Kernel: Add an LRU block cache.

This papers over some of the incredibly inefficient access patterns in the
Ext2FS implementation for a while longer. :^)

AK/InlineLRUCache.h

@@ -0,0 +1,70 @@
+#pragma once
+
+#include <AK/HashMap.h>
+#include <AK/InlineLinkedList.h>
+
+namespace AK {
+
+template<typename K, typename V>
+class InlineLRUCache {
+public:
+    ~InlineLRUCache()
+    {
+        while (size())
+            remove_last();
+    }
+
+    size_t size() const { return m_map.size(); }
+    size_t capacity() const { return m_capacity; }
+
+    void set_capacity(size_t capacity)
+    {
+        if (capacity == m_capacity)
+            return;
+        m_capacity = capacity;
+        while (size() >= capacity)
+            remove_last();
+    }
+
+    V* get(const K& key)
+    {
+        auto it = m_map.find(key);
+        if (it == m_map.end())
+            return nullptr;
+        V* entry = (*it).value;
+        m_entries.remove(entry);
+        m_entries.prepend(entry);
+        return entry;
+    }
+
+    void put(K&& key, V&& value)
+    {
+        auto it = m_map.find(key);
+        if (it != m_map.end())
+            return;
+        V* new_entry = new V(move(value));
+        m_entries.prepend(new_entry);
+        m_map.set(key, new_entry);
+
+        while (size() >= capacity())
+            remove_last();
+    }
+
+private:
+    void remove_last()
+    {
+        V* entry = m_entries.tail();
+        ASSERT(entry);
+        m_entries.remove(entry);
+        m_map.remove(entry->m_key);
+        delete entry;
+    }
+
+    InlineLinkedList<V> m_entries;
+    HashMap<K, V*> m_map;
+    size_t m_capacity { 16 };
+};
+
+}
+
+using AK::InlineLRUCache;

Kernel/DiskBackedFileSystem.cpp

@@ -1,8 +1,49 @@
 #include "DiskBackedFileSystem.h"
 #include "i386.h"
+#include <AK/InlineLRUCache.h>
 
 //#define DBFS_DEBUG
 
+struct BlockIdentifier {
+    unsigned fsid { 0 };
+    unsigned index { 0 };
+
+    bool operator==(const BlockIdentifier& other) const { return fsid == other.fsid && index == other.index; }
+};
+
+namespace AK {
+
+template<>
+struct Traits<BlockIdentifier> {
+    static unsigned hash(const BlockIdentifier& block_id) { return pair_int_hash(block_id.fsid, block_id.index); }
+    static void dump(const BlockIdentifier& block_id) { kprintf("[block %02u:%08u]", block_id.fsid, block_id.index); }
+};
+
+}
+
+class CachedBlock : public InlineLinkedListNode<CachedBlock> {
+public:
+    CachedBlock(const BlockIdentifier& block_id, const ByteBuffer& buffer)
+        : m_key(block_id)
+        , m_buffer(buffer)
+    {
+    }
+
+    BlockIdentifier m_key;
+    CachedBlock* m_next { nullptr };
+    CachedBlock* m_prev { nullptr };
+
+    ByteBuffer m_buffer;
+};
+
+Lockable<InlineLRUCache<BlockIdentifier, CachedBlock>>& block_cache()
+{
+    static Lockable<InlineLRUCache<BlockIdentifier, CachedBlock>>* s_cache;
+    if (!s_cache)
+        s_cache = new Lockable<InlineLRUCache<BlockIdentifier, CachedBlock>>;
+    return *s_cache;
+}
+
 DiskBackedFS::DiskBackedFS(RetainPtr<DiskDevice>&& device)
     : m_device(move(device))
 {
@@ -37,6 +78,12 @@ ByteBuffer DiskBackedFS::read_block(unsigned index) const
 #ifdef DBFS_DEBUG
     kprintf("DiskBackedFileSystem::read_block %u\n", index);
 #endif
+    {
+        LOCKER(block_cache().lock());
+        if (auto* cached_block = block_cache().resource().get({ fsid(), index }))
+            return cached_block->m_buffer;
+    }
+
     auto buffer = ByteBuffer::create_uninitialized(block_size());
     //kprintf("created block buffer with size %u\n", block_size());
     DiskOffset base_offset = static_cast<DiskOffset>(index) * static_cast<DiskOffset>(block_size());
@@ -44,6 +91,10 @@ ByteBuffer DiskBackedFS::read_block(unsigned index) const
     bool success = device().read(base_offset, block_size(), buffer_pointer);
     ASSERT(success);
     ASSERT(buffer.size() == block_size());
+    {
+        LOCKER(block_cache().lock());
+        block_cache().resource().put({ fsid(), index }, CachedBlock({ fsid(), index }, buffer));
+    }
     return buffer;
 }
 

Kernel/FileSystem.h

@@ -147,8 +147,7 @@ namespace AK {
 
 template<>
 struct Traits<InodeIdentifier> {
-    // FIXME: This is a shitty hash.
-    static unsigned hash(const InodeIdentifier& inode) { return Traits<unsigned>::hash(inode.fsid()) + Traits<unsigned>::hash(inode.index()); }
+    static unsigned hash(const InodeIdentifier& inode) { return pair_int_hash(inode.fsid(), inode.index()); }
     static void dump(const InodeIdentifier& inode) { kprintf("%02u:%08u", inode.fsid(), inode.index()); }
 };