text: /*
   * 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 <AK/StringView.h>
  #include <Kernel/Arch/i386/CPU.h>
  #include <Kernel/Devices/BlockDevice.h>
  #include <Kernel/FileSystem/DiskBackedFileSystem.h>
  #include <Kernel/FileSystem/FileDescription.h>
  #include <Kernel/KBuffer.h>
  #include <Kernel/Process.h>
  
  //#define DBFS_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(DiskBackedFS& 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 DiskBackedFS flush here,
              //       not some DiskBackedFS 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:
      DiskBackedFS& m_fs;
      size_t m_entry_count { 10000 };
      KBuffer m_cached_block_data;
      KBuffer m_entries;
      bool m_dirty { false };
  };
  
  DiskBackedFS::DiskBackedFS(BlockDevice& device)
      : m_device(device)
  {
  }
  
  DiskBackedFS::~DiskBackedFS()
  {
  }
  
  bool DiskBackedFS::write_block(unsigned index, const u8* data, FileDescription* description)
  {
  #ifdef DBFS_DEBUG
      klog() << "DiskBackedFileSystem::write_block " << index << ", size=" << data.size();
  #endif
  
      bool allow_cache = !description || !description->is_direct();
  
      if (!allow_cache) {
          flush_specific_block_if_needed(index);
          u32 base_offset = static_cast<u32>(index) * static_cast<u32>(block_size());
          device().write_raw(base_offset, block_size(), data);
          return true;
      }
  
      auto& entry = cache().get(index);
      memcpy(entry.data, data, block_size());
      entry.is_dirty = true;
      entry.has_data = true;
  
      cache().set_dirty(true);
      return true;
  }
  
  bool DiskBackedFS::write_blocks(unsigned index, unsigned count, const u8* data, FileDescription* description)
  {
  #ifdef DBFS_DEBUG
      klog() << "DiskBackedFileSystem::write_blocks " << index << " x%u" << count;
  #endif
      for (unsigned i = 0; i < count; ++i)
          write_block(index + i, data + i * block_size(), description);
      return true;
  }
  
  bool DiskBackedFS::read_block(unsigned index, u8* buffer, FileDescription* description) const
  {
  #ifdef DBFS_DEBUG
      klog() << "DiskBackedFileSystem::read_block " << index;
  #endif
  
      bool allow_cache = !description || !description->is_direct();
  
      if (!allow_cache) {
          const_cast<DiskBackedFS*>(this)->flush_specific_block_if_needed(index);
          u32 base_offset = static_cast<u32>(index) * static_cast<u32>(block_size());
          bool success = device().read_raw(base_offset, block_size(), buffer);
          ASSERT(success);
          return true;
      }
  
      auto& entry = cache().get(index);
      if (!entry.has_data) {
          u32 base_offset = static_cast<u32>(index) * static_cast<u32>(block_size());
          bool success = device().read_raw(base_offset, block_size(), entry.data);
          entry.has_data = true;
          ASSERT(success);
      }
      memcpy(buffer, entry.data, block_size());
      return true;
  }
  
  bool DiskBackedFS::read_blocks(unsigned index, unsigned count, u8* buffer, FileDescription* description) const
  {
      if (!count)
          return false;
      if (count == 1)
          return read_block(index, buffer, description);
      u8* out = buffer;
  
      for (unsigned i = 0; i < count; ++i) {
          if (!read_block(index + i, out, description))
              return false;
          out += block_size();
      }
  
      return true;
  }
  
  void DiskBackedFS::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());
              device().write_raw(base_offset, block_size(), entry.data);
              entry.is_dirty = false;
          }
      });
  }
  
  void DiskBackedFS::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());
          device().write_raw(base_offset, block_size(), entry.data);
          ++count;
          entry.is_dirty = false;
      });
      cache().set_dirty(false);
      dbg() << class_name() << ": Flushed " << count << " blocks to disk";
  }
  
  void DiskBackedFS::flush_writes()
  {
      flush_writes_impl();
  }
  
  DiskCache& DiskBackedFS::cache() const
  {
      if (!m_cache)
          m_cache = make<DiskCache>(const_cast<DiskBackedFS&>(*this));
      return *m_cache;
  }
  
  }
  