(This page and everything below is horribly out of date. It needs to be updated to reflect the implementation.)
In write path, VFS's function __block_write_full_page() is called with special get_block() method which doesn't allocate block immediately, but reserve it only and mark bh Delay.
In flush path, VFS's function mpage_da_writepages() scans mapping for dirty pages, collects dirty non-allocated bhs, tries to allocate them with minimal calls to get_block() method, applies allocated blocks to delayed bhs and drops Delay state.
ext4 implements two different get_block() methods:
- one for write(2) path
* checks whether block is already allocated * if it's not, then reserves block(s) needed for data and metadata and mark bh Delay
- another one for flush path
* starts transaction * allocates blocks * updates on-disk size * closes transaction