Kernel: Split BlockBasedFileSystem off FileBackedFileSystem

FileBackedFileSystem is one that's backed by (mounted from) a file, in other
words one that has a "source" of the mount; that doesn't mean it deals in
blocks. The hierarchy now becomes:

* FS
  * ProcFS
  * DevPtsFS
  * TmpFS
  * FileBackedFS
    * (future) Plan9FS
    * BlockBasedFS
      * Ext2FS

Kernel/FileSystem/BlockBasedFileSystem.cpp

@@ -0,0 +1,299 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <Kernel/FileSystem/BlockBasedFileSystem.h>
+#include <Kernel/Process.h>
+
+//#define BBFS_DEBUG
+
+namespace Kernel {
+
+struct CacheEntry {
+    time_t timestamp { 0 };
+    u32 block_index { 0 };
+    u8* data { nullptr };
+    bool has_data { false };
+    bool is_dirty { false };
+};
+
+class DiskCache {
+public:
+    explicit DiskCache(BlockBasedFS& fs)
+        : m_fs(fs)
+        , m_cached_block_data(KBuffer::create_with_size(m_entry_count * m_fs.block_size()))
+        , m_entries(KBuffer::create_with_size(m_entry_count * sizeof(CacheEntry)))
+    {
+        for (size_t i = 0; i < m_entry_count; ++i) {
+            entries()[i].data = m_cached_block_data.data() + i * m_fs.block_size();
+        }
+    }
+
+    ~DiskCache() { }
+
+    bool is_dirty() const { return m_dirty; }
+    void set_dirty(bool b) { m_dirty = b; }
+
+    CacheEntry& get(u32 block_index) const
+    {
+        auto now = kgettimeofday().tv_sec;
+
+        CacheEntry* oldest_clean_entry = nullptr;
+        for (size_t i = 0; i < m_entry_count; ++i) {
+            auto& entry = const_cast<CacheEntry&>(entries()[i]);
+            if (entry.block_index == block_index) {
+                entry.timestamp = now;
+                return entry;
+            }
+            if (!entry.is_dirty) {
+                if (!oldest_clean_entry)
+                    oldest_clean_entry = &entry;
+                else if (entry.timestamp < oldest_clean_entry->timestamp)
+                    oldest_clean_entry = &entry;
+            }
+        }
+        if (!oldest_clean_entry) {
+            // Not a single clean entry! Flush writes and try again.
+            // NOTE: We want to make sure we only call FileBackedFS flush here,
+            //       not some FileBackedFS subclass flush!
+            m_fs.flush_writes_impl();
+            return get(block_index);
+        }
+
+        // Replace the oldest clean entry.
+        auto& new_entry = *oldest_clean_entry;
+        new_entry.timestamp = now;
+        new_entry.block_index = block_index;
+        new_entry.has_data = false;
+        new_entry.is_dirty = false;
+        return new_entry;
+    }
+
+    const CacheEntry* entries() const { return (const CacheEntry*)m_entries.data(); }
+    CacheEntry* entries() { return (CacheEntry*)m_entries.data(); }
+
+    template<typename Callback>
+    void for_each_entry(Callback callback)
+    {
+        for (size_t i = 0; i < m_entry_count; ++i)
+            callback(entries()[i]);
+    }
+
+private:
+    BlockBasedFS& m_fs;
+    size_t m_entry_count { 10000 };
+    KBuffer m_cached_block_data;
+    KBuffer m_entries;
+    bool m_dirty { false };
+};
+
+BlockBasedFS::BlockBasedFS(FileDescription& file_description)
+    : FileBackedFS(file_description)
+{
+    ASSERT(file_description.file().is_seekable());
+}
+
+BlockBasedFS::~BlockBasedFS()
+{
+}
+
+bool BlockBasedFS::write_block(unsigned index, const u8* data, size_t count, size_t offset, bool allow_cache)
+{
+    ASSERT(m_logical_block_size);
+    ASSERT(offset + count <= block_size());
+#ifdef BBFS_DEBUG
+    klog() << "BlockBasedFileSystem::write_block " << index << ", size=" << data.size();
+#endif
+
+    if (!allow_cache) {
+        flush_specific_block_if_needed(index);
+        u32 base_offset = static_cast<u32>(index) * static_cast<u32>(block_size()) + offset;
+        file_description().seek(base_offset, SEEK_SET);
+        auto nwritten = file_description().write(data, count);
+        if (nwritten < 0)
+            return false;
+        ASSERT(static_cast<size_t>(nwritten) == count);
+        return true;
+    }
+
+    auto& entry = cache().get(index);
+    if (count < block_size()) {
+        // Fill the cache first.
+        read_block(index, nullptr, block_size());
+    }
+    memcpy(entry.data + offset, data, count);
+    entry.is_dirty = true;
+    entry.has_data = true;
+
+    cache().set_dirty(true);
+    return true;
+}
+
+bool BlockBasedFS::raw_read(unsigned index, u8* buffer)
+{
+    u32 base_offset = static_cast<u32>(index) * static_cast<u32>(m_logical_block_size);
+    file_description().seek(base_offset, SEEK_SET);
