Ext2FS: allocate_blocks allocates contiguous blocks (SerenityOS#1095)

This implementation uses the new helper method of Bitmap called
find_longest_range_of_unset_bits. This method looks for the biggest 
range of contiguous bits unset in the bitmap and returns the start of
the range back to the caller.

AK/Bitmap.h

@@ -157,6 +157,46 @@ class Bitmap {
  return -1;
  }
 
+ int find_longest_range_of_unset_bits(int max_length, int& found_range_size) const
+ {
+ int first_index = find_first_unset();
+ if (first_index == -1)
+ return -1;
+
+ int free_region_start = first_index;
+ int free_region_size = 1;
+
+ int max_region_start = free_region_start;
+ int max_region_size = free_region_size;
+
+ // Let's try and find the best fit possible
+ for (int j = first_index + 1; j < m_size && free_region_size < max_length; j++) {
+ if (!get(j)) {
+ if (free_region_size == 0)
+ free_region_start = j;
+ free_region_size++;
+ } else {
+ if (max_region_size < free_region_size) {
+ max_region_size = free_region_size;
+ max_region_start = free_region_start;
+ }
+ free_region_start = 0;
+ free_region_size = 0;
+ }
+ }
+
+ if (max_region_size < free_region_size) {
+ max_region_size = free_region_size;
+ max_region_start = free_region_start;
+ }
+
+ found_range_size = max_region_size;
+ if (max_region_size > 1)
+ return max_region_start;
+ // if the max free region size is one, then return the earliest one found
+ return first_index;
+ }
+
  explicit Bitmap()
  : m_size(0)
  , m_owned(true)
@@ -181,7 +221,6 @@ class Bitmap {
  }
 
 private:
-
  int size_in_bytes() const { return ceil_div(m_size, 8); }
 
  u8* m_data { nullptr };

Kernel/FileSystem/Ext2FileSystem.cpp

@@ -1080,12 +1080,49 @@ Vector<Ext2FS::BlockIndex> Ext2FS::allocate_blocks(GroupIndex preferred_group_in
  dbg() << "Ext2FS: allocate_blocks:";
 #endif
  blocks.ensure_capacity(count);
- for (int i = 0; i < count; ++i) {
- auto block_index = allocate_block(preferred_group_index);
- blocks.unchecked_append(block_index);
+
+ bool found_a_group = false;
+ GroupIndex group_index = preferred_group_index;
+
+ if (!group_descriptor(preferred_group_index).bg_free_blocks_count) {
+ group_index = 1;
+ }
+
+ while (blocks.size() < count) {
+ if (group_descriptor(group_index).bg_free_blocks_count) {
+ found_a_group = true;
+ } else {
+ if (group_index == preferred_group_index)
+ group_index = 1;
+ for (; group_index < m_block_group_count; ++group_index) {
+ if (group_descriptor(group_index).bg_free_blocks_count) {
+ found_a_group = true;
+ break;
+ }
+ }
+ }
+ ASSERT(found_a_group);
+ auto& bgd = group_descriptor(group_index);
+ auto& cached_bitmap = get_bitmap_block(bgd.bg_block_bitmap);
+
+ int blocks_in_group = min(blocks_per_group(), super_block().s_blocks_count);
+ auto block_bitmap = Bitmap::wrap(cached_bitmap.buffer.data(), blocks_in_group);
+
+ BlockIndex first_block_in_group = (group_index - 1) * blocks_per_group() + first_block_index();
+ int free_region_size = 0;
+ int first_unset_bit_index = block_bitmap.find_longest_range_of_unset_bits(count - blocks.size(), free_region_size);
+ ASSERT(first_unset_bit_index != -1);
 #ifdef EXT2_DEBUG
- dbg() << " > " << block_index;
+ dbg() << "Ext2FS: allocating free region of size: " << free_region_size << "[" << group_index << "]";
 #endif
+ for (int i = 0; i < free_region_size; ++i) {
+ BlockIndex block_index = (unsigned)(first_unset_bit_index + i) + first_block_in_group;
+ set_block_allocation_state(block_index, true);
+ blocks.unchecked_append(block_index);
+#ifdef EXT2_DEBUG
+ dbg() << " allocated > " << block_index;
+#endif
+ }
  }
 
  ASSERT(blocks.size() == count);