Chapter 9: The Extended Filesystem Family
Iterate until atleast the number
of blocks requiredfor indirection hasbeen allocated
ext2_alloc_branch
ext2_alloc_blocks
ext2_alloc_blocks
ext2_new_blocks
Set up indirection structures
Figure 9-12: Code flow diagram for
ext2_alloc_branch.
Decide if reservations should be used
ext2_new_blocks
ext2_has_free_blocks
ext2_get_group_desc
ext2_try_to_allocate_with_rsv
Disable preallocation if not enough free blocks are available
Allocation successsful? Update statistics and return
Allocation successsful?
Try different block groups
Update statistics and return
Retry from beginning without reservations
Yes
No
Yes
No
Figure 9-13: Code flow diagram forext2_new_blocks.
Recall that Ext2 supports pre-allocation, and this needs to be partly handled inext2_new_blocks.Since
the mechanism is already complicated enough without the details of pre-allocation, let’s first consider it
without this extra complexity. We will come back to how pre-allocation works exactly afterward.
Consider the prototype ofext2_new_blocks(note thatext2_fsblk_tistypedef’d tounsigned longand
represents a block number).
fs/ext2/balloc.c
ext2_fsblk_t ext2_new_blocks(struct inode *inode, ext2_fsblk_t goal,
unsigned long *count, int *errp)
{
...