+    auto nread = file_description().read(buffer, m_logical_block_size);
+    ASSERT((size_t)nread == m_logical_block_size);
+    return true;
+}
+bool BlockBasedFS::raw_write(unsigned index, const u8* buffer)
+{
+    u32 base_offset = static_cast<u32>(index) * static_cast<u32>(m_logical_block_size);
+    file_description().seek(base_offset, SEEK_SET);
+    auto nwritten = file_description().write(buffer, m_logical_block_size);
+    ASSERT((size_t)nwritten == m_logical_block_size);
+    return true;
+}
+
+bool BlockBasedFS::raw_read_blocks(unsigned index, size_t count, u8* buffer)
+{
+    for (unsigned block = index; block < (index + count); block++) {
+        if (!raw_read(block, buffer))
+            return false;
+        buffer += logical_block_size();
+    }
+    return true;
+}
+bool BlockBasedFS::raw_write_blocks(unsigned index, size_t count, const u8* buffer)
+{
+    for (unsigned block = index; block < (index + count); block++) {
+        if (!raw_write(block, buffer))
+            return false;
+        buffer += logical_block_size();
+    }
+    return true;
+}
+
+bool BlockBasedFS::write_blocks(unsigned index, unsigned count, const u8* data, bool allow_cache)
+{
+    ASSERT(m_logical_block_size);
+#ifdef BBFS_DEBUG
+    klog() << "BlockBasedFileSystem::write_blocks " << index << " x" << count;
+#endif
+    for (unsigned i = 0; i < count; ++i)
+        write_block(index + i, data + i * block_size(), block_size(), 0, allow_cache);
+    return true;
+}
+
+bool BlockBasedFS::read_block(unsigned index, u8* buffer, size_t count, size_t offset, bool allow_cache) const
+{
+    ASSERT(m_logical_block_size);
+    ASSERT(offset + count <= block_size());
+#ifdef BBFS_DEBUG
+    klog() << "BlockBasedFileSystem::read_block " << index;
+#endif
+
+    if (!allow_cache) {
+        const_cast<BlockBasedFS*>(this)->flush_specific_block_if_needed(index);
+        u32 base_offset = static_cast<u32>(index) * static_cast<u32>(block_size()) + static_cast<u32>(offset);
+        file_description().seek(base_offset, SEEK_SET);
+        auto nread = file_description().read(buffer, count);
+        if (nread < 0)
+            return false;
+        ASSERT(static_cast<size_t>(nread) == count);
+        return true;
+    }
+
+    auto& entry = cache().get(index);
+    if (!entry.has_data) {
+        u32 base_offset = static_cast<u32>(index) * static_cast<u32>(block_size());
+        file_description().seek(base_offset, SEEK_SET);
+        auto nread = file_description().read(entry.data, block_size());
+        if (nread < 0)
+            return false;
+        ASSERT(static_cast<size_t>(nread) == block_size());
+        entry.has_data = true;
+    }
+    if (buffer)
+        memcpy(buffer, entry.data + offset, count);
+    return true;
+}
+
+bool BlockBasedFS::read_blocks(unsigned index, unsigned count, u8* buffer, bool allow_cache) const
+{
+    ASSERT(m_logical_block_size);
+    if (!count)
+        return false;
+    if (count == 1)
+        return read_block(index, buffer, block_size(), 0, allow_cache);
+    u8* out = buffer;
+
+    for (unsigned i = 0; i < count; ++i) {
+        if (!read_block(index + i, out, block_size(), 0, allow_cache))
+            return false;
+        out += block_size();
+    }
+
+    return true;
+}
+
+void BlockBasedFS::flush_specific_block_if_needed(unsigned index)
+{
+    LOCKER(m_lock);
+    if (!cache().is_dirty())
+        return;
+    cache().for_each_entry([&](CacheEntry& entry) {
+        if (entry.is_dirty && entry.block_index == index) {
+            u32 base_offset = static_cast<u32>(entry.block_index) * static_cast<u32>(block_size());
+            file_description().seek(base_offset, SEEK_SET);
+            file_description().write(entry.data, block_size());
+            entry.is_dirty = false;
+        }
+    });
+}
+
+void BlockBasedFS::flush_writes_impl()
+{
+    LOCKER(m_lock);
+    if (!cache().is_dirty())
+        return;
+    u32 count = 0;
+    cache().for_each_entry([&](CacheEntry& entry) {
+        if (!entry.is_dirty)
+            return;
+        u32 base_offset = static_cast<u32>(entry.block_index) * static_cast<u32>(block_size());
+        file_description().seek(base_offset, SEEK_SET);
+        file_description().write(entry.data, block_size());
+        ++count;
+        entry.is_dirty = false;
+    });
+    cache().set_dirty(false);
+    dbg() << class_name() << ": Flushed " << count << " blocks to disk";
+}
+
+void BlockBasedFS::flush_writes()
+{
+    flush_writes_impl();
+}
+
+DiskCache& BlockBasedFS::cache() const
+{
+    if (!m_cache)
+        m_cache = make<DiskCache>(const_cast<BlockBasedFS&>(*this));
+    return *m_cache;
+}
+
+}