[0/2] btrfs: more explaination on extent_map members

Message

Qu Wenruo April 2, 2024, 6:23 a.m. UTC

Btrfs uses extent_map to represent a in-memory file extent.

There are severam members that are 1:1 mappe in on-disk file extent
items and extent maps:

- extent_map::start	==	key.offset
- extent_map::len	==	file_extent_num_bytes
- extent_map::ram_bytes	==	file_extent_ram_bytes

But that's all, the remaining are pretty different:

- Use block_start to indicate holes/inline extents
  Meanwhile btrfs on-disk file extent items go with a dedicated type for
  inline extents, and disk_bytenr 0 for holes.

- Extra members for fsync optimization
  I'm still not 100% sure how mod_start and mod_len really works though.

- Weird block_start/orig_block_len/orig_start
  In theory we can directly go with the same file_extent_disk_bytenr,
  file_extent_disk_num_bytes and file_extent_offset to calculate the
  remaining members (block_start/orig_start/orig_block_len/block_len).

  But for whatever reason, we didn't go that path and have a hell of
  weird and inconsistent calculation for them.

I do not have the confidence to handle the mess yet, but as the first
step, I would add comments for those members mostly according to
btrfs_extent_item_to_extent_map(), and hopefully we can improve the
situation in not-far-away future.

Qu Wenruo (2):
  btrfs: add extra comments on extent_map members
  btrfs: simplify the inline extent map creation

 fs/btrfs/extent_map.h | 62 ++++++++++++++++++++++++++++++++++++++++++-
 fs/btrfs/file-item.c  | 14 ++++++----
 2 files changed, 70 insertions(+), 6 deletions(-)