Linux Kernel Architecture

Chapter 9: The Extended Filesystem Family

Compute start block

alloc_new_reservation

search_reserve_window

find_next_reservable_window

return

Old reservation window available Update settings

No further window? Return with error code

Retry next reservable space

Free block in reserve window? Yes

No

Figure 9-17: Code flow diagram foralloc_new_reservation.

First,alloc_new_reservationdetermines the block from which the search for a reservation window
starts.

fs/ext2/balloc.c

static int alloc_new_reservation(struct ext2_reserve_window_node *my_rsv,

ext2_grpblk_t grp_goal, struct super_block *sb,

unsigned int group, struct buffer_head *bitmap_bh)

{

struct ext2_reserve_window_node *search_head;

ext2_fsblk_t group_first_block, group_end_block, start_block;

ext2_grpblk_t first_free_block;

struct rb_root *fs_rsv_root = &EXT2_SB(sb)->s_rsv_window_root;

unsigned long size;

int ret;

group_first_block = ext2_group_first_block_no(sb, group);

group_end_block = group_first_block + (EXT2_BLOCKS_PER_GROUP(sb) - 1);

if (grp_goal < 0)

start_block = group_first_block;

else

start_block = grp_goal + group_first_block;

size = my_rsv->rsv_goal_size;

...

If the inode is already equipped with a reservation window, the allocation hit counter is evaluated and
the window resized accordingly:

fs/ext2/balloc.c

if (!rsv_is_empty(&my_rsv->rsv_window)) {

/*

* if the old reservation is cross group boundary

* and if the goal is inside the old reservation window,