);
@ -47,136 +82,64 @@ PhysicalRegion PhysicalRegion::take_pages_from_beginning(unsigned page_count)
// TODO: find a more elegant way to re-init the existing region
m_pages = 0;
m_bitmap = {}; // FIXME: Kind of wasteful
finalize_capacity();
auto taken_region = create(taken_lower, taken_upper);
taken_region.finalize_capacity();
auto taken_region = try_create(taken_lower, taken_upper);
if (!taken_region)
return {};
taken_region->finalize_capacity();
return taken_region;
}
NonnullRefPtrVector<PhysicalPage> PhysicalRegion::take_contiguous_free_pages(size_t count, size_t physical_alignment)
{
VERIFY(m_pages);
VERIFY(m_used != m_pages);
// FIXME: Care about alignment.
(void)physical_alignment;
auto rounded_page_count = next_power_of_two(count);
auto order = __builtin_ctz(rounded_page_count);
Optional<PhysicalAddress> page_base;
for (auto& zone : m_zones) {
page_base = zone.allocate_block(order);
if (page_base.has_value())
break;
}
if (!page_base.has_value())
return {};
NonnullRefPtrVector<PhysicalPage> physical_pages;
physical_pages.ensure_capacity(count);
auto first_contiguous_page = find_contiguous_free_pages(count, physical_alignment);
for (size_t index = 0; index < count; index++)
physical_pages.append(PhysicalPage::create(m_lower.offset(PAGE_SIZE * (index + first_contiguous_page))));
for (size_t i = 0; i < count; ++i)
physical_pages.append(PhysicalPage::create(page_base.value().offset(i * PAGE_SIZE)));
return physical_pages;
}
unsigned PhysicalRegion::find_contiguous_free_pages(size_t count, size_t physical_alignment)
{
VERIFY(count != 0);
VERIFY(physical_alignment % PAGE_SIZE == 0);
// search from the last page we allocated
auto range = find_and_allocate_contiguous_range(count, physical_alignment / PAGE_SIZE);
VERIFY(range.has_value());
return range.value();
}
Optional<unsigned> PhysicalRegion::find_one_free_page()
{
if (m_used == m_pages) {
// We know we don't have any free pages, no need to check the bitmap
// Check if we can draw one from the return queue
if (m_recently_returned.size() > 0) {
u8 index = get_fast_random<u8>() % m_recently_returned.size();
Checked<PhysicalPtr> local_offset = m_recently_returned[index].get();
local_offset -= m_lower.get();
m_recently_returned.remove(index);
VERIFY(!local_offset.has_overflow());
VERIFY(local_offset.value() < (FlatPtr)(m_pages * PAGE_SIZE));
return local_offset.value() / PAGE_SIZE;
}
return {};
}
auto free_index = m_bitmap.find_one_anywhere_unset(m_free_hint);
if (!free_index.has_value())
return {};
auto page_index = free_index.value();
m_bitmap.set(page_index, true);
m_used++;
m_free_hint = free_index.value() + 1; // Just a guess
if (m_free_hint >= m_bitmap.size())
m_free_hint = 0;
return page_index;
}
Optional<unsigned> PhysicalRegion::find_and_allocate_contiguous_range(size_t count, unsigned alignment)
{
VERIFY(count != 0);
size_t found_pages_count = 0;
// TODO: Improve how we deal with alignment != 1
auto first_index = m_bitmap.find_longest_range_of_unset_bits(count + alignment - 1, found_pages_count);
if (!first_index.has_value())
return {};
auto page = first_index.value();
if (alignment != 1) {
auto lower_page = m_lower.get() / PAGE_SIZE;
page = ((lower_page + page + alignment - 1) & ~(alignment - 1)) - lower_page;
}
if (found_pages_count >= count) {
m_bitmap.set_range<true>(page, count);
m_used += count;
m_free_hint = first_index.value() + count + 1; // Just a guess
if (m_free_hint >= m_bitmap.size())
m_free_hint = 0;
return page;
}
return {};
}
RefPtr<PhysicalPage> PhysicalRegion::take_free_page()
{
VERIFY(m_pages);
auto free_index = find_one_free_page();
if (!free_index.has_value())
return nullptr;
return PhysicalPage::create(m_lower.offset((PhysicalPtr)free_index.value() * PAGE_SIZE));
}
void PhysicalRegion::free_page_at(PhysicalAddress addr)
{
VERIFY(m_pages);
if (m_used == 0) {
VERIFY_NOT_REACHED();
for (auto& zone : m_zones) {
auto page = zone.allocate_block(0);
if (page.has_value())
return PhysicalPage::create(page.value());
}
Checked<PhysicalPtr> local_offset = addr.get();
local_offset -= m_lower.get();
VERIFY(!local_offset.has_overflow());
VERIFY(local_offset.value() < ((PhysicalPtr)m_pages * PAGE_SIZE));
auto page = local_offset.value() / PAGE_SIZE;
m_bitmap.set(page, false);
m_free_hint = page; // We know we can find one here for sure
m_used--;
dbgln("PhysicalRegion::take_free_page: No free physical pages");
return nullptr;
}
void PhysicalRegion::return_page(PhysicalAddress paddr)
{
auto returned_count = m_recently_returned.size();
if (returned_count >= m_recently_returned.capacity()) {
// Return queue is full, pick a random entry and free that page
// and replace the entry with this page
auto& entry = m_recently_returned[get_fast_random<u8>()];
free_page_at(entry);
entry = paddr;
} else {
// Still filling the return queue, just append it
m_recently_returned.append(paddr);
for (auto& zone : m_zones) {
if (zone.contains(paddr)) {
zone.deallocate_block(paddr, 0);
return;
}
}
VERIFY_NOT_REACHED();
}
}